Regarding so called “Short barreled ammo”, for umm, well let’s just say for a Glock 27… 
or a 26, or ppk, or whatever of that size…
Is it loaded with MORE powder to push the same weight bullet to proper speed and expansion from a short barrel, or loaded with LESS powder so it can be controlled better in a smaller frame? Specs seem to be about the same 1000 fps on most.
Or just that they use a 155g bullet instead of a 180g (in .40).
What’s the actual deal here?
My understanding, from what I have read about this type of ammo, is that it has a faster burn rate, therefore, more of the powder will burn before the bullet exits the barrel. Rifle ammo has slower burn rates due to the longer barrel. The powder should still be burning, and therefore, gases expanding to propel the bullet until the bullet exits the barrel. If not, the bullet would slow down before exiting the barrel. The physics behind burn rates, powder formulas, powder sizes and shapes and barrel designs and lengths is quite interesting.
Unburned powder or burned after exiting the barrel is wasted energy not used to propel the bullet, which reduces bullet speed. Regular ammo does not attain the advertised velocities in short-barreled handguns because of that. Manufacturers have now found a new niche market for concealed carry with this ammo that we are willing to pay extra for. Previously it was hollow-point, then filled hollow-points (legal in New Jersey); frangible; pressed metal powder bullets, and now cartridges for short-barreled handguns.
I think different manufacturers do different things. Any combination of different powder, different power/velocity, different bullet weight, and even different HP design for lower velocities could be used. If you are looking at 9mm short barrel loads, ShootingTheBull410 does some extensive tests on short barrelled pistols and different loads.
@Dave17 - Excellent explanation, being a reloader sometimes this really becomes
a chore getting it all right. I will hunt the internet sometimes and then again I will
just call Hornady or whoever and say - ok what is best formula to shoot in this
new thing.
Be safe, be healthy, and always carry.