The Differences Between an M4 and an AR15

When choosing to build an AR15 many DIY builders typically pick out parts and accessories for their upper receivers and lowers that are commercially available and suited to their budgets. Upper receivers are made in a variety of price points, check out this website’s upper receivers and you’ll see the multitude of options.

On rare occasions, these rifle builders may want to choose between building an M4 instead of an AR15. While the similarities between the two rifles are many, one aspect usually helps a builder decide to build an AR15.

Since a true M4 is explicitly built for military use, the only way you’re going to get your hands on one is by making a trip to your local recruiting station and signing up. That may be a deal-breaker for some, but there are a few other notable differences between the M4 and the AR15. These differences, however, aren’t that huge of a deal when you stop to think about them.

Shorter Barrel


One thing that quickly sets the M4 apart from its commercial cousin, the AR15, is the barrel length. While a DIY rifle builder may choose to install barrels between sixteen and eighteen inches long, the M4’s barrel is only fourteen and a half inches in length.

What this means is the M4 is an SBR rifle or short-barreled rifle. Anyone wanting an SBR must apply for a tax stamp and fork out a pair of Benjamins based on the current legal landscape. If you don’t want to pay for a tax stamp, you can build a 14.5” rifle and have a gunsmith pin and weld a muzzle device to the end to ensure the length surpasses 16”.

Let’s assume you’re willing to up the ante on your build game by factoring in the extra money. There are still a few essential differences between the M4 and AR15 you need to consider.

Shorter Barrel Means Shorter Gas Tube


If you’ve been building rifles and pistols for a bit, you know by now that picking the right length of gas tube is proportionally critical to the size of the barrel.

Since the M4 barrel is one and a half inches shorter than the minimum legal barrel you can install in an AR15, the M4 gas tube will be, of course, smaller. A shorter gas tube means reduced gas pressure, and we all know what happens when there isn’t enough pressure to cycle the bolt carrier group properly.

Jams and misfires.

Feed Ramps and Grenade Launchers


To compensate for the reduced pressure created by a shorter gas tube, the M4 has a much more pronounced feed ramp than an AR15. The enhanced feed ramp of the M4 ensures each subsequent round is seated correctly during operation.

This difference alone is essential when firing the M4 on full auto. Although, most newer AR-15s come with M4 style feed ramps. Additionally, the M4 barrel has a slot cut into it that allows the operator to attach an M203 grenade launcher.

Don’t trick yourself into believing you can find an  AR15 barrel with this feature. Whatever part of the country you live in, grenade launchers aren’t available to the civilian population unless they are 37mm.

Three Round Bursts and Full Auto

One of the more significant differences between the M4 is the way they both fire. The difference between semi-automatic and fully automatic is not only the number of times you have to pull the trigger, but the number of rounds fired when you do.

An M4 rifle has either a three-round selection or a fully automatic selection. When set to fire three rounds with a single trigger pull, the M4 provides bit bursts of firepower for controlled tactical firing.

As many an AR15 owner will tell you, although you can move your trigger finger fast, there’s no inexpensive way to empty the magazine of an AR15 quicker than someone shooting an M4 on full auto.

If you’re hoping to build your own short-barreled, prepare to invest at least two hundred extra dollars to make it legal. That’s four hundred dollars that will hit your budget before you finally decide to purchase the rest of what you want to add to your rifle build.

Similar but Different


There are many similarities the M4 and AR15 share. Aside from the enhanced ammunition feed assembly, the internal operations of both rifles are the same. The bolt and bolt carrier groups are interchangeable, as are the trigger and firing systems. Externally, both can share the same type of handguard and accessory rail.

In essence, both the M4 and the AR15 share many standard operational features, including the type of ammunition fired. Aside from launching a grenade, a shorter barrel, and gas tube and possessing a three-round or fully automatic firing, the M4 and AR15 rifles are the same.

If you’re on a tight budget and the additional four hundred dollars to purchase tax stamps for a rifle with a short barrel and the ability to clear a loaded magazine in a few seconds just isn’t going to work, you may want to make yourself an AR15 instead. You can find 80 lowers to build an AR 15 by visiting

There are many differences between the AR-15 and the M4 carbine used by the military but there are also many similarities as well. The M4 and the AR15 are so much alike you can legally make your AR 15 rifle without the need to purchase four hundred dollars’ worth of tax stamps.

Whether it’s an M4 or an AR15, both rifles will provide you the same shooting pleasure at the range or on those hunting trips. If you’re interested in owning a military M4 rifle, buying or building an AR15 is as close as you can get without signing up for a four or six-year stint in the military.

In essence, only a couple of minor differences separate the M4 from the AR15, and you can manage all but one if you’re willing to spend additional money for tax stamps.