Given that most of us don’t always have perfect access to truly “genuine” mil-spec, mil-standard parts (those actually used in issued fighting tools), we’re shopping based on a little part trust and a bigger part knowledge. The trust part is accepting claims of “USGI-spec” parts actually being done to not only blueprint dimensions, but also made from the correct materials treated to the same processes. That’s receivers, bolts, and on down the list of the 100 or so parts that can make up an AR-15.
That’s true overall and good to keep in mind. The reason I decided to write about this topic at all was because there are some “mil-spec” features advertised on aftermarket parts that aren’t even really mil-spec. Feed ramps on barrel extensions are a good example.
M4 and A2 Feed Ramps
The mil actually only specs “M4 feed ramps” for use in its M4s. The rifle-platform M16s don’t use them. However! It’s rare now to find an upper receiver or a ready-to-go barrel that doesn’t have the M4 cuts. And that is the important thing! The barrel extension and the receiver both have to have the cuts. Issues can arise when receivers and barrel extensions are mismatched. Whether they materialize or not depends on the direction of the mismatch.
Backing up a ton: Any and all AR-15 barrel extensions have feed ramps. They guide the bullet up and into the chamber. The difference between standard feed ramps and M4 ramps is easy to see. The ramps on the regular rifle-type extension are cut just to the edge of the extension. The M4 cut extends down beyond the edge of the barrel extension (has a scalloped appearance), and a correspondingly deeper cut in an M4-spec upper receiver effectively extends the ramps downward (they’re lowered). So, an M4 feed ramp includes the barrel extension and the upper.
As suggested, it’s best to match the upper and barrel extension—no worries that way. However, if you’re wanting to rebarrel a non-M4-cut upper with an M4 feed ramp equipped barrel, or going the other direction: you have a new upper that’s been relieved to fit with the M4 ramps, and a non-M4-cut barrel extension, and that won’t work! First, the M4 barrel and non-M4 upper isn’t likely to cause issues, but due to the relief cut on an M4-spec upper, there will be a gap that works like an edge that most definitely can snag the bullet and stub the round. Some suggest taking a grinder to the extension area. I can’t advise on that one folks. I guess it depends on what your relationship to the necessary shop activities is.
Why is there an “M4 feed ramp”?
They are better! They provide a lower, wider, and smoother entry for any round of ammo, whether it’s necessary or not. I recommend putting any AR-platform firearm together with a matched M4 barrel extension and upper receiver set, just because you can. It’s a little assurance. Some, me included, have said that the M4 ramps are a help to some of the larger-diameter cartridges in use, such as 6.5 Grendel or 22 Nosler. With a larger diameter case body sitting down in the magazine, the tip of the bullet sits lower in relation to the chamber/bore centerline than with a .223-size round.
More About Ramps
Additionally, in this increasingly specialized world of AR-15 parts that we live in, there are proprietary feed ramp cuts used for some alternate cartridges, such as .458 SOCOM. These are most welcome! Again, you’ll find videos a-plenty on spooling up hand grinders to remedy feed ramp and ejection port issues with these larger cartridges, but I strongly suggest going factory-done aftermarket.
Now, there’s nothing wrong at all with a little polish on any and all feed ramps, which can not only be done fairly easily, it can be successfully done by most at home. This is a big tip, by the way, for pistol-caliber ARs. Bullets tend to bump, snag, and stub on these guns.
Do you have an AR barrel or feed ramp tip? Share it in the comments section.