GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Ribbonstone on March 26, 2016, 02:53:51 PM
-
My old friend is feeling kind of sick. It’s my fault, I gutted it and used the striker/striker spring assembly in a different rifle, and now its rejecting the new parts.
Still a 12-13 foot pound rifle running on an 850psi output 13ci tank, but now gets a lot less shots per tank full than it did. Was good for about 140-160 shots per fill, now only manages 100.
No leaking air at rest and can’t find any air leaks at the transfer port or bolt probe, so it’s likely squirting out “extra” air down the barrel too late to add any speed.
BIG clue is that it sounds funny…like a little “buzz”…which would be hammer bounce. Got to cure that to get the old-girl running the way she was running when healthy.
Have been playing with really heavy weight .177’s in a BAM 50 (going for 30 foot pounds) and got some very encouraging results with some of the really heavy .177 pellets.
Then got to testing the slower (20 and 12 foot pound) .177’s. This AR2078 is the last of those tests.
Pellets: (fired/recovered showing rifling)
(http://i157.photobucket.com/albums/t50/ribbonstone/f93148e1-0cc1-4ba7-91d2-0898059c7d93.jpg) (http://s157.photobucket.com/user/ribbonstone/media/f93148e1-0cc1-4ba7-91d2-0898059c7d93.jpg.html)
Rifle:
(http://i157.photobucket.com/albums/t50/ribbonstone/AR2078/421a4164-5525-43fd-acee-2318e9250fa4.jpg) (http://s157.photobucket.com/user/ribbonstone/media/AR2078/421a4164-5525-43fd-acee-2318e9250fa4.jpg.html)
Target:
(http://i157.photobucket.com/albums/t50/ribbonstone/AR2078/c16307d7-a08a-4ee0-a5d3-80b43aabc249.jpg) (http://s157.photobucket.com/user/ribbonstone/media/AR2078/c16307d7-a08a-4ee0-a5d3-80b43aabc249.jpg.html)
Energy normally (like in the 8 other PCP .177 rifles tested) increases. Even in the really slow match type rifles, they’d gain energy with weight. Not in this rifle.
So…thinking on it…if the “extra” air was squirting out all at the same time at the shot, would have seen that increase. As there was no increase in energy with weight, then the air isn’t coming out in one blast.
So we have these clues that indicate hammer bounce:
1.Output gauge still reading 850psi, so the regulator isn’t acting up.
2.Greater air use for the same energy.
3.No leaking at the shot.
4.No leaking in storage.
5.Slight “brap” sound rather than a clear “pop”.
6.Heavy weight pellets not gaining energy.
I’ll get it cured, get back to about 150 shots per tank.
-
A sticky poppet valve or an over lubed hammer slows things down and can indeed keep valve open longer w/o adding any pellet speed.
If it is bounce, indeed a spring with a higher spring rate tho likely less preload will speed up the hammer strike and if preload stays low poppet can still close quickly too.
Darn balance issues were always trying to figure out :P
-
Will pull it apart and take a look later today (have to head to the accountant to be raked over the tax-coals) or tomorrow. Will try a lighter striker first, as that's worked out in other guns...if it gets worse, will know that's the wrong direction to head.
Of course, when I have it apart, will certainly take a hard look-see at anything that looks suspect...lube/valve stem/spring/etc.
Only running on 850 air certainly doesn't take a mega spring to get the valve opened.
Kind of like a laxative commercial....just getting everything in balance.
BTW: about the fired pellets. Has that on/off between the tank and the tube. Can empty the tube, slowly bleed in about 250-350PSI of air from the bottle, and fire a pellet into a pile of soft fluffy cotton without damaging it past the point of seeing rifling.
-
Had a chance to look at it today, but light was going fast so long chronograph strings were “out” if I was going to get this done today.
So I had to do the next best (less time consuming) thing.
With that gauged on/off, can fill the tube and then turn it to “off”. Gauge will read the air pressure in the tube.
Fire one shot and see how much the pressure dropped in the tube.
So by opening and closing the on/off for each shot, could see how much air each shot uses. Averaged 7 shots.
With that as a guide, the posted tune used something like 140-150psi of airtube air per shot. Still had that slight blurred “popppp” that likely disguised a bounce problem.
Swapped to a lighter striker and a slightly heavier spring (which still isn’t heavy as all it’s got to do is open against 850psi of valve closing pressure).
Air use dropped to 100-110psi per shot, the report is a single quick “pop”.
Just enough light to get a few shots in…and It looks like I gained 10fps (to 670fps with the 13.4’s) and the gauge indicates less air use.
Will have to test, but if it works out as expected, will be a little up in power and the shot count about 25-30% longer (which should make it to 130 shots per fill….not up to the 150 I want, but in the right direction).
If you like to do math, can lend a helping hand here...I'm too tired to get this right.
I figure the distance between the tank block and valve to be about 4.25”…add a guess about the internal volume of the basically stock valve…and a little bit for the passage way thought the valve block and on/off. Basically something like 35cc’s of air volume. Pre-tune it would drop that volume from 850psi to about 710psi. After the above tune, drops that volume from 850psi to about 740psi per shot.
So, thinking proportionally, if the pressure in a given volume drops by about 13%, then it's the same change in pressure as if the volume changed by 13%, so the actually volume of air used (in that 35cc's) is something like 4.5-4.6cc's per shot? And the first (air wasting) tune changed by 17%, so the air use would end up something like 5.9-6cc?
Then again, just went though the Taxes with the accountant, and my mind is number-numb.