The good news is that a 7.62x39 upper will offer you some serious advantages over a 5.56 upper. The 7.62x39 upper will offer better stopping power as a result of its chambering, not to mention considerably better performance from a shorter barrel.
That being said, there are some concessions you might make with a 7.62x39 upper; not that you should.
It will produce more recoil and it might not have as many attachment points, especially if the overall build mimics an AK47. There are some ways around that, though, especially if you make some of the upgrades mentioned in this post.
A Muzzle Brake
The 7.62x39 upper you go with is probably going to come with a muzzle device, and that muzzle device is almost certainly a flash hider.
Now, a flash hider is relatively good at what it does, which is to obstruct the muzzle flash so that you don’t get temporarily blinded. This keeps the sight picture clear and helps you execute faster, more accurate, and more confident follow up shots.
But, beyond this, it doesn’t do much of anything else. And, building with a 7.62 upper means that you will already be making a concession in terms of recoil. That is, 7.62 Soviet produces a good bit more recoil than 5.56.
Many consider this worth it since you get so much more stopping power, and they have a point, but you still might want to do something with that recoil.
Well, one of the things you can do is as simple as upgrading your muzzle device from a flash hider to a muzzle brake.
Unlike a flash hider, a muzzle brake will change how the gasses are ported at the muzzle, redirecting the force of felt recoil. Generally (but not always) a muzzle brake will port the gasses upwards, which results in a net force down on the muzzle.
This lowers the force of felt recoil considerably, in some cases by as much as 50% (and without any affect at all on shot power) while also counteracting the forces of muzzle flip.
So when you add a muzzle brake to your 7.62 AR, you’re actually getting quite a few benefits. One is that you will enjoy fatigue-free shooting for longer as a result of the effect on recoil, and the other is that you will enjoy less muzzle flip so you can make faster, more accurate, more confident follow up shots.
Just be sure not to skip the hearing protection ever, because among other things, a muzzle brake will make your gun much, much louder.
An M-LOK Handguard
This is one you might not need to make, but if you’re building a 7.62 AR that sports the aesthetics of the Kalashnikov, you’d better take notes here.
The AK47 and rifles patterned on it had a wooden handguard, not too dissimilar from the “tropical” handguards found on HK subguns like the MP5. It does shield the hand, so as a textbook “handguard” it does its job. What it doesn’t do is provide attachment points.
So, in the event that your 7.62 upper comes with a traditional handguard that doesn’t have attachment points, swap that out for an M-LOK handguard.
You might be thinking, “Why not a Pic rail?” Well, there are some advantages of an M-LOK rail system over a Pic rail.
Among others, M-LOK rail systems are considerably lighter and allow for much better heat dissipation than Pic rail systems. And, on top of that, there are no ridges to snag on anything, either.
So, you can go with a Pic rail if you want, but don’t say you haven’t heard better things about alternatives.
Some Anodized or Skeletonized AR15 Parts
Lastly, if you are building with a 7.62x39 upper and you’re looking for AR parts that will lighten the overall build, consider either AR15 anodized parts or else skeletonized AR15 parts.
Anodized parts, which are made from aluminum, are going to be naturally lightweight. Skeletonized AR15 parts are even better, and are typically also made from aluminum.
Skeletonized parts will allow more ingress for dust and dirt, and create more surface area so they can be harder to clean, but that is often worth the massive advantage of weight savings, which can be quite considerable.