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Posted: 8/28/2012 5:33:04 PM EDT
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I have a Frankford Arsenal registered lower with an 11" upper and a car butt stock. Gun functions fine in this setup. That's the way I bought it.
I bought a second upper, head spaced it on same bolt, but the gun only works semi in that config. I can shoot semi all day long. Even did an Apple Seed shoot that way. When I try full auto, it fires one round, ejects empty, chambers the next, but won't fire. Trigger does nothing. Rotating the safety to semi will allow me to fire again. I can also retract the bolt and fire again, but still only fires one round. The new upper is a flattop witha 16" barrel. The lower has KNR anti rotation pins that I added, but I haven't made any other changes. I tried replacing the autosear, thinking maybe it was worn, but that didn't change anything. Not sure what else to check. Any ideas? Thanks. |
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bolt bounce
New upper is under or over gassed compared to the working one; probably over-gassed, as many short uppers these days are, since it doesn't make a difference in semi, and that's their target market. Try a much heavier buffer - H2 or H3, or one of the Spike's T series |
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Quoted:
I have a Frankford Arsenal registered lower with an 11" upper and a car butt stock. Gun functions fine in this setup. That's the way I bought it. I bought a second upper, head spaced it on same bolt, but the gun only works semi in that config. I can shoot semi all day long. Even did an Apple Seed shoot that way. When I try full auto, it fires one round, ejects empty, chambers the next, but won't fire. Trigger does nothing. Rotating the safety to semi will allow me to fire again. I can also retract the bolt and fire again, but still only fires one round. The new upper is a flattop witha 16" barrel. The lower has KNR anti rotation pins that I added, but I haven't made any other changes. I tried replacing the autosear, thinking maybe it was worn, but that didn't change anything. Not sure what else to check. Any ideas? Thanks. When you function check it in FA, does the trigger reset correctly? Does the sear trip when you ease the bolt forward? |
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bolt bounce New upper is under or over gassed compared to the working one; probably over-gassed, as many short uppers these days are, since it doesn't make a difference in semi, and that's their target market. Try a much heavier buffer - H2 or H3, or one of the Spike's T series Thanks for your quick reply. I'll order a heavier buffer and see if that fixes it. Can you explain the mechanics of what you're saying? Is the bolt carrier actually moving backward a second time (bouncing) after the recoil and chambering of the next round and hitting the autosear? In addition to fixing it, I'd like to understand it. 
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Quoted:
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I have a Frankford Arsenal registered lower with an 11" upper and a car butt stock. Gun functions fine in this setup. That's the way I bought it. I bought a second upper, head spaced it on same bolt, but the gun only works semi in that config. I can shoot semi all day long. Even did an Apple Seed shoot that way. When I try full auto, it fires one round, ejects empty, chambers the next, but won't fire. Trigger does nothing. Rotating the safety to semi will allow me to fire again. I can also retract the bolt and fire again, but still only fires one round. The new upper is a flattop witha 16" barrel. The lower has KNR anti rotation pins that I added, but I haven't made any other changes. I tried replacing the autosear, thinking maybe it was worn, but that didn't change anything. Not sure what else to check. Any ideas? Thanks. When you function check it in FA, does the trigger reset correctly? Does the sear trip when you ease the bolt forward? Thanks for the quick reply. A manual function check works every time. The sear trips and the trigger resets correctly. Thats why I haven't been able to figure out what's going on. I can't duplicate it without firing it, and when I fire it, I can't see what actually happens and I can't check the status of the FCG without ejecting the live round in the chamber. When I do that, I've reset the status of the FCG
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Quoted:
Can you explain the mechanics of what you're saying? Is the bolt carrier actually moving backward a second time (bouncing) after the recoil and chambering of the next round and hitting the autosear? In addition to fixing it, I'd like to understand it.
It's actually the bolt carrier rebounding from the completion of its battery stroke. It moves back just enough that the hammer, moving forward already, hits the back of the bolt carrier, blunting its stroke, instead of getting a clean hit to the firing pin. In a properly timed automatic firing cycle, the hammer arrives at the firing pin almost exactly when the bolt carrier is furthest forward, so it doesn't have time to rebound any. In a semi firing cycle, it doesn't usually matter, because any rebound has time to settle out and be damped by the buffer and action spring. An overgassed upper cycles more violently, so the bolt carrier moves with a bit more speed than usual, and can have time to rebound before the hammer arrives in the automatic cycle. A heavier buffer both slows down the cycling speed of an overgassed upper, by giving the extra gas more mass to move, and better resists the rebounding forces at work with its greater inertia. The cost is a slower cyclic rate, heavier dead weight of the rifle itself, and possible cycling issues with weaker ammo or properly-gassed uppers. |
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Quoted:
Quoted:
Quoted:
I have a Frankford Arsenal registered lower with an 11" upper and a car butt stock. Gun functions fine in this setup. That's the way I bought it. I bought a second upper, head spaced it on same bolt, but the gun only works semi in that config. I can shoot semi all day long. Even did an Apple Seed shoot that way. When I try full auto, it fires one round, ejects empty, chambers the next, but won't fire. Trigger does nothing. Rotating the safety to semi will allow me to fire again. I can also retract the bolt and fire again, but still only fires one round. The new upper is a flattop witha 16" barrel. The lower has KNR anti rotation pins that I added, but I haven't made any other changes. I tried replacing the autosear, thinking maybe it was worn, but that didn't change anything. Not sure what else to check. Any ideas? Thanks. When you function check it in FA, does the trigger reset correctly? Does the sear trip when you ease the bolt forward? Thanks for the quick reply. A manual function check works every time. The sear trips and the trigger resets correctly. Thats why I haven't been able to figure out what's going on. I can't duplicate it without firing it, and when I fire it, I can't see what actually happens and I can't check the status of the FCG without ejecting the live round in the chamber. When I do that, I've reset the status of the FCG If the hammer is falling in live fire, but not firing the round, then I agree with Circuits' post above. You are getting some bolt bounce. And as he stated, changing to a heavier buffer will correct the issue. |
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