QB thoughts (long post...it's raining..can't shoot)

QB thoughts (long post...it's raining..can't shoot)

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

August 14th, 2010, 5:24 pm #1

1. Took awhile to re-find Mr. Archers post.
http://www.network54.com/Forum/113813/m ... ariability
(Going to use the term striker rather than hammer. For me, a striker moves in a straight line, a hammer has some kind of rotational movement. QBs use a striker in my mind.)

The differences impressed me. No idea on the strength variations, but suspect it would be proportional to the length variations. Thinking that a 54gram striker paired with a 59mm spring isnt going to give the same results as a 58gram striker and a 66mm spring. Not so much the 4grams as it would be a spring a bit more than ¼ inch longer.

Idea here is that two people can put together the same rifle, but because of manufacturing variations, get pretty widely different results.

So far, have found a lighter striker to help in getting more shots. Can play with the striker spring to get up to the vel. you want, but if the absolute max. speed (no matter how much extra gas it wastes) is you goal, keep a full weight striker.

Couldnt find a post, so will insert a picture. Not the max. reduction in weight, but results that will be noticeable.
[/IMG]

EXAPMPLE:
A tuned up QB .22 getting 16.4 foot pounds, but only 23 full power shots. Using a full weight striker.

With the striker weight reduced to 42grams (about a 30% reduction) and no other changes, rifle is down to 12.4 foot pounds but gets 48 shots before fall off.

Increasing the spring tension brings it up to 15.1 foot pounds for 34 shots

Cant quite get back up the absolute max a heavy hammer can give, but get pretty close and a significant greater shot count. Bonus is that its quieter as well.

Hopefully the power adjuster end cap will go on sale shortly, and if so, that will make things a lot easier than taking it all apart and shimming for pre-tension.
http://www.network54.com/Forum/113813/m ... +adjuster-


2. That transfer port is another point of variability. The QBs have gotten better about this, the new port material seems a bit tougher than the soft stuff used in the past, but they still can get squished in assembly. Have tried various sizes, from strangled off versions with a .08 passage to open ones with a .12 passage.

Last experiments were with a PCP QB, but the idea stays the same:
http://www.network54.com/Forum/113813/m ... the+QB+PCP

Understand, the transfer port actually just passes though the gas tube to seal against the valve body itself. The upper end seals to the bottom of the breech. In the Crosman system, the transfer port also seals to the valve body, but the Crosman system has the transfer port passing through the breech and sealing to the barrel itself. The QB system seals to the breech, and then seals the barrel to the breech though o-rings on the barrel.

Gas is strangled off by whatever the smallest opening it has to pass through. Wont do a bit of good to put a transfer port in place that is any larger than the hole in the breech/barrel. Enlarging the hole in the barrel of a .177 seldom does accuracy any good as if it is much larger, the pellet skirt will catch and cut itself on it during chambering. There is some room to play in .22, but be sure not to raise a burr on the inside.


3. Bolt probe. Im still a fan of the hollow bolt probe, but will allow that a small diameter solid one offers a larger gas-path.

Problem with a probe is that if you make them too small, then tend to help cock some pellets when chambering. Some pellets (like CPs) have a wide flat base inside a short skirt. If a skinny probe doesnt hit it dead center, tends to cock the pellet a bit off center.

Other problem with a probe is that it seats different pellets to different depths. The probe pushes on the middle of the pellet, so if there is a deep hollow base (like RWS pellets have) those pellets get seated less deep in the barrel. A more solid base pellet, like the CP, gets seated deeper. A short probe can leave a deep hollow base over hanging the transfer port.

Do like the fancy ball tip probes, because they do seat pellets better, but they're harder to make. If youre going with a probe, then go with one fatter than you originally planed on.

Hollow probes seat by the edge of the pellet, so deep hollow or solid base, theyre going in the same depth.

A lot of the factory bolt probes arent straight, youll find rub marks on one side or the other from them being cocked to the bore. Polishing outside can remove that contact and improve smoothness, and maybe offer a tiny gas path like a real fat probe. Slightly drilling the inside and opening the little hole offers a larger inside path.

(WARNING: the .177s have a very small diameter of steel under that O-ring and they are often heat treated and a bit brittle. Cleaning up a .177 bolt probe can break it off pretty easily if you get aggressive.)

Anyway, can enter bolt probe into the search function and get pages of results and ideas, often conflicting.

A little bolt-probe collection:
Polished outside, reamed inside .177 probe:
[/IMG]

This .177 one broke off when drilling for a probe. Had to add a chunk on ahead of the o-ring and shape it to a probe and o-ring seat (used on a 5mm conversion):
[/IMG]

A fat probe in .22:
[/IMG]

.25 conversion has plenty of room for a big hollow probe. With a larger bore diameter, can get a good sized hollow probe but still have walls thick enough to take the stress of chambering pellets:
[/IMG]


4. The valve and piercing assembly. If running a tanker or bulk fill, modify the valve to get rid of the piercing pin. IF there is no 12gr. to pierce, it just gets in the way of gas flow.

The issue valve seal can last a pretty log time. Its two natural enemies are dirt and over pressure. Because it is pretty soft, it can be sliced more easily by a stray bit of grit, and because it is pretty soft, over pressure can squish it out of shape.

It has an unnatural enemy as well: heat. If in modding the valve stem you let it over heat, can cook the seal material and kill it.

The first time I get into the valve, Ill pitch the fiber filter and brass screen. Good idea, but the issue parts will shed fibers and screw up the valve.

Once it dies, make plans for a tougher seal. Delrin seals are sold by at least a couple of custom shops (Mountain Air and probably others) or you can take a shot at making one.

Are lots of variations on valve seals, angles of seating, lengths, weights, etc. If you make your own, it can be an area for lots of experimentation. There are relationship s between the length of the seal and the strength of the return spring, the diameter of the seal and the gas pressure holding it closed, the diameter of the seal and the gas flow around it and the side of the valve, and the angle of the seat and the flow of the gas gets complicated, but fun to play with small variations.

This is a real long post on converting a 78 to a tanker78, but it has a picture of the simple tanker valve at the end:
http://www.network54.com/Forum/113813/m ... ictures%29

If you buy one already made, then better it mimics the size/shape of the issue valve seal as you can just make a direct replacement.

Often need to lap the new harder material seal to fully seal.

5. Breech to gas tube fit. The issue breech has a small foot print to the air tube; it only has a small area of contact. Make sure it gets full contact. If the breech wiggles or see-saws, it just wont keep its point of impact.

Will test fit the bare breech to the gas tube, finding nay high spots that keep full contact, and stoning the offending burr or high spot down until the breech fully contacts the air tube.

Can see the size of the actual fit of the breech to the gas tube. On this one, was a high spot at the rear end (see as bare metal). Working that down to an even contact helped keep the breech-tube fit stable.
[/IMG]

The transfer port seal has to be just right. Fully sealing the breech to the air tube, but not forcing the breech from full contact. Too short and it leakstoo long and the breech see-saws on the high spot.

6. Barrel to breech fit (2nd paragraph in #2 above to understand why). Are 3 o-rings on the barrel shank of a QB that seal it to the inside of the breech.

The inside of the breech is rough and there is often a burr where the set screw passes though from the top. That burr will slice the o-rings and ruin the seal. Polish he inside of the breech slightly (dont want to enlarge the hole, just get rid of the burrs).

If the rear barrel o-ring leaks, will squirt gas back from the same area as the bolt probe. Can fool you into thinking the bolt probe is leaking. If the front o-ring leaks, you wont feel it unless you put your finger on the joint between the breech and the barrel. Either kind of leak can both rob power and kill accuracy (because it doesnt leak the same amount o gas each time).

If the center o-ring leaks, you wont feel anything on the outside. The gas coming out of the transfer port can run around between the two other o-rings before finding the way into the barrel, and that can be somewhat variable. Better if all 3 are in good shape, not abraded or cut, and lightly lubed with silicon grease.

Are some barrels that are a poor fit to the breech, and theyll give accuracy problems or point of impact shifts form day to day (or both). Can try fatter o-rings, wrapping the central section with Teflon tape to built it up, or whatever other idea you may have.

My best fix of a too loose barrel shank has been to strip the barrel of o-rings, heat it, tin it with solder. Tinning where you get a good coat liquid flowing coating, and fling off the excess leaving just a thin coating of solder. Basically making the whole barrel shank a tiny bit larger. Clean out any solder in the air transfer hole, make sure the o-ring seats are smooth, put the o-rings back on (with silicon grease please), and reassemble.

7. Barrel ends.
The crown gets most of the attention, and there are on-line tutorials about recrowning, but Id test before recrowning. Crown doesnt have to be pretty to do its job, so a straight visual may give you the wrong impression.

Not that the crown is not important, it is, but that a good crown is one that releases the pellet evenly all the way around the base and doesn't amage the pellet on exit. It can do that and not look textbook pretty.

The breech end doesn't get the exam it should. A burr that catches the pellet during chambering is not going to do accuracy one bit of good. Often find a burr at the transfer port.

Best test for this is to chamber a pellet, then knock it back out with a rod from the muzzle. If it has a bit taken out of it, you got a breech-burr-problem.

Different rifle (A BAM 51) but same idea:
http://www.network54.com/Forum/113813/m ... he+obvious

8. I saved barrel bands for the end. If everything else is set up right, I've not found them to make a difference in accuracy. If there is an accuracy difference with one on vs. pure free floating, then suspect one of the above items is not right.

There does seem to be a difference in keeping the point of impact consistent from day to day, and here a barrel band can help.

But a barrel band should NOT hog-tie the barrel. They work best when the barrel is free to slide in and out, but there is nearly no side to side or up and down play. Having them fit tight to the gas tube is a good idea, for it the band can rotate on the gas tube, theyll rotate the barrel with them.

Simplest way to get the QB barrel band to not roatate is to file a couple of small flats on the bottom part of the band that are covered by the stock, and glass bed it to the stock (read up on glass bedding read up on release agents twice). Can use drilled dimples so long as they arent too deep. With the flats (or dimples), it wont rotate, and if it wont rotate, the barrel isnt tweeked by the band.

Another way to look at it is that the barrel band acts as a motion limiter. Barrel is free floated, but only by the tiny amount of space around the band. When the barrel bends from applied pressure, it only bends to the limit of the band.

Aftermarket breeches can have a much larger foot print to the gas tube, which is a big help with stability, and they often have a tighter breech to barrel fit, which is another big help. Having multiple barrel locking screws may help, but if the fit is right, one screw will do the trick.

Alternative method:

This hasnt helped or hurt accuracy (so long as the bands are fitted as outlined above) but has made for very stable rifles that have their zeros stay put for at least months.

Adding a barrel band just ahead of the breech works better on the Crosman system (Discovery) than the QB system. It adds support to the ass end of the barrel/breech and helps keep it stable. QBs have much beefier screws holding the breech to the air tube so the rear band doesnt help as much as it did on the Crosman. But Im not changing it on these three guns because it is working to keep POI the same from day to day (and month to month).
http://www.network54.com/Forum/581291/m ... arrel+band

Stopped raining... would rather shoot than write.







Last edited by gubb33ps on August 14th, 2010, 5:32 pm, edited 1 time in total.
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Joined: May 12th, 2001, 1:29 pm

August 14th, 2010, 5:48 pm #2

Here's a question about transfer port seals. Have you considered using a off-the-shelf ANSI 007 O-ring? Seems to work extremely well in my QB with no futzing around trying to get a clean square cut on plastic tubing.

http://www.network54.com/Forum/113813/m ... port+seal-

Meanwhile, thanks for the interest in the Archer Power Adjuster. It's already in Stephen's pipeline, but there are some other new products (e.g., Drop-In True 2-stage Trigger Kit for one and HDD for another) that got in line ahead of it and will probably delay the Adjuster's intro. But only for a month or so.

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

August 14th, 2010, 6:24 pm #3

Did try an o-ring seal on a PCP conversion. 1st shot seemed fine, leaked on the 2nd shot. Never did find the o-ring, belive it was blown through the transfer port and out the barrel along with the pellet of the first shot. Would indicate it needed a bit more compression to hold it in place.
Last edited by gubb33ps on August 14th, 2010, 6:24 pm, edited 1 time in total.
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Joined: May 12th, 2001, 1:29 pm

August 14th, 2010, 6:37 pm #4

...to that O-ring when you unthinkingly allow the hammer to tap the valve stem with the breech off - it DISAPPEARS!

But in the usual situation, the pressure of firing should push the O-ring away from - not into - the port and up against the walls of the hole in the main tube.

Was your experience perhaps with a modified (i.e., enlarged) port?

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

August 14th, 2010, 6:43 pm #5

Might be that it was loose, but it felt like it compressed when the breech was snuggled down. Could be the o-ring was a bit too restrictive, squeezing shut a tib, so the air just carried it way with it.

Next time I have the PCP version apart, will try an o-ring.
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Joined: May 12th, 2001, 1:29 pm

August 14th, 2010, 9:42 pm #6

So the 007 ring does protrude far enough to contact the breech and compress when the breech snugs down. It's also a press fit into the hole in the main tube. Both aspects of this fit help explain why it seals so nicely.

Unless the hole in the breech was really hogged out, it's hard to imagine how the O-ring could escape.

Steve
Last edited by pneuguy on August 14th, 2010, 10:59 pm, edited 1 time in total.
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Joined: March 28th, 2002, 6:54 pm

August 15th, 2010, 4:09 am #7

of the receiver at the port...plenty of room for an o-ring to extrude. Not a common problem luckily...
Last edited by classicalgas on August 16th, 2010, 12:59 am, edited 1 time in total.
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Joined: November 17th, 2006, 3:51 am

August 15th, 2010, 4:46 am #8

..haven't had a good reason to un-do what's been working, but the next time I need to take it apart, will give the o-ring seal a re-try. Never did find that o-ring, whole or pieces. Could just be as simple as the o-ring picked that moment to break and it shot out as a string.
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