Originally Posted by
MistWolf
Is that right?
The LMT bolt I had did indeed have failures to extract which manifested as bolt over the base of the cartridge next in the magazine. Why does a failure to extract manifest itself as a bolt over? Because the extractor loses control of the rim of the spent case. When that happens, the ejector cannot eject the empty. When the rifle attempts to feed a fresh round from the magazine but the empty case is still in the action. This brings the fresh round to a sudden halt before it's stripped from the magazine and the bolt rides up over the rim. Usually, the spent case is also caught in the action. Sometimes, it gets pushed out of the ejection port after causing the bolt over.
But couldn't the bolt over be caused by short stroking? No, it cannot. The bolt going forward cannot get a grip on the next round in the mag until it's completely behind the rim.
Ok, what about carrier speeds being so high it outruns the magazine? With modern magazines, it's unlikely, especially in a semi-auto AR. I built a pistol AR that had a grossly oversized gas port. Carrier speeds were so fast that the impact against the hammer sent a sharp sting through the trigger to my finger. Yet the AR never had any bolt overs.
Finally, the extractor springs were the issue with my LMT Enhanced Bolt. It caused failures to extract which lead to bolt overs. The bolt was given to me by a friend because he could not get it to work. He had the same problems. When I looked it over, it turned out the extractor spring tension was almost non-existent. Go read Grant's thread on tuning an SBR to run suppressed and unsuppressed. He had problems with the LMT Enhanced bolt.
Recently I dug the bolt out to see what I could do to fix it. The extractor springs were identical to the take down retainer pin spring except the LMT extractor springs were shorter. If cutting down the take down retainer pin springs to use as extractor springs is a "hack", it's the same hack LMT used without success. No "clicky-pens" lost their lives to fix the extraction problem the LMT Enhanced bolt had.
However, cutting down two take down retainer pin springs fixed the extraction problem. No more failures to extract, no more bolt overs during however many hundreds of rounds I've shot through that upper since the fix.
I've had bolt overs in other ARs. Wanna take a guess what fixed the problem in each case? That's right, replacing a weak extractor spring. I've helped other AR owners troubleshoot and fix bolt overs Wanna take another guess as to what the fix invariably was in each case?
Wanna guess what things did not fix the bolt over problems? Smaller gas ports, heavier buffers, new action springs or different magazines.