M4 receiver or no???

[Scoots]

I'm looking at rebuilding my AR 300 Fireball using Mega Machine Shop (MMS) upper & lower receivers:




What's the difference between an M4 cut upper receiver and a standard cut upper receiver .... and which one is better suited for a 300 Fireball?

[Colorado Osprey]

I'm probably wrong on this...but...here it goes...

M4's uppers need to have a M4 profiled threaded 14.5" barrel with front site base and bayo lug, flat top marked Picatinny rail, M4 barrel extension feed ramps and a bayo lug.

Courts ruled that any rifle resembling Colt's M4 may use that name.

I believe that to be a M4 upper reciever it needs a Mil-Spec Picatinny flat top rail with T-markings.

M4 feed ramps are part of the barrel extension, not part of the upper reciever, but makes a M4 upper, but not upper reciever.

Neither part upgrade will make a hill of beans difference in a 300 Fireball build. The feed ramps were only designed to help in full auto fire with a smaller diameter. Most people who like flat top style uppers don't care whether it has markings or not as they are usually covered by some type of siting systems.

Mega makes good recievers...you should be happy with those.

[pug]

If you are making a M4 clone upper then you would probably want to follow the different M4 specs like mentioned. As far as the feed ramps it sounded like you already have the barrel and you should check the barrel extension to see if it has M4 type ramps cut and if so I would use a M4 cut upper and if not use a standard upper. I have had better functioning with a broader range of magazines and bullet profiles by using the M4 cut. If you try to feed a round nose bullet out of a low sitting magazine it will hang on standard feed ramps because the ramp starts too high for the bullet to ride up on it. As far as the differences in the uppers the M4 feed ramps are cut lower in the barrel extension so the upper receiver has to have two small dimples cut out to match the same profile.

[DonT]

The M4 ramps extend down into the upper itself due to the different angle.

Randall at ar15barrels.com has a good picture on the difference (see link below):
http://ar15barrels.com/gfx/feedramps.jpg

I have gotten away using an M4 cut extension on the barrel and standard receiver and an M4 reciever and standard barrel with out problems. BUT this is unusual (or lucky), in any event I would makes sure the barrel extension is cut for the upper you will be using...This helps to take one more potential problem out of the loop.

Just one mans thoughts...
Happy

DonT

[Scoots]

Thanks for the clarification.

I knew about the M4 feed ramps on the barrel extension, but I was unfamiliar with an upper receiver being cut to "match" the cut on the barrel extension.

I'll double check, but I don't think my Oly barrel from DelTon is cut "the M4 way".