I think it's a combination of plenty of case volume and relatively low pressure. I haven't played with anything other than 230 grain bullets so far. Since I have the luxury of using Quickload, I can play with loads using the short barrels on two of my guns. Some of the loads that work well with a 5" barrel will most likely just increase muzzle blast with short barrels. I haven't played with it in Quickload yet. My poweder selection WAS based a lot on percentage of powder burned. That's going to be a function of burn rate and time in the barrel (I think).
I didn't chrony anything, but know that my loads were a little faster than factory because of the difference in recoil. There were no pressure signs on the brass, so I think i have them in the pocket. With my loads, slight differences in OAL made big differences in peak pressure (in Quickload). So, they're all loaded very close to max OAL.
I may play with different powders down the road, but will rely heavily on Quickload to tell me if I'm screwing up with my short barrels.