As far as upper receiver goes, the M4E1 offers two versions. The threaded upper receiver and the enhanced upper receiver. The enhanced upper uses a one piece mounting system for the handguard without using the barrel nut. This is more rigid to mount a free float handguard system to increase overall accuracy.

Aero Precision M4E1 

The M4E1 Lower Receiver is a special part we make that looks cool like a fancy version but is as strong as a regular one, and it doesn't cost a lot!

The magazine well is flared so the magazine can go in much easier than a standard AR15 lower receiver. The trigger guard is also built in as a part of the receiver, so it's easier to use, even with gloves on.

For a traditional AR15 lower receiver, the user has to use a 1/16" hex key and a roll punch to get the trigger guard installed. The M4E1 doesn't require that.

Instead of using a tricky pin to put the part that catches the bolt in place, we use a simple screw. This makes it easier to put together without needing special tools or worrying about scratching it. We give you the screw when you get the M4E1.

And lastly, the M4E1has the 90 degree fire selector marking as well as the 45 degree fire selector. The Radian fire selectors are highly recommended for a build like this.

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.

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.

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?

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

Billet

Forged

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.

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.

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.