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 Defective bolt? (Not Cycling)
undercoverwookie  [Member]
1/11/2012 5:36:13 PM
So I am a bit puzzled on this one. Right off the bat I'll let you know I am using a SI Defense BCG in a DPMS upper. I have considered breaking down the gas system as one suspect area for failure to cycle, but the upper runs flawlessly with a DPMS BCG, and a Bushmaster BCG. To test the SI Defense bolt carrier, I swapped out the SI bolt with the DPMS bolt, and again it cycled fine. I also tried to use the SI BCG in the Bushmaster, with the same problem. The gas rings are all staggered.

I figure the most obvious solution is to just use a DPMS bolt since that would cure it, but I'd prefer not to drop $50 if I don't have to. It's running on a 16" DPMS barrel, 55g American Eagle 5.56. It passed a field gauge test, and I cannot find any signs of abnormal wear or stress on the bolt from the 8 or so rounds that were fired. Any help is appreciated, though I suppose this is what I get for trying to save a few bucks. Thanks.
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a308garand  [Team Member]
1/11/2012 6:24:14 PM
Install new gas rings or return the bolt for replacement.....
Circuits  [Team Member]
1/11/2012 7:10:56 PM
Originally Posted By undercoverwookie:
So I am a bit puzzled on this one. Right off the bat I'll let you know I am using a SI Defense BCG in a DPMS upper. I have considered breaking down the gas system as one suspect area for failure to cycle, but the upper runs flawlessly with a DPMS BCG, and a Bushmaster BCG. To test the SI Defense bolt carrier, I swapped out the SI bolt with the DPMS bolt, and again it cycled fine. I also tried to use the SI BCG in the Bushmaster, with the same problem. The gas rings are all staggered.

I figure the most obvious solution is to just use a DPMS bolt since that would cure it, but I'd prefer not to drop $50 if I don't have to. It's running on a 16" DPMS barrel, 55g American Eagle 5.56. It passed a field gauge test, and I cannot find any signs of abnormal wear or stress on the bolt from the 8 or so rounds that were fired. Any help is appreciated, though I suppose this is what I get for trying to save a few bucks. Thanks.


You've pretty much narrowed it down to the SI bolt, though I wonder why you didn't also try the SI bolt in the DPMS carrier.

If the SI bolt works with one of the other carriers, then just swap the DPMS bolt into the SI carrier and vice versa.

If SI bolt works with the other carriers, then the issue would seem to be unusual tolerance stacking - with maybe the SI carrier a bit generous and SI bolt near min spec/and or undersized gas rings. Takes a LOT of slop to let enough gas spill by to prevent things from cycling, though.

If the SI bolt doesn't work with the other carriers, do as the poster above suggested, and try replacing the rings; or return the undersized bolt for replacement or refund;
undercoverwookie  [Member]
1/11/2012 8:03:13 PM
Thank you for the input. I will inquire about a return/exchange. In the meantime, I forgot to mention this is a commercial bolt, though from what I understand the only difference is the dimensions of the magazine cuts on the underside of the carrier, ruling out a mil-spec/commercial inconsistency? I do believe I ran the SI bolt on the DPMS carrier too, which resulted in the same problem. Apologies for forgetting to mention that. I'll give it a shot in the Bushmaster carrier if I do not hear back from the seller by the next range trip.