Understanding Compatibility of AR Components

Not all AR parts are created to work together—and that matters more than most builders realize. Whether you’re assembling a pistol-caliber carbine, a standard AR-15, or a large-frame AR-10, cross-platform compatibility isn’t guaranteed. From receiver dimensions and barrel extensions to bolt carrier group and buffer system differences, each class of AR brings its own design standards and limitations. This guide breaks down where compatibility ends and headaches begin—so you can avoid costly mistakes and build with confidence.

Compatibility is about Standardization

One of the best things about the AR platform is the standardization and interchangeability of components. This standardization makes it easy for aftermarket component manufacturers to offer tremendous variety to the consumer. You can pretty much find every major part in multiple designs, materials, colors, and finishes. Because of the standardization, any conformant part that you buy will fit in any AR. If a part breaks, you can easily replace it. If you want to upgrade a single component, you probably have several, if not dozens of options.

That said, there are exceptions to the standardization of AR parts.

Only the AR-15 has a standard for the core components: the Mil-Spec. The large frame (AR-10 or LR308) has multiple standards, and even those are inconsistently implemented. The AR-9 does not have a standard.

Not every component is compatible with every class of AR. Some parts will work in the pistol caliber carbine (e.g. AR-9), AR-15, and AR-10/LR308. Some parts are class-specific and only work in one class. Understanding this compatibility can support informed decision making and purchasing.

To make matters worse, not every component is compatible with every platform manufacturer. For example, the Aero Precision M5 lower receiver is slightly wider at the pivot pin lug than most AR-10/LR308 lowers. As a result, there are Aero Precision M5-specific pivot/takedown pins, which are slightly longer.

Some manufacturers have proprietary designs that only work with their own systems. For example, Wilson Combat manufactures some barrels that have gas systems with unique lengths. As a result, you MUST buy one of their gas tubes for those specific gas system lengths. That may not seem like a big deal. But when you read product reviews, it can become obvious how frustrating NOT having that information can be to a consumer.

The tables below illustrate the compatibility of components across AR platforms.

Platform Component Compatibility

Platform components are almost entirely specific to the AR class.

Component	EPC	M4E1	M5
Lower Receiver	EPC	M4E1	M5
Upper Receiver	EPC	M4E1	M5
Handguard	AR-15	AR-15	M5

Part/Component Compatibility

Most small parts that are not caliber-specific are interchangeable between the AR classes.

Component	EPC	M4E1	M5
Muzzle Device	Cal. Specific	Cal. Specific	Cal. Specific
Crush Washer/ Spacer	Barrel Specific	Barrel Specific	Barrel Specific
Gas Block	N/A	Generic	Generic
Gas Tube	N/A	Generic	Generic
Bolt Carrier	PCC	AR-15	AR-10
Dust Cover	N/A	AR-15	AR-10
Forward Assist	N/A	Generic	Generic
Charging Handle	AR-15	AR-15	AR-10
Pivot Pin	AR-15	AR-15	AR-10/M5¹
Takedown Pin	AR-15	AR-15	AR-10/M5¹
P/TD Pin Detent	Generic	Generic	Generic
P/TD Pin Spring	Generic	Generic	Generic
Magazine Catch	N/A	Generic	Generic
Magazine Catch Spring	N/A	Generic	Generic
Magazine Release Button	N/A	Generic	Generic
Bolt Catch	Generic	Generic	Generic
Bolt Catch Buffer	Generic	Generic	Generic
Bolt Catch Spring	Generic	Generic	Generic
Trigger Group	PCC/Generic	Generic	Generic
Hammer/ Trigger Pins	Generic	Generic	Generic
Safety Selector Switch	Generic	Generic	Generic
Safety Selector Detent	Generic	Generic	Generic
Safety Selector Spring	Generic	Generic	Generic
Pistol Grip	Generic	Generic	Generic
Pistol Grip Bolt	Generic	Generic	Generic
Pistol Grip Lock Washer	Generic	Generic	Generic
Receiver Extension	Generic	Generic	Generic
RE End Plate	Generic	Generic	Generic
RE Castle Nut	Generic	Generic	Generic
Buffer Retainer Pin	Generic	Generic	Generic
Buffer Retainer Spring	Generic	Generic	Generic
Buffer	PCC	AR-15	AR-10
Buffer Spring	AR-10	AR-15	AR-10
Buttstock	Generic	Generic	Generic

M5 pivot and takedown pins are slightly longer than AR-10 pins.

Frequently Asked Questions

Can you use AR-15 parts on a pistol caliber build?

Some parts like fire control groups*, receiver extensions, and handguards can be shared between AR-15s and pistol caliber carbines (PCCs), especially when the PCC is based on the AR-15 lower footprint. However, uppers, bolts, and mags are typically unique to pistol caliber platforms.

* Direct blowback pistol bolts may require more robust hammers.

What are the differences between AR-15 and AR-10 component compatibility?

AR-10s (large frame) and AR-15s (small frame) differ in nearly every major dimension—uppers, lowers, barrels, bolts, buffer systems, and magazines are all different. However, some triggers, grips, buffer tubes, some small parts, and buttstocks may carry over depending on the design standard (DPMS vs Armalite).

Refer to our guide above.

What parts are interchangeable between small frame and large frame ARs?

Interchangeable parts are limited, but typically include:

Triggers
Pistol grips
Receiver extensions
Buttstocks
Some small parts

Refer to our guide above.

Are AR-9 and AR-15 lowers the same?

No. While many AR-9s are modeled after the AR-15 lower, they require different magazine wells (for Glock, Colt, etc.) and often use a dedicated mag block or integral design. They also require different bolt carrier groups and often use blowback operation, not gas.

Can you convert an AR-15 to shoot pistol calibers?

Yes—with the right components:

Dedicated AR-9 upper
Mag block or PCC lower
Blowback bolt carrier
Heavy buffer

We cover these conversion paths in our Caliber Conversion article.

Do AR-10 and AR-15 share any parts?

Only a few—triggers, grips, buffer tubes, some small parts, and some muzzle devices may carry over. Most other parts—especially uppers, lowers, and bolts—are completely different.

What buffer system do AR-9 and AR-10 rifles use?

AR-9s typically use a carbine buffer system with a heavier buffer or even a dedicated PCC buffer due to the blowback action.
AR-10s often use longer buffers and rifle-length tubes, and are sensitive to gas port and buffer tuning.

What parts are cross-compatible between AR platforms?

Cross-compatible components across platforms (PCC, AR-15, AR-10) include:

Triggers
Safety selectors
Charging handles (sometimes)
Pistol grips
Receiver extensions (with caveats)

Will this part fit my AR-10 or AR-15?

If the part doesn’t list compatibility, assume it’s not cross-platform. Many sellers specify “AR-15 only” or “large frame compatible.” When building or upgrading, match all components to the same platform type to ensure function and safety.

What should I know about AR-15, AR-10, and AR-9 compatibility before building?

Each AR platform uses unique parts. Beginners should understand that while some components (like triggers and grips) carry over, most critical parts—uppers, bolts, mags, buffers—are platform-specific. Use this guide to plan your build intelligently.

Are AR-15 parts compatible across different brands?

Yes—most AR-15 parts are designed to be Mil-Spec compatible, meaning they follow standardized dimensions and tolerances. This includes components like:

Upper and lower receivers
Buffer tubes and stocks
Triggers and safety selectors
Barrels and handguards (in many cases)

However, some brands use proprietary designs for features like:

Barrel nut threading (e.g., Geissele, Daniel Defense)
Receiver shapes (billet receivers may not fit all uppers)
Ambidextrous controls or unique gas systems

If you’re building from mixed-brand components, always double-check:

Whether the upper and lower match in spec and fit
The handguard-to-upper interface
Barrel nut thread pitch and torque specs