Father son AR15 platform project, need some input


Apr 3, 2015
Preface: So I got into a project with my son this last summer. We decided to have an AR put together that would be a budget Upper swap project. Nothing fancy just plain jane nitride black. We did not want a vanilla 223/5.56 so I found a local shop that specializes in budget ARs to see the options. These would be for ringing steel, punching paper and just a fun gun LEGO project for he and I to spend time on.
I set the budget for a rifle complete and an additional complete upper in a different caliber at $800. I knew the guy that has the shop would be at the next gun show so off I went the day of the show. He had a broad selection and so I told him what I wanted to do and within minutes I had a full rifle in 6.5 Grendel 18" 1:8 barrel 16" carbine length gas system keymod handguard single stage trigger and a B5 stock and comp style grip, with one mag for less than $500. Next was a the complete additional upper. I chose 300 Blackout in a 16" 1:8 barrel with 15" keymod handguard pistol length gas system. The bill with tax ended up at $878, busted the budget but got what I wanted.
Topped the Grendel with a 2-10x Mil dot and the BLK with a 4x ACOG. We went to the range to function test and they both ran flawless. As a budget build I wasn't expecting top of the line accuracy. Boy did I get a surprise. the grendel with Wolf 100gr steel case ran right at an 1", HDY Black 123 and some 123 SMK loads from the show were sub inch. The BLK ran just over an inch with 147 fmj and at an inch with 125gr. pleased as punch. My son rang the steel at 200 and 300 over and over giggling the whole time.
Now comes to my ask on input. We have decided to add another upper and this time we want a big bore. I know a few that have some AR15 platform thumpers so I ask of the 450 Bushmaster, 458 SOCOM, and the 50 Beowulf what are the pros and cons here. Remember this is fun gun and not intended for game(being a traditionalist I don't think I could hunt with an AR) though you never know. These will be 16" carbine length gas to run on our existing lower. let me know your thoughts