Getting lower velocity and air in the face with Bam 51

Getting lower velocity and air in the face with Bam 51

Joined: September 19th, 2010, 11:38 pm

August 9th, 2012, 1:07 pm #1

I have been getting a blast of air in the face when shooting my Bam 51 for a while now.

Placing my fingers at different locations while shooting seems to indicate it was coming from between the upper and lower and the bolt cocking slot.

Two days ago, not needing the riser because I no longer have a shroud, I removed the riser and shortened the viynl tube between the upper and lower and was able to stop all or most of the blast that came from between the upper and lower by tightening it better. I put divers grease on the probe O ring and don't seem to get it from that part.

However I am now seeing it come from the elongated hole where the lower tube. the cocking bolt enters.

I lost 20 FPS, 920 to 900, when I removed the riser for some reason, I am thinking I have an Oring that is blown from some where inside that lower tube and removing the riser and tighening the upper and lower is just redirecting it out the bolt cocking slot.

Any ideas?
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Joined: November 17th, 2006, 3:51 am

August 9th, 2012, 9:33 pm #2

Probably will like this one:

If the transfer port isn't truely sealed to the VALVE (supose to just pass through the air tube and seal on the valve body itself) it could let a burst of air blow back between the air tube and the valve head.

Won't like this one

If the valve stem is a loose fit through the head of the valve, will let air leak past the stem/rod in aback towards the striker (and out throught the openings in the rear of the air tube).

Have had old-old well used Benjamins do this. The valve stem hole in the valve head worn oval, so the air blasts down the stem and towards the striker. Cure was to either make a fatter falve stem and ream the hole in the valve head to match OR to drill out and bush the oval hole smaller so a standard valve stem would fit.


Of the two...hope it is the first.
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Joined: September 19th, 2010, 11:38 pm

August 10th, 2012, 12:32 am #3

I have been getting a blast of air in the face when shooting my Bam 51 for a while now.

Placing my fingers at different locations while shooting seems to indicate it was coming from between the upper and lower and the bolt cocking slot.

Two days ago, not needing the riser because I no longer have a shroud, I removed the riser and shortened the viynl tube between the upper and lower and was able to stop all or most of the blast that came from between the upper and lower by tightening it better. I put divers grease on the probe O ring and don't seem to get it from that part.

However I am now seeing it come from the elongated hole where the lower tube. the cocking bolt enters.

I lost 20 FPS, 920 to 900, when I removed the riser for some reason, I am thinking I have an Oring that is blown from some where inside that lower tube and removing the riser and tighening the upper and lower is just redirecting it out the bolt cocking slot.

Any ideas?
Either in the tube or barrel, I suspect tube.

I really went after those 4 reciever screws, then put on painters tape on the joint between barrel and upper, upper and lower and the oblong cut for the cokcking bolt where it was blowing before.

I fired it and the tape was intact everywhere. Before I tightened it, it would have blown that tape off easily.

Thanks for the advice on potential leaks.
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Joined: May 29th, 2004, 2:30 am

August 10th, 2012, 10:38 am #4

I have been getting a blast of air in the face when shooting my Bam 51 for a while now.

Placing my fingers at different locations while shooting seems to indicate it was coming from between the upper and lower and the bolt cocking slot.

Two days ago, not needing the riser because I no longer have a shroud, I removed the riser and shortened the viynl tube between the upper and lower and was able to stop all or most of the blast that came from between the upper and lower by tightening it better. I put divers grease on the probe O ring and don't seem to get it from that part.

However I am now seeing it come from the elongated hole where the lower tube. the cocking bolt enters.

I lost 20 FPS, 920 to 900, when I removed the riser for some reason, I am thinking I have an Oring that is blown from some where inside that lower tube and removing the riser and tighening the upper and lower is just redirecting it out the bolt cocking slot.

Any ideas?
The B50 I had used a brass transfer port with shoulders cut on each end for o-rings. Your vinyl tubing transfer port may not be completely sealing. It may also be slightly deformed causing a slight restriction in airflow (lower MV). I used to make transfer ports with different hole sizes to "tune" the MV of the rifle. If the leak continues you may want to look into replacing the vinyl transfer port with an OEM solid brass one.



Dan

''If everything seems under control, you're not going fast enough!'' Mario Andretti
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Joined: October 25th, 2002, 3:46 pm

August 14th, 2012, 5:53 pm #5

I have been getting a blast of air in the face when shooting my Bam 51 for a while now.

Placing my fingers at different locations while shooting seems to indicate it was coming from between the upper and lower and the bolt cocking slot.

Two days ago, not needing the riser because I no longer have a shroud, I removed the riser and shortened the viynl tube between the upper and lower and was able to stop all or most of the blast that came from between the upper and lower by tightening it better. I put divers grease on the probe O ring and don't seem to get it from that part.

However I am now seeing it come from the elongated hole where the lower tube. the cocking bolt enters.

I lost 20 FPS, 920 to 900, when I removed the riser for some reason, I am thinking I have an Oring that is blown from some where inside that lower tube and removing the riser and tighening the upper and lower is just redirecting it out the bolt cocking slot.

Any ideas?
If the B50/51 valve was counterbored deeper it would seal with a polytube 'better'

cheers
Walter.....
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Joined: February 16th, 2006, 6:03 am

August 15th, 2012, 7:12 am #6

The only place that leaking air could come from is the O-ring around firing valve shaft...

.

The brass part on the left screws into the body and clamps the O-ring onto the shaft to prevent air from leaking around the shaft.... Do not clamp too much because the shaft has to move freely in the body.... Secure the threaded part with PTFE tape to prevent it from moving in the body....
Do not mistake a slight whiff of air from the slot which could be caused by the hammer assembly moving along when trigger is activated and the air is displaced inside the shaft....


Malan
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Joined: September 19th, 2010, 11:38 pm

August 19th, 2012, 2:46 pm #7

I have been getting a blast of air in the face when shooting my Bam 51 for a while now.

Placing my fingers at different locations while shooting seems to indicate it was coming from between the upper and lower and the bolt cocking slot.

Two days ago, not needing the riser because I no longer have a shroud, I removed the riser and shortened the viynl tube between the upper and lower and was able to stop all or most of the blast that came from between the upper and lower by tightening it better. I put divers grease on the probe O ring and don't seem to get it from that part.

However I am now seeing it come from the elongated hole where the lower tube. the cocking bolt enters.

I lost 20 FPS, 920 to 900, when I removed the riser for some reason, I am thinking I have an Oring that is blown from some where inside that lower tube and removing the riser and tighening the upper and lower is just redirecting it out the bolt cocking slot.

Any ideas?
I have been busy on other projects.

I tore it down last night, the poly tube was deformed on the lower end, so I shorted it a little more and put it all back together. The poly tube is hard and fits very tight in the lower.

I have not chronied it because it is still early morning here and the lighting is not quite good enough, but I still get the air leak. But after examining my chrony on another project, I noticed that the chrony was not open as far as it should have been because of the tripod mounting, so 900 FPS could be the velectiy it was shooting before at a 2000 PSI fill.

But some more facts about this leak.

It has been there since I got the rifle.

It is felt lightly on the left forearm.

The saftety has been removed from this gun and the threaded hole is open, I don't seem to get air from that hole.

Malan, you said that I should not be confused with air movement thru the side caused by the hammer racing forward on firing, so is this perhaps a natural thing with the Bam 50 series to have some air escape from that slot?

If I replaced the poly with the transfer tube insert with the orings, are they all the same size? If not being as I am not a machinist, where would I get one.

I hesitate to spend a lot more money on the gun, I paid too much already, and I have a Royale 500 carbine that does everything the BAM does but does it a lot better.

But then I guess that is just air gunning.
Last edited by unrepentantsinner on August 19th, 2012, 2:47 pm, edited 1 time in total.
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Joined: September 19th, 2010, 11:38 pm

August 19th, 2012, 3:30 pm #8

I just shot it over the chrony starting with a 2200 PSI fill.

The velocity with Kings started at 735FPS, after perhaps 20 shots it went up to 807 and then started going back down, this is 100 FPS lower than before.

When I got the rifle it was doing 975 FPS with a 2800 PSI fill...

So I shortened the spring to slow it down. Which put me at 935 or so. I think the spring has been compromised due to shortening it, it is just not opening up that valve up.

Now on the air leak. I put painters tape on the area where the poly tube goes from the lower to the upper on both sides, I put it on the cocking slot and I put it on the hole on the rear of the reciever where the safety was.

When I shot it only the tape at the cocking slot was disturbed and then only slightly. If I put my fingers ont hat slot when firing, I feel a slight air pulse. So perhaps I am only experiencing the air displacement from the hammer movement?
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