I have an upper receiver cut for 450 Bushmaster (slightly enlarged ejection port) that I no longer need to use for that caliber, so would it be fine to run 5.55 NATO instead using that receiver with the proper barrel etc?

I’m thinking it should work fine as it’s just a standard AR upper with a slightly larger ejection port cutout.

Am I missing anything, or am I just overthinking it?

Attached File


I’d prefer to keep it for a build as the fit is perfect on every lower I have, but I could trade it for a comparable upper.

As long as only the ejection port was changed, you’ll be fine running it as a 223/5.56.

Upon closer inspection, the feed ramp area is cut specifically for a 450 Bushmaster barrel.

So that answers my question, this has to be used on a larger caliber single stack feed as there is a single wide feed ramp.

I figured there had to be something else to it.

I guess its going to EE...

BCA?  I don't know of anyone else who would do that.  There should be no reason to cut into the receiver for 450 BM unless the feed ramp on the barrel is oversized and BCA is known for that.

Not sure who’s it was, could be though.  

I got it several years ago thinking I was going to move back to  Ohio but never did.

I’ll have to do a search of my email tomorrow to see who I got it from.