i prefer 7.62.39 to the 5.56 which im not much of a fan of. bigger heavier bullet.
If being 7.62 was all it took, then a 120 gr .380 would do just as much. Right?
Obviously not, and I make the comparison to point out that the amount of powder in the case is what does the work - not the diameter or weight of the bullet. Check the ballistics tables again, and you'll see the x39 doesn't make even with the .30-30 - because the ratio of powder to bullet weight means it's underloaded.
Other factors to consider are the case taper - the x39 requires a fully curved mag to feed reliably, exactly what the AK had, and what the AR jinks up with it's straight mag well. If anything, the better AR mags are designed to feed ammo in a curved stack too, and there's just enough room to get away with it. We'd actually be better off if the mag well was slant cut and we had proper curved mags overall.
an adjustable stock would seem to enhance shooting comfort no matter what the length.
What length the stock would be is largely a matter of what position you're shooting in, prone, kneeling, offhand, or tactically squared up, and whether you have an armored vest on. Being comfortable is something else entirely - an M4 adjustable at a good position may still have the wrong footprint shape, hit the shoulder at the angle, and hopefully you don't get a really good cheekweld shooting it in subzero temps when you put your warm face against a -20 aluminum buffer tube. Note very carefully the adjustable stock is not used in the Artic, precisely where heavy coats would seem to need the accommodation.
I built a AR with A1 stock, rifle handguards, and in 6.8SPC, with a 16" barrel. I use it primarily to hunt in cold weather, wanted enough handguard to protect me and the barrel, wanted 40% more power, and knew that cartridge was designed for the 14.5" - so NFA legal 16" was sufficient.
What the market offers to the impulse AR buyer is either a full on varmint, or a lookitmee CQB fantasy rifle, which is why there's such a variety. If this gun that's getting built needs some sorting out, here's how to do it:
Pick the exact job it's going to do, either kill live targets or paper ones. That will select the best cartridge, don't compromise. That cartridge is optimum in a specific barrel length, don't compromise. Screw that barrel on a A3 flattop, you can attach any optic you want. The stock? If you are ranked in the top 10% in that type shooting, you don't need a recommendation, and if you're not, you aren't good enough to tell the difference. It really means that little. Until then, a standard fixed stock will do. Same for grip. The handguard could be freefloated, a full length rail is useless for scope mounting as you can't bridge it. If you picked a barrel that is guaranteed to shoot under 1MOA, fine, otherwise, that $300 can go to something more productive, like the optic. Trigger last, an adjustable take up screw is about all you need unless it's a competition range gun. Goes back to will $250 make you twice as good a shooter? Not really, it just feels twice as good.
Cartridge, barrel, upper, stocks then trigger. It all flows from what amount of gunpowder and power you need downrange, not what diameter bullet seems to be.
