Figuring Out Feed Ramps

Saturn M4 feed ramp for AR-15 rifle

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.

AR-15 rifle receiver
Most all barrel assemblies and upper receivers on the market now feature M4 ramps.

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.

M4 rifle feed ramps
An older-style (some call a “rifle”- or A2-style) feed ramp is dished out only on the extension itself (top).

Conclusion: Feed 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 plenty of videos 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.

Editors Note: This article was originally published in January of 2019.

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (18)

  1. First of I’m not a gunsmith or firearms professional. However I’ve built numerous ar15 and lr308 rifles and pistols. Feed ramps being m4 or. 30 cal. I prefer them. I won’t buy a barrel that isn’t a wylde 223 556 but I’m only now noticing. 308 m5 using m4 feed ramps, which I belive are for the smaller round like. 338 and others not the .308 762×51 or 6.5 creedmore which o think should have a.30cal feed ramp.? Does anyone have a follow up that confirms or denies my claim?

  2. Man, it’s crazy how many of ya’ll have absolutely no idea why you do what you do. The M4 was the first (allegedy) carbine length gas system. Shorter length equates to much higher pressure in the upper receiver. Higher pressure equte to parts (bcg, buffer, buffer spring, ROUNDS, etc) experiencing much higher wear/tear. With the bcg moving that much faster, the magazine springs were having a hard time supplying the rounds fast enough, AS WELL AS the tips of the rounds were digging into the feed ramps and catching. So the engineers “lowered” the feed rampa into the receiver. Absolutely nothing to do with casing diameter, or whatever other weird sht you guys imagine.

  3. All things being equal and having used both weapons extensively, I’d rather have the feed Ramps. Everything that you have on your side to make any weapons more reliable and work when it absolutely needs to is a plus. Feedramps, proper staking of castle nuts, forward assist (yep went there)S-happens seen it. Proper maintenance and lubrication. The last is using the best and right ammo for the job. When you need to nail something you use a hammer not a wrench. The M-16/M4 are great weapons. They were not intended to be super long range man killers. That’s why AR10’s- And Scar heavies exist for.

  4. So, what you’re saying is that an “M16 upper (not M4 cut) will work with either barrel, but an M4 cut upper will only work with an M4 cut barrel. Seems counter intuitive to recommend an M4 cut upper, especially if the barrel I’m looking at has the ‘rifle’ feed ramp (not M4 cut).

  5. Thanks for explaining the feed ramp topic in a couple of different ways. For someone who is new at trying to understand feed ramps it helps to read a couple different explanations.

  6. I found the article very informative. I’m an old vet, now retired and looking to build some AR’s as a hobby to keep me doing something that I will enjoy. My experience with AR’s is all A-1’s. from my service in the ’70’s to my personal weapons, all of my AR’s are early 1970 slab sided Colt SP1’s. I am glad to know that I need to look for changes in design and the differences in the A2 and A4 feed ramp / rails. again, Thanks.

  7. I too built a .458 SOCOM carbine . Caught a deal on a matched barrel and bolt. I didn’t think anything about it having M4 feed ramps. I bought a .458 upper to go with it, with the extra internal clearance cut before anodizing. When I function tested it the first time with live half-jacketed rounds I had the worst FTF jam I have ever experienced. Went back to the sellers website and found I was not the only one with problems. The manufacturer said they had sold hundreds of barrels with out problems reported. Then the manufacturer discontinued the barrel. I replaced the M4 feed ramp barrel with a “monolithic” feed ramp barrel and no more problems. Any body need a anchor for a very small boat?

  8. Having been in military supply, I know that what the mil-spec means: it gets the minimum job done for thousands (millions) of parts/systems. The parts are interchangeable.
    If I was concerned with precision, I would seek out parts from well-known gun makers and customizers – Baer comes to mind, Hart barrels, etc etc. instead of mil-spec. Mil-spec is good for the masses, and if you want the job done it is a great start. If you want precision, don’t look to the mil-spec gun, look to the one that is popular in 3-gun competitions, at Thunder Ranch, etc.

  9. So, I had purchased a 458 upper. Would not feed to save its life! Gun smith said , ” not head spaced properly”. Factory said “its fine ” . Sent back again. After two months of back and forth, cleaned up the feed ramps. Little polish. Guess what? It’s a miracle! Feeds flawlessly. I should have taken my own advice for once.

  10. You wrote that much to make a point that should have taken about 100 words? You also repeated yourself.

    Go back to writing school. Brevity matters.

  11. Interesting reading. Thank you for answering the question do well. Being a “Newbie” AR owner ((I’ve been an avid gun owner for nearly 50 years), I appreciate the information and visual aids that I’m sure will be a big help in enjoying and using this gun platform.

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