I was trying to think of a good place to put this and figured it would get more exposure here. (Mods feel free to move it to NFA if needed).
The weapon in question is a Bushmaster 14.5" select-fire M4gery. Ammunition used was new production commercial Lake City M855 (no it is not pull down or reject ammo) according to the email from the Lake City-ATK rep.
Operator was on the range conducting BZO of carbine. Weapon was zeroed and no malfunctions were encountered. Operator switched into full-auto and that’s when the fun began. It would fire random burst and then stop. After it was reported I went out to the range to observe first hand and test it myself. What I saw was that the weapon would stop firing, a live round was in the chamber (primer had been lightly struck) and the hammer was forward.
Having just inspected the weapon and gaged it I was rather dumbfounded as to what was going on. I assumed that the hammer was somehow tripping late and was following the bolt carrier forward however there was not enough energy to ignite the primer. Weapon went back to shop and was torn down. I didn’t like the look of some of the parts so I replaced the entire fire control group, replaced the buffer spring (was still within spec) rechecked the BCG, replaced the gas rings and checked the gas tube, etc… and couldn’t find anything wrong.
Went back out to range after completion of work and test fired again. Weapon malfunctioned as previous. Now I am scratching my head because I have changed everything that I can think of with no change in results.
Went back to the shop checked everything again-all was good. I had a sneaky suspicion as to what the problem was but I was in denial. I tested my theory by getting another carbine, removing the upper that had just had the barrel replaced, chamber reamed, gaged, cleaned, etc… and we swapped them over. Went back out to the range and conducted a test fire.
As I suspected the new upper was 100% and no malfunctions. The other upper on the different lower did the exact same thing as previous. Fired a few burst and then stopped. My final conclusion was the gas port in the barrel was eroded and it was undetected until the weapon was fired in full-auto. Unless I missed something there is no other logical explanation.
I would like to hear from Grant, Robb, and any others out there to see what their take is on this. Earlier today a second carbine did the same thing so I will be validating my results in the next day or so with another test.
How old is the carbine?
ID like to see what conclusions others would come to as I too own a F/A Bushy.
Havent shot it enough but thankfully so far so good. I was amazed, however, that even with everything brand new, it cycled normally with an H2 buffer. Makes me wonder how big Bushy gas ports really are to begin with…:eek:
This weapon came into service at the end of 2004 or early 2005. Judging by the gas tube and FSB area (which is covered by the gay YHM sight) I would say that it has been hammered. Unfortunately no one maintained any maintenance records and no round counts on any of the weapons.
This is exactly what happens when the sear isn’t catching the hammer hook. The hammer will ride the carrier forward and function at about 90% fire rate.
It’ll periodically stop as it’s barely igniting most of the primers.
You may be able to salvage the setup by swapping in heavier buffers until it runs right. This is obviously not the best solution, and only a band-aid, but it will work. I had the same thing on a 10.3" CMMG barrel that had been beat on.
The hammer would follow as you describe. I went from H2 to H3 and it worked fine until I got the new barrel installed, then it worked on both H2 and H3.
I am actually in the process of re-barreling it but since the POS jam nuts on the YHM rail systems are so hard to remove I had to cut it off. Almost every one that I have re-barreled or had to remove has required me to Dremel the nut off because they didn’t use anti-seize and they applied some “goop” to the barrel nut. Very frustating.
Did that also. The hammer seemed to drop right as the bolt locked into the chamber as it should. The other gun was working fine until we swapped uppers. Once I did, it did the exact same thing.
i’d go with todd, sounds like bolt bounce. a heavier buffer will slow the carrier down and prevent it from happening. some otherwise identical guns will do it and some won’t, it seems to come down to individual tolerances within the upper and its component parts.
So are you thinking that if I add an H2 or H3 buffer it will solve the problem? My other question is if it worked fine when it first came in what causes it to do this now?
I think the heavier buffer would certainly band-aid a situation like this, but that’s treating the symptom and not the real problem. The real problem is that it’s getting too much gas – likely from an eroded, or bigger than necessary, gas port.
I’ve seen some people’s suppressed guns do this in auto, but work perfectly in semi and without the can. They put in a heavier buffer or a regulated gas block and it “cured” the ailment.
I had the same thing happen to brand new RockRiver m-4’s w/14.5 in barrels. Less than 300 rounds through the guns and it happened to 6 out of our agencies 20 guns. I went through the guns and noticed the gas rings were bad in two of them, so they were replaced. That didn’t help. They went back to the factory. I was originally told it was a gas problem. Enough pressure to cycle the next round, but not enough to get a good primer hit. (that didn’t make sense to me?) Then I was told they switched to an h3 buffer and all functioned reliably. Just got them back (with 20 h3 buffers) and have not had time to function check them yet. all guns fuctioned on semi auto. By the way this never happened to any of our Colt’s.
Bolt bounce sez me. I’ve seen this a few times although I don’t get around autos much. In one case the heavier buffer did the trick, I don’t know what became of the others. But everything was in perfect working order inside.
I am lobbying right now for a borescope so I can inspect the port like we used to do at another location. I really want a permanent fix rather then something temporary so if it means a new barrel so be it.
