ar15 in 450 bushmaster, having problems

[bigdog454]

I have a ar15 in 450 bushmaster that is having problems ejecting and reloading a shell. I have cleaned the gas port, the bolt carrier, replaced the gas tube, to no avail. sometimes after cleaning it will cycle 1 or 2 ok. Any ideas or tips will be apreciated.
BD

[Texas by God]

Has it ever worked with that ammo? If so, oil the bolt carrier thru those two little holes and resume firing. Honestly we need more info to start guessing.

[Gazz]

I have a AR in .458 SOCOM with a similar problem. So far everyone I have talked to about the problem tells me it is under gassed. In other words I need a larger gas port. I did bump it up one # drill size but forget which one and it still does not work. i'm going to try again with next size up drill bit. I was worried about opening it up to much but I can always make an adjustable gas block or gas tube.
Your rifle needs to have just enough gas to lock the bolt back after shooting one round from the magazine.

[bigdog454]

Gazz: I was wondering about that. A num 50 drill eill just fit, but I don' know what size it should be.
BD

[Gtek]

Built or purchased upper? Have you checked ejection port size? Type of gas system? Lower type? Buffer? I have several that I have assembled including SOCOM's and they run perfect except for the weak link magazine thing.

[pmer]

I have a 300-221 that 1 time had a bad bolt carrier; it would shoot 8 or 9 rounds and then something happened to the bolt carrier where the bolt would get stuck inside and would not rotate freely. Replaced the bcg and it worked great again.

[Moleman-]

450BM carbine gas system I go with a #47 bit (.0785") after predrilling with a slightly smaller one, standard spring and buffer. Usually they come over gassed.

[bigdog454]

#50 drill bit =.0700. maybe a #47 would open it up enough may try it. I've been shooting Hornady factory ammo with a 250 gr FTX jacketed bullet. I bought the whole system, upper and lower together. I've tried a different lower, same results maybe I could cange out the bolt carrier with a different ar that I have, Ive tried everything else.
Thanks for the information guys, I'll let you know if I get it goin well.
BD

[Gtek]

Have you confirmed ejection port has been opened up to about .620"? Have heard tales of extractor/barrel extension internal binding issues, who assembled rifle? Have you single round fed for extraction check bolt hold open?

[bigdog454]

drilled out the gas hole in the barrel to .0785, as moleman suggested, seems to have solved the problem.
BD

[Moleman-]

Excellent! When I was making my first 450BM barrel I searched online and found guesses from .063" all the way up over .096" for a carbine gas system. So, I started small and made the gas port larger until it would eventually lock back on a mag which was a #48 at .076". Cold, dirty, ect for a little wiggle room I opened it up .0025" more with the #47 (.0785") bit. Got 5 blanks on the work bench right now that will end up as 450bm barrels.

[bigdog454]

A tech at Bronells, suggessted up to .082
BD

[koger]

I have worked on 13 different AR's, about half of them factory rifles, that all of them just had the pilot hole drilled in the barrel. Drill the correct size hole, and they ran fine. None of them was the .450, all were 223 and 300bo!

[Tripplebeards]

Yep as others say undergassed or it could be an improper crimp on your round. Have you tried factory loads to see if they cycle.

I was improperly crimping my 450 BM loads and they would cycle or feed in my Ruger American bolt action. Taper only with the 450 BM or you’ll have stuck rounds and feeding issues. I would take apart your upper and make sure your rounds drop loosely into the chamber. I don’t believe there is a Lyman ammo checker type gauge to see if the round is the correct diameter for proper feeding.


I shot my 243 AR for the first time last month and stuck the fourth round in the chamber. I made the mistake of trying a round I loaded long for another rifle. Besides loaded too long my cases were case hardened and sprung back to big in diameter. The bullet was stuck in the lands and case was stuck in the chamber. The bolt carrier wasn’t shut and open by about a half inch. I’ve since bought a Lyman 243 case checker and a small base die.

[hp246]

I had a similar problem. Turned out it was a rough chamber. I polished the chamber and it took care of the problem.