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Posted: 12/13/2011 4:21:01 PM EDT
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I'm recently purchased a new Bushmaster M4A2. Grew up a trap shooter and, well, this is my first rifle. I've been pretty excited about it. I've put a low number of rounds through it already (approx 300) and I've had a pretty good number of jams. I could use some advice from some of the more experienced guys around here if that's not too much trouble.
I'd say at this point I've had about 25 jams. They are all the same variety: When stripping off a round from the cartridge, the bolt doesn't feed the round into the chamber. Instead, the bolt leave a large dent in the subsequent round. I've had this problem even on the first round from a new magazine. I've been using Remington UMC. I recognize that there are higher quality rounds... but this number of failures is unacceptably high given the circumstance. I lubricated it liberally... in fact, I've probably gone overboard. I've not customized the gun at all (yet). It's bone stock. Any advice would be greatly appreciated. |
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Are you having a Bolt Over Base malfunction, where the bolt partially picks up the round from the magazine, but does not feed the round into the chamber fully, leaving the bolt over the top of the cartridge case and the cartridge partially in the magazine and partially in the chamber? If you are having this type of malfunction its main cause is a poor magazine. When your bolt is locked to the rear and you release the bolt catch, if your carbine will not feed a round directly from the magazine, this can also be caused by poor magazines. |
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Have you tried only loading 28 rounds in the mag instead of 30?
Are they good mags? A common problem with bad mags is failures to feed usually occuring in the first few rounds of a fully loaded magazine? Is your magazine catch properly installed? If possible tighten it one turn. FWIW i have not had good luck with Rem UMC ammo. I bought 1600 rounds of it once and will never buy anymore. Even Wolf WPA is better ammo than Rem UMC in my opinion. This one is a reach: Measure you action spring (aka buffer spring) i once had a simular problem and it was an out of spec action spring. A action spring for a collapsable stock should measure at least 10 1/6" long. ETA: at one point Bushmaster was shipping CProducts mags. I had terrible results with those mags. I had better luck with D&H, PMAGs and NHTMG mags. |
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After reviewing a number of comparable internet pictures, it definitely appears that I'm getting the "bolt-over-base" malfunctions. The photos definitely seem consistent with RedFalconBill's diagnosis. The bolt advances about 50% forward leaving the half lodged in the chamber.
I'm using the BFI magazine that came with my rifle. As far as load-outs, the gun has regularly jammed with only 5 rounds in the magazine... AR15fan's suggestion that it could be the action spring seems intuitive... but I also agree with him that type of flaw seems far-fetched. I very much appreciate all your input. What a wonderful world we live in where experts on this stuff can help out someone in a jam. If you have any suggestions on the BFI mag, I'd appreciate it. I can probably pick up a different magazine and give it a shot this weekend. |
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Bolt over base is usually either a bad magazine or some sort of hard mechanical issue like an out-of-spec lower, bad mag catch, or something like that. Can also be a result of very excessive cyclic rate or shortened bolt travel, where the bolt rebounds from the back of the buffer tube and starts closing before the ammo stack has time to move up in the magazine.
Easy enough to check the bolt travel, if you pull the charging handle all the way to the rear the bolt face should be approximately 1/4" behind the bolt catch. Different magazine would be easy enough to change, another thing to try would be a heavier buffer, something like an H2. |
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