I'd pick a gun with a 21/22" barrel in a vent rib version. It would easily work for both 3 gun and HD, only 3" longer than the "tactical" guns, and would be way better for 3 gun. VM, 1301, and M2 all come in said configuration.
If you feel like the one on the stag is not to your liking, then switch it out. If you don't like the AR stoner then pick one of the many quality comps out there and get one. The Miculek is a great comp for the price if money is tight.
The answer is not as simple as what can you load to. How long of a load will the chamber hold is a better question. And that will be different with different styles of bullets.