Yes.  I was bored and bought a POS ATI 16" .450 upper.  BCG included.  Runs fine hand cycling but fails to eject when firing.  It DOES lock the bolt back on the last round.  But it leaves a semi-mangled piece of brass in the ejection port.  I have tried a couple different buffer/spring combos and while it seems the lighter ones mangle the brass less, it still won't eject properly.  I have checked the ejector with the BCG out of the gun and it seems smooth, but it also looks like the extractor feels a little "sticky" while testing.  It's like the ejector can't get the brass out soon enough and it's jamming the case mouth into the front of the port.  Overgassed?  IDK.  It really does seem to be pretty damned accurate so I would love to get it to run.  Using C products mags BTW.  Does it on both 5 and 7 rounders.
Have tried Hornady American Whitetail and 250 gr FTX so far.

The ejection port is not relieved at all BTW.  Looks like a stock 5.56 opening.  I posted this originally in the "build it yourself" forum by mistake.  Any advice appreciated.  It's my first foray into big bore ARs.  Have tried both standard carbine buffers/springs and heavy.  Little change noted.

I have also noticed that it indeed does NOT run fine hand cycling most of the time.  Almost as though the port needs to be opened a little...

As in your other thread, the port really does need to be opened up.  Yes the round technically passes through the standard opening, but at ejection it will take off at slight angles and unless it threads the needle perfectly it will fail to eject.  Opening up the port is 100% a must for reliability.

Thank you.  Now I'm not sure what to do.  I DO have the tools to possibly do it myself.  I thought about sending it in as it is brand new but being as they built it like that in the first place, they probably wouldn't be too quick to do anything.

I had the same issue after purchasing a cheap 450 upper.  Swapped an Aero XL upper, didn't want to open it up myself.  This was years ago it was $75 when I bought it pre-covid, $150 now.  

https://www.aeroprecisionusa.com/ar15-xl-assembled-upper-receiver-anodized-black

You could get a .450 cut upper and swap the parts over.

Bushmaster themselves sell theirs for example: https://www.bushmaster.com/shop/upper-parts/bushmaster-a4-modified-complete-upper-receiver/

Paladin Machine Shop, tp555(at)vfemail.net, can do it and it will be done right and the cover will still latch shut.  Cost is very minimal where shipping back and forth cost more than the labor.  You’ll have it back within a week to 10 days too.  Only catch is you need to strip it down and send him only the upper receiver.  While he has it, he can lathe square the front of the receiver also.

Well I'v already broke out the dremel and files lol.  Going to try just opening the front half and see how it does like that to save the door detent.  Uppers are cheap and I have the tools/know-how to swap it out if I screw it up.  I will say that with the tiny bit of testing that I have done so far, it seems to be VERY accurate.  Got a beautiful three-shot cloverleaf yesterday just off a tripod.  Only 75 yards but I was impressed.  Those big holes sure are easy to see lol.

I had to open up a standard ejection port several years ago. Study pictures online of what an enlarged opening looks like so you know how much material to take off. Rough cut with the Dremel, then finish with the files. It's not hard, just a little tedious. Works great, no issues.

In four decades of shooting AR’s recreationally, I have never NEEDED to latch the dust cover closed.

I’ve never been to a ”class” that required dust-cover manipulation.

Maybe one day when I get older, I’ll find myself in an environment where keeping the dust cover closed would be pretty important. Probably won’t have the .450 Bushmaster with me though.

I can run a wicked Dremel when conditions require.

I have a box of stripped uppers around here somewhere too. I need to organize this stuff.


Attached File

A lot of logic there.  Dust covers are important, and I have needed to use one.  I hope i don't need a dust cover again.

Back up irons are important too, but not on my 450 BM, they are on my 556 SOCOM Carbines.

Remove more off the bottom than top.  Roughly .010-.015” off the top and the rest on the bottom.
Attached File


Since the .450 BM case is smaller in diameter than the .458 SOCOM’s case, I doubt you’d need to open the port as wide as for the .458.

Well, at least some success!  Opened the port.  Looks OK.  Need some aluma black.  I'm a bit restricted on how much I can shoot in the back right now but I went out with one round.  Ejected fine and locked the bolt back.  Only went a couple feet though.  Need to get to the range soon.  MUCH better than yesterday though.  Thanks everyone!

P.S Man this thing ROCKS that poor AR 550 gong out there.  Really packs a wallop lol.