PCC's are fun but not all the practical aside from close range shooting.
Yes, a .357mag can gain a good amount of velocity with a longer barrel, however its still at 150 yards and in rifle.
I have a Kel Tec Sub2000 carbine in .40sw that I use as a stow away rifle on car trips, camping, etc. It allows me to have a PCC instead of just a handgun where a normal rifle wouldn't be "kosher".
I've recently built a 9mm AR primarily to finish off a lower that had no upper to make it work and partially because it is cheap to feed with my dillon 550b cranking out 115gr reloads at 250+ per hour.
It feeds from uzi mags and I've got a pencil profile colt barrel. It is light weight and pack 32 rds of 9mm fun.
Is it really practical for hunting or is it better than a 5.56 AR, nope.
But its cheap, fun, and I made it out of mish-mashed parts and its a hoot.