Half inch diameter for the bolts does seem on the small side. The .22 versions are roughly 7/16 and look at the difference in the diameter of the ammo. A bigger diameter, 5/8???, bolt would give more room for the pieces.
I know there are hardly any posts on the forum these days BUT..... I look daily and have a lot if interest in your project. I have the barrels and plan on a .410 shotgun, 6 barrel gun build next. As I always do, I expect to shamelessly steal your ideas. Your gun design elements should apply to any center fire version and ideas for those are a rare find.
I am fighting the temptation to inquire with Cutter about that extra set of 38/357 barrels he has.