Guns, Dogs and Blades QnA

What is a good upper barreled receiver for the money to go on a AR lower receiver? What would be a good caliber also to use? 9 mm, 7.62x39, 50 cal Beowulf?

For the money, 556. Easiest round to find in America. Cheapest parts to find and purchase.
Best war gun to own due to amount of ammunition in the home land.

A good caliber to use?
My favorite is 375.socom.
Second favorite is 6.8.
Third would be the 204.

All wildcat rounds. That is the only issue with these rounds. Difficult to find. Best if you have someone to load or you load yourself.

I laid down a 1000 lb. pig at 300 yards W/ 375 socom. Walked dead for about 40 yards.
Giant hole.
Dead.
(Another?) New AR15 Round - The .375 Reaper -
Review: Tromix .375 SOCOM - RifleShooter
.375 SOCOM - Wikipedia

6.8 works great for deer. This is my favorite all around bullet. Designed for the military.
https://gunnersden.com/6-8-remington-spc-ballistics-pros-cons/

204 is an incredibly fast round. Really reach's out and touch's. 4000fps plus depending upon your loading. .204 Ruger Ballistics GunData.org

9mm good on ar's for close up combat.
7.62x39. Problem here is that most bullets you will find are berdan primed (mostly non-reloadable), steel cased and communist rounds. The primers are hydroscopic. Tend to corrode and also corrode and rust your barrel as well.
50 beowolf. Have not had a 1000 lb pig to shoot yet. Seems a little unruly compared to the few other big bore 15's.
Don't get me wrong, would be nice to have one. Or any of the mentioned calibers for that matter.