It's my 1st build. I'm told not all recievers will work with all barrels. I have a 14.5 Bushmaster barrel and I need a reciever. I like the CM   t but I am told that a reciever with M4 cut outs won't work with a Bushmaster barrel. When I look at the barrel I see 2 what looks to me "feed ramps. I looked at my other Bushmaster(1999 model) that I just had a Bushy flat top reciever swaped out and it also has feed ramps. I don't see anything , cut outs, or any cuts on the reciever. Can anybody clue me in?

A non-M4 ramped upper receiver will typically work well with a M4 ramped barrel.  

A M4 ramped receiver with a rifle ramped barrel can cause jams because there's a lip of steel caused by the barrel extension being exposed because the receiver is relieved at the feed ramp area, this can cause the tip of a bullet to catch on.  

Thanks, that makes since. Without seeing the two together it looked like the feed ramps on my barrel would fit with the feed ramps on the reciever.

M4 ramped reciever and barrel extention

If your ramps don't mate up perfectly just like the pic above, GET ANOTHER UPPER. Too many dealers are selling "M4" uppers which are fake.

bushmaster doesnt have "extended" feedramps.


get a cmt upper with no ramps or the rifle version.


bigbore's write up on feedramps

Don't know how comfortable you would be with this, but I fixed mine by re-profiling the feed ramp geometry on the Bushmaster barrel, so it lined up well with my Colt M4 upper. I had the Bushmaster barrel left over from an earlier build, instead of buying another receiver or barrel, I just re profiled the extension with a Dremel.

Works like a dream, but you have to make sure that you polish the ramps well when you are done.

Does putting different parts together decrease the value of the rifle? Bushmaster parts are so much more expensive than other brands I see, but I don't want to have a questionable weapon to save a few bucks. I'll be training with this weapon on our department.

Just get a CMT/Stag upper without M4 cuts.  They make both you know.

I read the other thread on the M4 cuts and understand the barrel/upper matching and mating can be problematic.  But, the BigBore thread did not discuss WHY?  Why would the M4 designer make the bullet feed chute dimensions different from the rifle feed chute dimensions?  It seems that the M4 dimensions cuts away more of the 6 o'clock lug than the rifle configuration, so the battery strength is dimished by that amount.  What was the problem being addressed by the designer when the M4 variation was made?

poor/low positioning of the mags?
heavier/longer bullets?
cyclic rate of machine guns does not allow the mag spring to "raise" the round to proper height fast enough to feed through rifle ramps?

Most of what is cut away from the 6 lug is from the front side, the one closest to the round. If you compare the back side to the rifle cut, you see that there is little or none taken away from the area, and this is where the bolt will lock up anyway. If there is any weakening, and I doubt there is, it is to be so negligable as to not be worth talking about. There is so much real estate taken up by the other lugs that I doubt you would even miss the sixth, were it not even there.

I have one AR with M4 cuts. It makes not a shit of difference for me shooting semi-auto all day long between the rifle ramps and the M4. With full auto, I guess it makes a difference, but not for what you and I are going to do with the weapon. All my AR's shoot the same, M4 or no M4 cuts, I personally think it would be easier to go back to the rifle cuts to make it easier to mate with the vast preponderance of barrels out there, without having to monkey around with making a rifle extension work with an upper with the M4 ramps. Pain in the ass for me.

That's just my .02. YMMV.