There are so many AR uppers to covert to different rounds, and it pisses me off.

I can see a few rounds worth making uppers in 7.62x39, 9x19, .45 acp and maybe a higher power .22 round such 22-250. I guess .38 special, .357 mag and .44 mag would be okay too, being somewhat common.

other then that, what is with people making stupid uppers in wildcat rounds that you have chance in hell of finding in the real world...seriously, where the fuck are you going to find a 6.8mm Grendel in the Dem Rep of Congo?

what calibers do you see as truly practical to convert an AR to?

some 6mm rounds or "wildcats" are actually pretty efficient

I cant recall the name but theres one the US is considering not a whole lot bigger in overall size than the 5.56 cartridge so you can carry just as many but offers a heck of a lot more range and penetration

theres some built on the 6mmBR remingtonsomethingorother round its optimized design strictly for competition and cant be beat for consistency due its case size to powder volume ratio and shoulder angle/neck length (big cases with small amounts of propellent tend to have erratic performance)

projectiles in 6-7mm range have a high BC (ballistic coefficient) and is about optimum caliber in any hand held type rifle length barrel look at the length of a 5.56 projectile vs the length of most 6mm projectiles

I always thought Colt was missing the point when they offered the 9mm Luger upper when there are so many better 9mm sub mgs around why would you want to carry an AR in a 9mm but each to his own I guess

after re-reading your post I guess if youre concerned about practicality such as available supply and logistics and compatibility with local forces (ie Congo? which side are you planning to work for??) etc better stick with a NATO round

I'm talking for the logistics side of it. In conduction of operations where you may not get a re-supply so easy. Then a 7.62x39mm upper makes sense.

I know the 6.8 and 7mm rounds are good rounds, just the logistics of them is insane. Your not going to find them anywhere outside of a specialty gun shop.

Mind you that I reload for everything i own. The AR uppers that I recommend:

.458 Socom w/ 500gr tungsten core bullets
5.7x28 (uses p90 mags) for my wife
.22lr cheap and fun


I have been looking at 6.8 spc but it doesnt do anything special. It just shoots 110gr bullets a little farther and harder. My .458 can do the same thing. Now 6.5 creedmoor is just downright mean. Accurate as hell too. Shame you need an ar-10 lower to run a magazine of em.

If I were purchasing one that interchanged?
Just off the top of my head?
I'd want it to run these rounds, in this order:
5.56x45 NATO
5.45 Russian
7.62x54 Russian
7.62x51 NATO
7.62x39 Russian

This list based upon present price and availability.
You can purchase 3000rds of 5.45 for $400 right now.
Ditto 7.62x54 - 2400rds for $199.