Palmetto State Armory Stripped A3 Upper Receiver with M4 Feed Ramp Cuts

Most of you reading this should be able to discern between the various variants (A1, A2 etc.) but what the heck is an M4 feed ramp cut?

Lets start this off with a little military history lesson.

Origin of the M4 feed ramps;

Soon after the M4 carbine was adopted, the armed forces realized that the higher gas pressures caused by the shortened, carbine length gas tube system created unreliable cycling when fired on full auto (particularly when firing from worn magazines). It seems that M4s firing at 700 to 950 rounds per minute combined with the higher pressures move the bolt so fast that the next round in the magazine doesn't have enough time to come all the way to the top before the bolt starts pushing it towards the chamber. The Army extended the feed ramp in the barrel extension to the receiver bellow it to help scoop the round into the chamber.

Please note that there are three criteria that must be met simultaneously in order for the M4 feed ramps to correct the feeding issue;

1. A Carbine length gas system
2. Full-auto fire
3. A worn or weak magazine spring

If not all three items above are present, the M4 feed ramps are not required. Surely there will be those out there that want it for the "cool factor" or some other reason, better to have and not need than to need and not have, right?

Well, wrong actually!

The top of the feed ramp ends at the opening of the barrel extension and its location is fixed and cannot be bushed back or raised into the receiver. The M4 feed ramp simply extends the ramps by carving them deeper into the upper receiver beneath the barrel extension, thus only lowering the bottom edge of the feed ramp. so in keeping the upper edge of the ramp fixed, the M4 feed ramp effectively makes the entire ramp 17 degrees steeper in angle.

Whats the big deal with that?

Well, if you were to put a snap cap (dummy round) into an AR-15 and cycle the bolt by hand, you will see that the first part of the round that touches the ramp is the very tip of the bullet. The steeper angle of the M4 feed ramps provides a much harder blow to the bullet during the loading cycle. This can cause the tip of the bullet to become deformed and this will have a negative impact on overall accuracy. Now don't get me wrong, this slight deformation will hardly cause any issues at 100 yards if any at all, but stretch that out to 500 or even 1,000 yards and the impact will be much more obvious.

Installing a non-M4 feed ramp equipped barrel extension onto a receiver should help this issue as there will be less "dragging" of the bullet's tip up the ramp so the fact that an upper receiver has the cuts shouldn't be too much of a bad thing. Besides, I do not intend to use this upper receiver to build a long range (400 yards and beyond) rifle, but more of a tactical type short to mid range lightweight gun.

The Palmetto State Armory Stripped Upper Receiver;

It should come to no surprise that I got this receiver with my lower build kit listed last time (saved me shipping costs haha) and to be honest I wasn't expecting miracles with the $69.95 price tag.

It is forged from the same 7075-T6 that most other companies use, it is made to military specifications like everyone else does and it is a flat top (A3) with machined T-marks (for accessory placement consistency). It uses .250" diameter pivot and takedown pins and requires installation of a forward assist assembly and a port cover door assembly (not included with a stripped upper receiver).

It has the Palmetto logo machined onto the left side of the receiver just under the flat top (as you can see in the picture bellow), and as a nice unexpected bonus there is a dry lubricant coating on the entire inside of the receiver that is so smooth to the touch you almost cant feel your finger moving over it!

There is really only one small issue I found with this upper, that there is a little over spray onto the flat top from the white T-marks. Not a big deal but might be an issue for perfectionists. here is a picture to show you just how "terrible" the over spray is;




And here is a picture to show you what the M4 feed ramp cuts look like. Also note the dry lubricant coating on the inside of the receiver, its gray color is easily noticeable against the black anodized receiver. Also note that I installed a port cover door assembly (also known as a dust cover) before this picture was taken so that is not included with a stripped upper receiver.

Next time we will look at some tactical accessories, what they do, and how/why the work!