renegadecow Posted June 16, 2014 Report Share Posted June 16, 2014 Bit of a head scratcher here gents. So I was sorting out my 8mm Derringer, the lower barrel decided to roll out its bb when the upper barrel was fired. I concluded it was from a loose hop up as the slightest air leak dislodges a loaded bb. I get a section of o-ring as a shim to add a bit more pressure to the hop (it's a fixed hop up) which seemed to work fine. Then I proceeded to chrono it and got 200fps on the untouched barrel and the one I fixed read 230fps. That's using .34g which comes down to a surprising 0.30J increase. I was stunned so I tried shimming the upper barrel too to see if it was a fluke only the muzzle energy rose by about as much as well. General knowledge tells me that chronoing with less hop or turned off gives a higher, more accurate value to a gun's potential of muzzle energy and the converse is used sometimes to cheat the chrono. But this is for AEGs AFAIK and with the Derringer using gas, maybe there's some other voodoo magic going on. I'm genuinely stumped. Hooray for me since my pocket pistol is now shooting at the equivalent power of a full sized GBB pistol, but I can't get around to explaining how. Link to post Share on other sites
LordElpus Posted June 16, 2014 Report Share Posted June 16, 2014 I have also experienced this on 6mm AEGs but was never able to figure it. Close as I got was maybe the extra hop pressure was improving the airseal around the rubber. Link to post Share on other sites
scorch Posted June 16, 2014 Report Share Posted June 16, 2014 Back pressure has a chance to build up behind the BB before it reaches a breaking point and overcomes the pressure of the hop. Try keeping your hand still and pushing a coin off the table with your finger, then try again but flick the same coin. Same principle. Link to post Share on other sites
Stuey Posted June 16, 2014 Report Share Posted June 16, 2014 More hop holds the BB in while the pressure builds up behind it, eventually overcoming the resistance allowing the BB to 'ping' out at high speed. With less hop, it doesn't take much pressure to allow the BB to escape, so it's pushed slowly out of the barrel, rather than being held and released. IIRC in old classics you could adjust FPS by changing an o-ring in the chamber. Link to post Share on other sites
renegadecow Posted June 16, 2014 Author Report Share Posted June 16, 2014 I don't think I've observed that happenng on AEGs but it could just be from the general lack on air volume. Thing is, a lot of people chrono guns with the hop off to see whether or not it meets field limits. If what's happened to me is happening to just about every gas gun out there' would some be playing hot without their knowing? Link to post Share on other sites
Robinio Posted June 16, 2014 Report Share Posted June 16, 2014 Did you rechage any gas in the magazine/gun? might be that. Link to post Share on other sites
amateurstuntman Posted June 16, 2014 Report Share Posted June 16, 2014 IIRC in old classics you could adjust FPS by changing an o-ring in the chamber. Yes bullet valve style guns. They allowed the air pressure to develop before releasing the bb. I don't think I've observed that happenng on AEGs but it could just be from the general lack on air volume. Thing is, a lot of people chrono guns with the hop off to see whether or not it meets field limits. If what's happened to me is happening to just about every gas gun out there' would some be playing hot without their knowing? No, it doesn't happen with AEGs. There is a limited amount of air with an AEG and that restriction does not apply to gas guns. There will be a sweet spot after which you will see a loss of power. Link to post Share on other sites
Stuey Posted June 16, 2014 Report Share Posted June 16, 2014 This is why guns should be chrono'd with the BB the player intends to use with the hop set for play. Link to post Share on other sites
renegadecow Posted June 17, 2014 Author Report Share Posted June 17, 2014 The hard part about that is, for people intending to cheat it's too easy just to lie about bb weight. Now that I think about it, the exact same thing happened to me years ago when I was making shells for my Mateba. I found that making the part that holds the bb tighter increased fps by around 20 but it wasn't the hop so I thought nothing much of it. In the interest of maximizing the potential muzzle energy a gun would get, maybe a redesigned hop chamber for AEGs and gas guns alike where the bb is held not by the hop mound but by a separate uniform chamber (like the cartridge on a revolver) would help. I imagine just using the hop dialed all the way up to achieve the same effect would mean sending bbs to the moon. I'll try more testing in my part, at least for gas guns. Link to post Share on other sites
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