Thing is, with the inscriptions on the guns, it's hard to say what they would actually fire. Ever hit a tree with a baseball bat? That mechanic, with enough force behind it, could break bones. So firing one of Roland's mystical, inscribed, ancient revolvers, could easily break the arm of someone not from his world. It could also allow the guns to accept a cartridge they weren't necessarily made for. If the rounds hit someone with the visual effect equivalent to them being struck with a sledge hammer, simple physics of our world say the guns would have to fire at least a .50 caliber bullet with enough gun powder in the cartridge to launch it at 2k fps. The main problem is people trying to box the guns into makes/models/calibers of guns that exist in the real world, not Roland's world. Think outside the box and give King's universe a little respect.

The thing is, as with the inscriptions on the guns, it's hard to say what they would actually fire. 

Ever hit a tree with a baseball bat? That mechanic, with enough force behind it, could break bones. 

So firing one of Roland's mystical, inscribed, ancient revolvers, could easily break the arm of someone not from his world. It could also allow the guns to accept a cartridge they weren't necessarily made for. 

If the rounds hit someone with the visual effect equivalent to them being struck with a sledge hammer, simple physics of our world say the guns would have to fire at least a .50 caliber bullet with enough gun powder in the cartridge to launch it at 2k fps. 

The main problem is people trying to box the guns into makes/models/calibers of guns that exist in the real world, not Roland's world. Think outside the box and give King's universe a little respect.