Which AR15 and M16 Parts Are Different?

Notwithstanding the proliferation, and immense popularity, of the AR15 platform - America’s Rifle, as it were - the basic AR15 and M16 are both chambered around the 5.56x45mm NATO cartridge and are overwhelmingly more similar than they are different.

Yet you might be aware that the M16 is the military’s standard issue rifle and has been for over 60 years; it is capable of fully-automatic selective fire, making it an NFA weapon, whereas the AR-15, a strictly semi-auto platform, is not.

With that said, both of these rifles, when equipped with the same attachments, size barrels, and a carry handle, are more or less indistinguishable to the eye.

So, with that said, which AR15 and M16 parts are different? This short post will break most of that down.

The Lower Receiver and Lower Parts; The Bolt

We’ve already explained that the M16 is capable of full-auto selective fire and the AR15 is not. Therefore, most of the M16 parts that differ from the AR-15 are lower parts.

Let’s start with the lower receiver itself. At first glance, they look exactly the same, but the internal dimensions of the AR-15’s lower receiver are different from the M16’s, preventing the AR from accepting the latter’s auto sear; the internals for the fire controls are a different size.

Then we have the internal parts themselves. The main parts that are different are the trigger, disconnector, hammer, and safety selector lever. The sear of an M16 is also known as an “auto sear” which enables the rifle to fire three-round bursts or in a fully-automatic mode of fire.

Though very similar, another M16 part that is slightly different from the AR is the bolt. The M16’s bolt is a little bit different and has more mass at the rear, which adjusts the manner in which it interacts with the sear as it cycles; the extra mass at the rear of the bolt helps to absorb and thereby mitigate recoil when the rifle is fired in full-auto.

This just about covers which M16 parts are different from the AR15. So now, what about what they have in common?

Alright, So What AR15 and M16 Parts Are Not Different?

As has already been stated, just about everything else is the same between an M16 and an AR15, assuming the latter is a standard model chambered in 5.56, like the M16. It is equally important to recognize that given the AR’s popularity it has been reconfigured around a staggering number of cartridges, including .22 rimfire cartridges, which necessarily changes some of the parts used.

With that said, if both are built around 5.56, then you can expect the same:

• Barrel: Assuming, again, that both platforms are chambered in 5.56.
• Gas block and tube: Since these parts are mated to the barrel they are interchangeable across the two platforms.
• Charging handle: This part enables manual cycling of the bolt carrier group and is the same between the two platforms.
• Firing pin: This internal component of the bolt carrier group is the same between both platforms.
• Handguard: Many M16s are made with tropical-style handguards, whereas many ARs are outfitted with Picatinny, M-LOK, and KeyMod handguards. They are interchangeable, however.
• Buffer tube: While you might want different weights depending on load data, the buffer tubes used between AR15 and M16 rifles are the same.
• Buttstock: This part’s compatibility hinges on the former (the dimensions of the buffer tube, not on caliber). Therefore they are interchangeable between platforms.
• Grip: The pistol grips of both platforms are not dependent on action type or caliber, and so are interchangeable.
• Magazines: This goes without saying since they both have basically the same action and are chambered in the same cartridge.
• Roll pins and hardware: As a general rule, the nuts, pins, and other hardware inside of the M16 and AR15 can be interchanged.

As you can see, the vast majority of the parts are interchangeable. With that said, the key differences in the lower receiver and lower receiver parts make it impossible to make an AR-15 full auto without permanent adjustments (which are illegal to make).

Nonetheless, the two platforms are remarkably similar and do in fact utilize a lot of the same parts.

Here for AR15 or M16 Parts?

Whether you’re here for a complete AR15 kit or just need some miscellaneous M16 parts (bearing in mind that many parts are interchangeable between the two platforms) we have what you need.

Check out our full collections of AR15 parts and M16 parts via the previous links, and don’t miss our full collection of gun parts kits, too, some of which can be used as build kits. We also carry magazines as well as shooting accessories and attachments, too. If you can’t find what you’re looking for make sure to get in touch with us at 610-250-3960.