Are All AR15 Lowers The Same

Most AR15 lowers are the same in terms of functionality. Most lower receiver accessories will fit. however, machining tolerances may vary from one company to another, including the trigger group pin holes.

Yes, you can mix and match an AR15 upper from one company and match it with a lower receiver from other, but they may wobble and not have the tightest fit. In worst case scenario, some none mil-spec receivers are marketed as mil-spec, and they don't quite fit well with a mil-spec upper.

There can be slight differences in fit between parts from different manufacturers. This doesn't mean they won't fit together, just that some might be easier to assemble than others.

misaligned lower receiver pin hole

For example, this particular Aero Precision lower doesn't fit well with a Daniel Defense upper or a Mil-Spec billet VLTOR upper. Maybe there was a bad batch, maybe, maybe not. but machining tolerances are definitely NOT consistent in the AR world. 

Why Mix Brands?

If you know the AR15 well, you might prefer not to mix and match different brands, especially when you're crafting a high-precision AR15. Usually, receivers from the same company fit together best.

But, not all AR15 builders are focused on creating super accurate AR15s. Many builders are just fine with making something that fits their budget, can shoot up to 100 yards, and they're satisfied with that.

Some people care about discount deals, customization, style, and color options. So they get whatever they can get their hands on.

Do Trigger Pin Holes Tolerance Matter?

Yes they can affect sear engagement performance if out of spec. Trigger pin holes are generally 0.154" in diameter. In the AR15 world, they are called the small pin holes. 

Generally, trigger pin holes have a diameter of 0.154". In the AR15 world, people refer to them as the small pin holes.

Different receiver sets can vary in tolerances. Though it might not be noticeable to the naked eye, it can impact aftermarket trigger upgrade performance.

timney ar15 competition drop in trigger

That's why drop-in trigger sets are popular among AR15 builders, as they come as a unit with the tightest tolerance. This way, the trigger company doesn't have to rely on the imperfections of lower receiver manufacturers in the market.

They do this to ensure the best and consistent trigger pull and trigger reset performance.

KNS anti walk pins

The trigger pull weight and performance might differ if the trigger holes become worn out or loosen after heavy use. We highly recommend using anti walk pins to prevent the steel pins from rotating and grinding and stressing on the aluminum trigger pin holes.

Will Any AR15 Upper Fit Any Lower?

ar15 bcg bolt head

The answer is almost always yes. The key term to look for is "Mil-Spec," which stands for military specification. Any part that's designated as Mil-Spec should fit together with other Mil-Spec parts.

There are a few exceptions, but these are rare. If something doesn't fit or the upper receiver pin hole doesn't line up with the lower receiver pin hole, it has to do with tolerance problem.

Billet VS CNC Lower Receiver



here can also be differences in style between parts from different manufacturers. For example, a lower receiver that's machined from an aluminum billet might have a different finish than a forged upper receiver.

Billet receiver generally has more complex cuts to make the receiver look better than a standard lower receiver. It takes more time to machine, and therefore the cost for a billet receiver maybe more expensive.

Some billet receivers even feature ambidextrous magazine release and proprietary bolt catch that a standard receiver may not have. 

Does Mixing Brands Affect Accuracy?

Some people believe that mixing brands can affect the accuracy of the rifle. This is a myth. The accuracy of an AR-15 is determined by the barrel, bolt carrier group (BCG), and handguard, ammo choice, shooter skills, and all of which are part of the upper receiver.

ar15 lower receiver fit

It's highly recommended to match the barrel to the BCG brand for a solid lock-up and to reduce wear. You see, when they come from the same company and are machined at the same shop, it helps keep the tolerance under control.

ar15 bcg bolt head

Now, if the chamber doesn't meet the same tolerance as the bolt head, the rifle ends up having friction issues, leading to accuracy problems and damage."

The lower receiver's main job is to house the fire control assembly and magazine, and to hold the upper receiver. As long as you're using high-quality parts, mixing brands won't affect accuracy.

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