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Is Your Lower Parts Kit Compatible?

Maybe You’re Just Getting Into Building AR-15s As A Hobby Or You Want To Make Sure The Lower Kit You Have Will Work For Your Build.

Either way, Lower Parts Kits can be confusing, especially with all the AR-15 variants out there.  We’re going to go over which parts kits work with your AR-15/AR-308/AR-9 or similar firearm.

What is in an AR-15 Lower Parts Kit?

A Lower Parts Kit is what you need to finish the “from-scratch” build you might have. The Upper Receiver and Lower Receiver are easy to understand, but they won’t do much without the Lower Parts Kit.

The Lower Parts Kit consists of every internal part needed to have a functional firearm. This ranges from the trigger, sear, and hammer to the magazine catch and other springs. Depending on which kit you get, a pistol grip and buffer tube assembly are usually included, but that will vary from kit to kit. These are still necessary parts to have, be sure to have your own on hand if they are not included.

Essentially the Lower Parts Kit includes every spring, detent, or other part you don’t think about that is required for the AR-15 to function. Here’s a list of what you can primarily find in a lower parts kit.

Parts included in a Lower Parts Kit


Trigger and sear


Bolt catch

Front pivot pin

Rear takedown pin

Magazine catch

Springs and retaining pins


Which Lower Parts Kits work with which AR-15 variants?

Compatibility is a big concern because of how many different AR-15s there are, especially when you find out that some AR-308 receivers are not compatible. But when it comes to Lower Parts Kits this is less of a concern. That’s because most of the internals will be the same across most AR-15 variants.

Just to be sure, let’s look at some of the major variants you’ll encounter.

Standard AR-15s

If you have a mil-spec lower receiver or any other lower within the normal specifications of an AR-15, your Lower Parts Kit will work. You won’t have any issues unless some of the parts are damaged or out of spec.

Non-Standard AR-15s

For AR-15s that are not in the standard chambering, you will be fine. Most of the parts found in the kit do not directly deal with the caliber and are not impacted by that difference. The parts that are impacted will most likely be the buffer assembly since they help cycle the gun.

If you are looking for a specific parts kit that goes with your caliber of a choice, the appropriate buffer assembly will be included. This ranges from .22 LR all the way to .450 bushmaster.


While there are several variants of the AR-10, the most common are the DPMS variants which are the LR-308s. These patterns make up the majority of modern AR-308s. This still does not mean they use different parts kits.

The buffer assembly included will be designed to handle the .308 chambering. These are usually marketed as a “.308 parts kit”.


For the 9mm version of the AR-15, the AR-9 does have some compatibility issues. It will use roughly 90% of the AR-15 Lower Parts Kit with the only differences being the use of a modified ejector and a different magazine catch.

These differences result from the direct blowback action of the AR-9. Since the cycling of the gun is different and does not use the normal gas-blowback system of the AR-15, the AR-9 needs a different extractor.

Because many AR-9s use Glock magazines, the magazine catch needs to be compatible with the magazine in order to retain it. The standard AR-15 catch would not work and the appropriate catch is included in AR-9 parts kits.

Rifle style

Compatibility w/Lower Parts Kit

Standard AR-15


Non-Standard AR-15 (Caliber Difference)

Compatible with appropriate buffer assembly

AR-308, AR-10, SR-25, DPMS

Compatible with appropriate buffer assembly


Compatible with appropriate ejector/magazine release/buffer assembly

Non-Compatible Parts

Just in case you were wondering what parts are not compatible, these are mostly found in the Upper Receiver. These will be things like the bolt head, the gas systems, and of course the barrel depending on which caliber your particular rifle is chambered in.

The other non-compatible parts will be magazines. Depending on the case length, your magazine will not fit in the standard AR-15 mag well. But there will be some cartridges that work with the standard AR-15 magazine.

These rounds are .300 Blackout and .458 Bushmaster, among others. Using the base magazine design for other calibers is an easy way to ensure your magazine will function, but this raises safety concerns. If you use the wrong caliber with the incorrect barrel it could cause the gun to explode and injure you or someone else. The chance of this increases when your magazines can accept multiple calibers. To avoid this make sure to check your ammunition and dedicate specific magazines for specific calibers.