This is my first post, so bear with me if I'm outside of the norm for posting here. I have an issue with my AR described below:
14.5 w/ mini Y comp
milled front sight
A3 upper all factory parts, with both and carrier from bushmaster
standard bushmaster lower
ACE skeleton shorty stock (entry size)
I've had the lower for a long time on a 20" upper and recently swapped it out to build this one. The only thing I changed on the lower is the stock to the ACE shorty stock, which came with DPMS CAR SPRING AND BUFFER.
The upper is brand new from bushmaster.
Upon firing i have a malfunction with some rounds. I mainly fire Lake City .223 FMJ 62grn green tip. They obviously have a little punch, and lock the action open and fire flawlessly 100% of the time.
Recently, i had opportunity to fire a variety of standard FMJ .223 55grn rounds varying from American Eagle, Federal, and Surplus. Opon firing these rounds, the weapon failed to lock to the rear 95% of the time and appeared to be shorstroking with a FTF about 50% of the time. I attempted loading 1 at a time to test lock back, and 2-5 rounds for feed and continued to have issue. I switched back to 62grn without incident.
I took the rifle to a gunsmith, the only one in the area that I know of to claim to be a AR smith, who stated that the bolt was fouled in the rear, and might not have been seating down to the rear all the way causing the FTF. But could not explain the failure to lock back.
On the range, I conducted a few test to attempt to locate the problem.
I switched bolts without change
I loaded one at a time to test lockback
Same results with lighter bullet
I loaded 2-5 rounds to test feed
I conducted an overtravel inspection of the carrier, which appeared to be unremarkable. The rear screw in the stock was removed to be safe, with no change.
The only problem that I can see is the the ACE stock came with DPMS internatls (IE: spring and buffer). The spring is black in color and appears to be heavier that a Bushmaster CAR spring. I have a BM CAR spring and buffer, and will attempt to change them out, but range time is unfortunately postponed to late in the week due to distance.
Please help me out, throw me some ideas on what it could be, or just entertain my dilema.
Did my post get missed?
Just looking for a little help..................
well, without looking at the rifle and assuming the problem is not related in some way to worn parts/bad butstock tube/etc., it appears as though the b/c are not travelling back far enough back to fully cycle. this will eject the round most of the time, but will not lock back or reliably pick up the next round. now that the obvious is out of the way, you can try to find out why.
your rifle is most likely either underfunctioning (b/c simply doesn't get enough gas to cycle appropiately) or overfunctioning (b/c cycles too quickly, before enough gas pressure reaches the gas key).
the most likely culprits to the former can be a bad gas tube, leaking gas key, worn rings, improper allignment of gas tube and gas key, misaligned FSB, or an out of spec gas port. drilling a gas port, btw, should be a LAST resort after you've eliminated everything else.
as far as the latter problem, try different buffer weights, springs. your current set up may not be holding the b/c in place long enough for the gas pressure to build to the proper functioning level.
as i've said in other posts, i've noticed a trend lately with others on my dept. who carry bushmasters. the barrels are constantly having stuck cases and shortstroking even with good, hot ammo. good luck with your problem and keep us updated.
Sounds like you're not getting enough gas to the bolt carrier with the lighter rounds. Since the heavier rounds travel slower and allow the gas to divert into the gas tube for a longer period you get normal funtion. Lighter bullets travel faster which means you have a shorter dwell time for the gas to operate the bolt carrier. Check your gas system for leaks around the gas key and rings. You mentioned you tried a different bolt, was that a bolt and carrier or just the bolt? What condition are the gas rings like on that one as well? Last thing is to check the gas port size on the barrel. If it's too small that would explain why the heavier bullets work and the lighter ones don't.
+1 on checking the gas key screw tightness, and a thorough clean/lube. I never had much luck with the American Eagle ammo, but it sounds like it's not the only brand that's having the issue...
Sounds like a gas leak or weak ammo. Check the gas key for looseness and make sure Bushmaster set the bolts correctly. It has been a problem in the past. My Colt preban Hbar doesn't like weak ammo. It will hum all day on higher pressure loads like it was intended to shoot. Good Luck, WarDawg
Thank you to all that responded:
The problem seems to have resolved itself.
Taking the advice of this message board, and an email from bushmaster, I attempted to test a few components.
I conducted a test of the gas key screws, they appeared to be torqued to spec and staked appropriately. I also tested the alignment of the gastube in relation to the BC, which appeared normal. I also tested the seal as per bushmaster test with the bolt facing upwards, into the BC and dropped some oil into the key. Then using compressed air, found no oil coming out of the BC. I've never heard of this test, but BM recommended it.
Anyway, took the buffer spring out (DPMS-black spring) and compared it to a BM CAR spring. The DPMS spring def appeared heavier, and weighed in about 2 pounds heavier in strength when tested.
Then I re-cleaned everything, oiled appropriately (including inside of the buffer tube, and reassembled (WITH THE BM BUFFER SPRING).
Test fire on the range today revealed 0% malfunction using 4 different types of ammo including SS109, Surplus ball 55grn, wolf 55 grn, and federal factory 55grn ball. Totaled out at about 100 rounds today.
Hopefully that is all, and its back up and running.
Thanks again for the input!!!