Question about air (pcp) in tube only on a 78

Question about air (pcp) in tube only on a 78

Joined: November 27th, 2006, 1:04 am

September 16th, 2010, 5:15 pm #1

I asked Mike about that and he said not exceed 850 in the tube.

Has anyone tried to fill the tube with air to 850 and if you did, how many shots could be expected?

I would find 4 to be plenty. I hunt, not paper punch and the limit on squirrels is 3 per day. So that would satisfy me.

TIA
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Joined: June 7th, 2010, 12:14 am

September 16th, 2010, 6:51 pm #2

Through a regulator.

I have no experience myself, but look at the CO2 curves to get a feel for practical maximum "as designed" pressures.

Heck, You might get on extra shot to chase the critter to your side of the tree.

Cal
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Joined: November 17th, 2006, 3:51 am

September 16th, 2010, 9:24 pm #3

I asked Mike about that and he said not exceed 850 in the tube.

Has anyone tried to fill the tube with air to 850 and if you did, how many shots could be expected?

I would find 4 to be plenty. I hunt, not paper punch and the limit on squirrels is 3 per day. So that would satisfy me.

TIA
From an old post:


Gave up on the mix-match of spare parts and just broke out a little .22 QB78 carbine. Rifle was set for a pretty close to factory 580fps/14.3 gr. when running on co2.

Test rifle:
[[/IMG]

WARNINGS:
1. Am about to pressurize a QB co2 bulk fill tube with something besides co2 (air). STOP READING NOW if that is an upsetting thought for you.
2. Selected 1100psi as the absolute max. Figure for 50% of the readers, that will be too much to subject a co2 tube to, for 25% it will be too conservative, about 23% wont know what the hell Im talking about, and maybe 2% will agree with the 1100psi figure. 1100psi about the pressure that has been measured when the co2 tube was charged with co2 and the temperature was very close to 90F (which is pretty much all summer long down here).
3. For goodness sake, DO NOT use a tank to fill. WAY WAY too easy to over fill with a tank when shooting for only 1100psi.
4. If you screw up, use a poor condition rifle, or over fill, have a good chance of turning the rifle into a grenade.


THINGS THAT CAN GO WRONG:
1. Valve stem failure; better to use a Delrin valve stem.
2. Check over the bulk fill end cap, its the most likely point of failure as it is held to the tube by that external collar. Collar fails, the end cap becomes a big ugly (deadly) bullet.
3. Test for leaks over night., air is thinner than co2 and does not replace its pressure from liquid as co2 does, so leaks show up (probably leaked co2 as well, but would take days to notice it). Pump the system up to 800psi and put it away for the evening. After several hours check the pressure. There is no gauge, so will have to re-connect the pump, pump slowly, and listen for the tick of the fill valve opening to know the internal pressure.
4. IF there is a leak, hunt it down and fix it, then restest for leaks.
5. Valve set back is no more likely at this pressure than when using co2 in the summer. There is a big steel ring that the valve assembly butts up against. That ring is where the stock bolt threads in from below, and the Phillips head breech screw threads in from above, so it is well secured.

All vel. readings taken at 5 yards.., can estimate MV to be 20-30fps higher.

14.3gr. CP @ 5 yards
Stock: 1100psi start:
747
736
727
713
689
662
(tests stopped) 700psi

PROBLEM: Too damned fast, too wasteful of air, too inconsistent.

Observation: the output is already in serious decline when starting at 1200psi. A higher starting pressure would certainly give something more like a bell curve, but thats not a direction I want to go with a stock co2 gas tube. Only 3 shots in 30fps of each other.

Test solution:
Reduce the hammer strike to lower the pressure level (and power level) that gives more consistent result.

Both a lightened striker and a shortened/lighter spring.

Striker was easy, had a spare that was already lightened.
[/IMG]


14.3gr. CP @ 5 yards
580 - 1100psi
602
627
640
657
661
676
682
679
672
648
629
614
595
(test halted) 700psi

Observation: this is certainly in the pressure area Id be willing to subject a CO2 gas tube to (peaking at about 950-1000psi) but not the consistency with a 30fps shot count of only 6 shots. Power is more than really desired for a back yard gun, and if I can trade a little less gas use and energy, while still running at 1100psi max fill pressure, would be happier.

Solution: SLIGHTLY increase spring tension to try for a 1100PSI peak and choke off the transfer port to try for less gas use/more good shots. With a little luck, the increase from spring tension will be more than off set by the strangled off transfer port.

Spring tension increased by adding one shim (.06 thick washer).

Choking the transfer port.

In this case, modified a metal transfer port from a Crosman to work through the poly-port of the QB. Flange machined to fit tightly in the breech, poly port still doing the sealing. No leaks noted throughout the tests.
[/IMG]
[/IMG]
Adjusted spring tension, lighted hammer, choked off poly port:

14.3gr. @ 5 yards
649 1100psi
661
672
676
673
674
668
669
653
642
630
621
(test halted) 750psi

(Same as above, but with 21gr. pellets to see if back pressure conserves significant air.)
21gr. @ 5 yards
550 1100psi
557
571
579
578
572
562
561
551
548
519
502 700psi


Light or heavy, am only getting 9-10 shots in the 30fps range.

Tested both velocity (measured at 5 yards) and accuracy (targets at 20 yards), looking for a visual representation of the point of impact difference over 20 shots (4 groups of 5 shots). As luck would have it, ran out of printed targets, so the below are hand drawn.

[/IMG]

Targets #1 - #4 (20 shots) were on one fill from 1100psi to about 700psi. Targets #5-#8 (20 shots) wee on a second fill of the same range. Takes 15 pumps of the hand pump to raise pressure from 700psi to 1100psi. For those with good eyes, the little numbers writer are not group sizes, are estimates of point of impact.

In both cases, the first 15 shots were pretty close to the same spot (within 1/10, from .46 to .53 high), the last 5 shots 1/5-1/4 lower on the target (.32 high). Not unexpected from the chronograph results. For back yard plinking, for 12-15 shots its acceptable.

There is more that can be done, but it will be valve work and not simply adjusting the spring/striker weight/ transfer port diameter.

More testing halted until daylight tomorrow too many error readings on the chronograph. Next trail will be to increase the valve stem return spring tension slightly. Again, will drop some power, but may pick up a few more consistent shots by decreasing valve dwell. Goal would be 15-18 shots at about 12 foot pounds.


Another day, some final tests.

Figuring the sweet spot to be about 7-10 shots long, tried to stay inside of that area for a few accuracy/point of impact tests at 20yards.
[/IMG]

Pretty much confirmed that it takes a crap-load of variation for anything noticeable to show up at 20yards. Limited testing, but this is right in the ballpark of what the same rifle will do on co2, where vel. variations are under 10fps.

The impact differences here are not significant; would become more of a vertical string with range, but for short range shooting variations in accuracy are covering the small variations in point of impact.

(Againvel. readings taken at 5 yards. Can make whatever correction you feel is about right for muzzle vel.)

1100psi to 750psi/ 10 shots (two 5-shot groups):

CPs:
612
616
624
616
621

623
617
611
588
588 36fps variation / 8 shots under 20fps variation

JSB 15.8:
550
561
566
567
578

575
580
575
570
566 30fps variation / 8 shots under 20fps variation

RWS Hobby
642
638
639
645
641

643
639
633
630
618 27fps variation / 9 shots under 20fps variation


LPA IS ALL ABOUT VOLUME:

There just isnt enough volume to run a QB sized tube using a more or less standard valve at low air pressure and get a significant number of shots. For safety reasons, running at higher than co2 pressure should be avoided; get a new high pressure rated air tube, its both made for HPA and generally longer/more volume. The basic Co2 valve is designed to move a lot of gas, which could be made serve well for making a powerful high pressure rifle (add the Delrin valve stem/lose the 12gr. piercing assembly).

With a tiny volume running at low pressure, each shot decreases air pressure significantly. First shot is at about 1100psi, second shot at about 1060, third shot at 1020, etc. Over the 10 shots, only about 350psi was lost, but that is about 1/3 of the available pressure.

Basically, a co2 valve just cant self-adjust well enough to keep things constant.

If a constant supply of air was provided (with a regulated HPA bottle), could forget about making the valve self regulate and just adjust the valve/hammer/transfer port to work at one single pressure. Should be able to adjust to 20-21 foot pounds with a constant supply of 1100psi air and get 45-65 full power shots from a 13CI HPA bottle.

BOTTOM LINE:

You can run a QB78 (or other good condition bulk fill co2 rifle) on LPA, just keep the pressure no greater than typical warm weather co2 pressure.

You will get very few (maybe only 2) shots that are reasonably close together in velocity.

Typical valve adjusted for good performance on co2 is likely to be very air-wasteful, making great power but using more air per shot that would be optimum.
Last edited by gubb33ps on September 17th, 2010, 12:21 am, edited 1 time in total.
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Joined: January 30th, 2005, 11:00 pm

September 16th, 2010, 11:16 pm #4

Good Post Robert NT:
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Joined: June 7th, 2010, 12:14 am

September 16th, 2010, 11:21 pm #5

From an old post:


Gave up on the mix-match of spare parts and just broke out a little .22 QB78 carbine. Rifle was set for a pretty close to factory 580fps/14.3 gr. when running on co2.

Test rifle:
[[/IMG]

WARNINGS:
1. Am about to pressurize a QB co2 bulk fill tube with something besides co2 (air). STOP READING NOW if that is an upsetting thought for you.
2. Selected 1100psi as the absolute max. Figure for 50% of the readers, that will be too much to subject a co2 tube to, for 25% it will be too conservative, about 23% wont know what the hell Im talking about, and maybe 2% will agree with the 1100psi figure. 1100psi about the pressure that has been measured when the co2 tube was charged with co2 and the temperature was very close to 90F (which is pretty much all summer long down here).
3. For goodness sake, DO NOT use a tank to fill. WAY WAY too easy to over fill with a tank when shooting for only 1100psi.
4. If you screw up, use a poor condition rifle, or over fill, have a good chance of turning the rifle into a grenade.


THINGS THAT CAN GO WRONG:
1. Valve stem failure; better to use a Delrin valve stem.
2. Check over the bulk fill end cap, its the most likely point of failure as it is held to the tube by that external collar. Collar fails, the end cap becomes a big ugly (deadly) bullet.
3. Test for leaks over night., air is thinner than co2 and does not replace its pressure from liquid as co2 does, so leaks show up (probably leaked co2 as well, but would take days to notice it). Pump the system up to 800psi and put it away for the evening. After several hours check the pressure. There is no gauge, so will have to re-connect the pump, pump slowly, and listen for the tick of the fill valve opening to know the internal pressure.
4. IF there is a leak, hunt it down and fix it, then restest for leaks.
5. Valve set back is no more likely at this pressure than when using co2 in the summer. There is a big steel ring that the valve assembly butts up against. That ring is where the stock bolt threads in from below, and the Phillips head breech screw threads in from above, so it is well secured.

All vel. readings taken at 5 yards.., can estimate MV to be 20-30fps higher.

14.3gr. CP @ 5 yards
Stock: 1100psi start:
747
736
727
713
689
662
(tests stopped) 700psi

PROBLEM: Too damned fast, too wasteful of air, too inconsistent.

Observation: the output is already in serious decline when starting at 1200psi. A higher starting pressure would certainly give something more like a bell curve, but thats not a direction I want to go with a stock co2 gas tube. Only 3 shots in 30fps of each other.

Test solution:
Reduce the hammer strike to lower the pressure level (and power level) that gives more consistent result.

Both a lightened striker and a shortened/lighter spring.

Striker was easy, had a spare that was already lightened.
[/IMG]


14.3gr. CP @ 5 yards
580 - 1100psi
602
627
640
657
661
676
682
679
672
648
629
614
595
(test halted) 700psi

Observation: this is certainly in the pressure area Id be willing to subject a CO2 gas tube to (peaking at about 950-1000psi) but not the consistency with a 30fps shot count of only 6 shots. Power is more than really desired for a back yard gun, and if I can trade a little less gas use and energy, while still running at 1100psi max fill pressure, would be happier.

Solution: SLIGHTLY increase spring tension to try for a 1100PSI peak and choke off the transfer port to try for less gas use/more good shots. With a little luck, the increase from spring tension will be more than off set by the strangled off transfer port.

Spring tension increased by adding one shim (.06 thick washer).

Choking the transfer port.

In this case, modified a metal transfer port from a Crosman to work through the poly-port of the QB. Flange machined to fit tightly in the breech, poly port still doing the sealing. No leaks noted throughout the tests.
[/IMG]
[/IMG]
Adjusted spring tension, lighted hammer, choked off poly port:

14.3gr. @ 5 yards
649 1100psi
661
672
676
673
674
668
669
653
642
630
621
(test halted) 750psi

(Same as above, but with 21gr. pellets to see if back pressure conserves significant air.)
21gr. @ 5 yards
550 1100psi
557
571
579
578
572
562
561
551
548
519
502 700psi


Light or heavy, am only getting 9-10 shots in the 30fps range.

Tested both velocity (measured at 5 yards) and accuracy (targets at 20 yards), looking for a visual representation of the point of impact difference over 20 shots (4 groups of 5 shots). As luck would have it, ran out of printed targets, so the below are hand drawn.

[/IMG]

Targets #1 - #4 (20 shots) were on one fill from 1100psi to about 700psi. Targets #5-#8 (20 shots) wee on a second fill of the same range. Takes 15 pumps of the hand pump to raise pressure from 700psi to 1100psi. For those with good eyes, the little numbers writer are not group sizes, are estimates of point of impact.

In both cases, the first 15 shots were pretty close to the same spot (within 1/10, from .46 to .53 high), the last 5 shots 1/5-1/4 lower on the target (.32 high). Not unexpected from the chronograph results. For back yard plinking, for 12-15 shots its acceptable.

There is more that can be done, but it will be valve work and not simply adjusting the spring/striker weight/ transfer port diameter.

More testing halted until daylight tomorrow too many error readings on the chronograph. Next trail will be to increase the valve stem return spring tension slightly. Again, will drop some power, but may pick up a few more consistent shots by decreasing valve dwell. Goal would be 15-18 shots at about 12 foot pounds.


Another day, some final tests.

Figuring the sweet spot to be about 7-10 shots long, tried to stay inside of that area for a few accuracy/point of impact tests at 20yards.
[/IMG]

Pretty much confirmed that it takes a crap-load of variation for anything noticeable to show up at 20yards. Limited testing, but this is right in the ballpark of what the same rifle will do on co2, where vel. variations are under 10fps.

The impact differences here are not significant; would become more of a vertical string with range, but for short range shooting variations in accuracy are covering the small variations in point of impact.

(Againvel. readings taken at 5 yards. Can make whatever correction you feel is about right for muzzle vel.)

1100psi to 750psi/ 10 shots (two 5-shot groups):

CPs:
612
616
624
616
621

623
617
611
588
588 36fps variation / 8 shots under 20fps variation

JSB 15.8:
550
561
566
567
578

575
580
575
570
566 30fps variation / 8 shots under 20fps variation

RWS Hobby
642
638
639
645
641

643
639
633
630
618 27fps variation / 9 shots under 20fps variation


LPA IS ALL ABOUT VOLUME:

There just isnt enough volume to run a QB sized tube using a more or less standard valve at low air pressure and get a significant number of shots. For safety reasons, running at higher than co2 pressure should be avoided; get a new high pressure rated air tube, its both made for HPA and generally longer/more volume. The basic Co2 valve is designed to move a lot of gas, which could be made serve well for making a powerful high pressure rifle (add the Delrin valve stem/lose the 12gr. piercing assembly).

With a tiny volume running at low pressure, each shot decreases air pressure significantly. First shot is at about 1100psi, second shot at about 1060, third shot at 1020, etc. Over the 10 shots, only about 350psi was lost, but that is about 1/3 of the available pressure.

Basically, a co2 valve just cant self-adjust well enough to keep things constant.

If a constant supply of air was provided (with a regulated HPA bottle), could forget about making the valve self regulate and just adjust the valve/hammer/transfer port to work at one single pressure. Should be able to adjust to 20-21 foot pounds with a constant supply of 1100psi air and get 45-65 full power shots from a 13CI HPA bottle.

BOTTOM LINE:

You can run a QB78 (or other good condition bulk fill co2 rifle) on LPA, just keep the pressure no greater than typical warm weather co2 pressure.

You will get very few (maybe only 2) shots that are reasonably close together in velocity.

Typical valve adjusted for good performance on co2 is likely to be very air-wasteful, making great power but using more air per shot that would be optimum.
And the daily limit is still 3

I picked up an Evanix 6-shooter (renegade) figuring six shots would be a good day.

Maybe I should have just pumped up the AR.

It's ALL good!
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Joined: November 17th, 2006, 3:51 am

September 17th, 2010, 12:37 am #6

It's a good way to gauge the performance of your rifle at different pressure levels (or valve set ups). That first test was pretty close to a stock valve; the shot count was super low but the jump from 580fps on co2 was more than expected.

Don't know if it's truly useful unless you get it to the point of not leaking even a little bit. Very small volume, so a small leak runs the pressure down real fast.

My bag limit is still 8 (tree squirrels here are mostly grays) and possession of 16. Still, I hunt mostly the little places where firearms wouldn't be wise, and in those little areas, there aren't 8 squirrels to be shot.
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Joined: June 7th, 2010, 12:14 am

September 17th, 2010, 12:44 am #7

But....I need to pick up a hand pump for the "revolver"

Any suggestions? PA has many choices, and some are out of stock.

Cal



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Joined: November 17th, 2006, 3:51 am

September 17th, 2010, 1:12 am #8

An old-old FX and a Hill. If they both were to crap out, would probably buy another FX. Hill isn't a bad unit at all, but have serious doubts about the dry-pac system doing much of anything unless you change out the dry-pac about every month (seems to be a chemical reaction to moisture, so it won't just dry back out from baking the substance). The Hill is probably the easier of the two to get apart for a rebuild (they do sell the rebuild kits for both).

I do notice that both of these are on the upper end of hand pump prices, which probably isn't a random chance.

Here's a British comparsion of the two... pretty much agree with the conclusions.
http://gunmart.net/accessories_review/hills_v_fx_pumps/
Last edited by gubb33ps on September 17th, 2010, 1:13 am, edited 1 time in total.
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Joined: June 7th, 2010, 12:14 am

September 17th, 2010, 1:28 am #9

Thanks for the link, good read! NT
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Joined: November 27th, 2006, 1:04 am

September 17th, 2010, 7:45 pm #10

From an old post:


Gave up on the mix-match of spare parts and just broke out a little .22 QB78 carbine. Rifle was set for a pretty close to factory 580fps/14.3 gr. when running on co2.

Test rifle:
[[/IMG]

WARNINGS:
1. Am about to pressurize a QB co2 bulk fill tube with something besides co2 (air). STOP READING NOW if that is an upsetting thought for you.
2. Selected 1100psi as the absolute max. Figure for 50% of the readers, that will be too much to subject a co2 tube to, for 25% it will be too conservative, about 23% wont know what the hell Im talking about, and maybe 2% will agree with the 1100psi figure. 1100psi about the pressure that has been measured when the co2 tube was charged with co2 and the temperature was very close to 90F (which is pretty much all summer long down here).
3. For goodness sake, DO NOT use a tank to fill. WAY WAY too easy to over fill with a tank when shooting for only 1100psi.
4. If you screw up, use a poor condition rifle, or over fill, have a good chance of turning the rifle into a grenade.


THINGS THAT CAN GO WRONG:
1. Valve stem failure; better to use a Delrin valve stem.
2. Check over the bulk fill end cap, its the most likely point of failure as it is held to the tube by that external collar. Collar fails, the end cap becomes a big ugly (deadly) bullet.
3. Test for leaks over night., air is thinner than co2 and does not replace its pressure from liquid as co2 does, so leaks show up (probably leaked co2 as well, but would take days to notice it). Pump the system up to 800psi and put it away for the evening. After several hours check the pressure. There is no gauge, so will have to re-connect the pump, pump slowly, and listen for the tick of the fill valve opening to know the internal pressure.
4. IF there is a leak, hunt it down and fix it, then restest for leaks.
5. Valve set back is no more likely at this pressure than when using co2 in the summer. There is a big steel ring that the valve assembly butts up against. That ring is where the stock bolt threads in from below, and the Phillips head breech screw threads in from above, so it is well secured.

All vel. readings taken at 5 yards.., can estimate MV to be 20-30fps higher.

14.3gr. CP @ 5 yards
Stock: 1100psi start:
747
736
727
713
689
662
(tests stopped) 700psi

PROBLEM: Too damned fast, too wasteful of air, too inconsistent.

Observation: the output is already in serious decline when starting at 1200psi. A higher starting pressure would certainly give something more like a bell curve, but thats not a direction I want to go with a stock co2 gas tube. Only 3 shots in 30fps of each other.

Test solution:
Reduce the hammer strike to lower the pressure level (and power level) that gives more consistent result.

Both a lightened striker and a shortened/lighter spring.

Striker was easy, had a spare that was already lightened.
[/IMG]


14.3gr. CP @ 5 yards
580 - 1100psi
602
627
640
657
661
676
682
679
672
648
629
614
595
(test halted) 700psi

Observation: this is certainly in the pressure area Id be willing to subject a CO2 gas tube to (peaking at about 950-1000psi) but not the consistency with a 30fps shot count of only 6 shots. Power is more than really desired for a back yard gun, and if I can trade a little less gas use and energy, while still running at 1100psi max fill pressure, would be happier.

Solution: SLIGHTLY increase spring tension to try for a 1100PSI peak and choke off the transfer port to try for less gas use/more good shots. With a little luck, the increase from spring tension will be more than off set by the strangled off transfer port.

Spring tension increased by adding one shim (.06 thick washer).

Choking the transfer port.

In this case, modified a metal transfer port from a Crosman to work through the poly-port of the QB. Flange machined to fit tightly in the breech, poly port still doing the sealing. No leaks noted throughout the tests.
[/IMG]
[/IMG]
Adjusted spring tension, lighted hammer, choked off poly port:

14.3gr. @ 5 yards
649 1100psi
661
672
676
673
674
668
669
653
642
630
621
(test halted) 750psi

(Same as above, but with 21gr. pellets to see if back pressure conserves significant air.)
21gr. @ 5 yards
550 1100psi
557
571
579
578
572
562
561
551
548
519
502 700psi


Light or heavy, am only getting 9-10 shots in the 30fps range.

Tested both velocity (measured at 5 yards) and accuracy (targets at 20 yards), looking for a visual representation of the point of impact difference over 20 shots (4 groups of 5 shots). As luck would have it, ran out of printed targets, so the below are hand drawn.

[/IMG]

Targets #1 - #4 (20 shots) were on one fill from 1100psi to about 700psi. Targets #5-#8 (20 shots) wee on a second fill of the same range. Takes 15 pumps of the hand pump to raise pressure from 700psi to 1100psi. For those with good eyes, the little numbers writer are not group sizes, are estimates of point of impact.

In both cases, the first 15 shots were pretty close to the same spot (within 1/10, from .46 to .53 high), the last 5 shots 1/5-1/4 lower on the target (.32 high). Not unexpected from the chronograph results. For back yard plinking, for 12-15 shots its acceptable.

There is more that can be done, but it will be valve work and not simply adjusting the spring/striker weight/ transfer port diameter.

More testing halted until daylight tomorrow too many error readings on the chronograph. Next trail will be to increase the valve stem return spring tension slightly. Again, will drop some power, but may pick up a few more consistent shots by decreasing valve dwell. Goal would be 15-18 shots at about 12 foot pounds.


Another day, some final tests.

Figuring the sweet spot to be about 7-10 shots long, tried to stay inside of that area for a few accuracy/point of impact tests at 20yards.
[/IMG]

Pretty much confirmed that it takes a crap-load of variation for anything noticeable to show up at 20yards. Limited testing, but this is right in the ballpark of what the same rifle will do on co2, where vel. variations are under 10fps.

The impact differences here are not significant; would become more of a vertical string with range, but for short range shooting variations in accuracy are covering the small variations in point of impact.

(Againvel. readings taken at 5 yards. Can make whatever correction you feel is about right for muzzle vel.)

1100psi to 750psi/ 10 shots (two 5-shot groups):

CPs:
612
616
624
616
621

623
617
611
588
588 36fps variation / 8 shots under 20fps variation

JSB 15.8:
550
561
566
567
578

575
580
575
570
566 30fps variation / 8 shots under 20fps variation

RWS Hobby
642
638
639
645
641

643
639
633
630
618 27fps variation / 9 shots under 20fps variation


LPA IS ALL ABOUT VOLUME:

There just isnt enough volume to run a QB sized tube using a more or less standard valve at low air pressure and get a significant number of shots. For safety reasons, running at higher than co2 pressure should be avoided; get a new high pressure rated air tube, its both made for HPA and generally longer/more volume. The basic Co2 valve is designed to move a lot of gas, which could be made serve well for making a powerful high pressure rifle (add the Delrin valve stem/lose the 12gr. piercing assembly).

With a tiny volume running at low pressure, each shot decreases air pressure significantly. First shot is at about 1100psi, second shot at about 1060, third shot at 1020, etc. Over the 10 shots, only about 350psi was lost, but that is about 1/3 of the available pressure.

Basically, a co2 valve just cant self-adjust well enough to keep things constant.

If a constant supply of air was provided (with a regulated HPA bottle), could forget about making the valve self regulate and just adjust the valve/hammer/transfer port to work at one single pressure. Should be able to adjust to 20-21 foot pounds with a constant supply of 1100psi air and get 45-65 full power shots from a 13CI HPA bottle.

BOTTOM LINE:

You can run a QB78 (or other good condition bulk fill co2 rifle) on LPA, just keep the pressure no greater than typical warm weather co2 pressure.

You will get very few (maybe only 2) shots that are reasonably close together in velocity.

Typical valve adjusted for good performance on co2 is likely to be very air-wasteful, making great power but using more air per shot that would be optimum.
It was noisy though. Got 6 shots. I dont have a chrony but it penetrated a 2x4 a bit more than the co2 did after I replaced the valve stem.
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