You need to put a magnum 3" 12ga 15 pellet shell through a stoeger side by side 
. Hell, if you're ever in the Omaha / Bellevue area, I'll gift a few shells to you .
I'll shoot anything once...maybe twice if I'm feeling froggy. I've shot .454 Casull and 450 Nitro Express and neither one was too bad. My 45-70 with a steel buttplate will leave a bruise if you shoot it enough, and RobertH's Super Shorty with 3" magnums was......interesting. That smarted a little, but the worst thing I ever shot was a Scandium .357 J frame. That's the only gun I ever took one shot and handed it back to the owner. I was done.
Dave, have you ever shot my .22 upper? If not hit me up some time and we'll go dump some rounds through it. I really think you should consider that, unless you want something bigger for hunting or long range shooting. It's a great way to get a lot of practice on the cheap, especially since you can set it up just like your 5.56 AR. Nothing like shooting all day long for less than $20.
I love my 300 Blackout, but to be honest it doesn't make a whole lot of sense if you're not going to use it suppressed. The 6.8 SPC really is a better hunting round. I had a 6.8 and it was a good gun, but I decided I wanted to go quiet so I sold that barrel and rebuilt it as a 300. Neither caliber has much in the way of recoil, but I always thought the 6.8 had excessive muzzle blast. It was just plain LOUD and it would part the hairs on the head of the guy at the next bay. The high cost of 6.8 brass compared to 300 Blackout was also a factor. I save all the Lake City brass I find and reserve that for forming into 300.
On paper the 6.5 Grendel looks like an even better cartridge, especially for longer range shooting. Until recently Alexander Arms decided to be buttheads and keep it a proprietary cartridge so the cost of getting set up has been higher than other calibers, and there are less offerings out there than the 6.8.
I've never really looked much into the differences between .50 Beowulf, .450 Bushmaster, and .458 Socom, but I'm under the impression that they all have pretty much the same performance, about the same as a middle of the road 45-70 load. Again, these probably won't be practical to shoot much unless you reload. Even then, I would stick with either of the .45 caliber offerings since there are a lot of great bullets available in that caliber, including (gasp) cast bullets. Yes, you can shoot cast bullets through an AR just fine, I do it all the time with my 300. It
would be pretty cool to have an AR capable of taking down anything in North America.....
Another option is to build another .223/5.56, but set it up differently. How about a bull barrel, fixed stock, and higher magnification scope? You already have a lightweight carbine, so maybe a precision rifle would be a nice change.
