This question about loading pistol calibers for carbines comes up from time to time.
It doesn't matter what you shoot the round in, it has to be at or under SAMMI max pressures, which means we have to follow our load books and online PDF's from the powder makers.
In general the powder that gives the best velocity in a short barrel, does so in the long barrel. I have seen where it wasn't a couple of times though in my limited testing with carbines and pistol calibers. Mostly 9MM, but a little .40 and .44 Mag. When the fastest short barrel load was not the fastest long barrel load, it was still very close to the top velocity in the long barrel.
I have had some time off lately and have shot around a dozen different 9MM loads with two different bullet weights through a 3" EMP, a 4 1/2" TZ, and a 16" AR. Every single time the fastest in the EMP was the fastest in the AR, as has been in the past.
Stay within the limits set by the load books etc. Test your different loads in the carbine to see which gives the most velocity and which is accurate.
Of the three I shot yesterday with a 125 Gr jacketed bullet, the one that surprised me as far as velocity was N 340. The EMP and the AR were very accurate with it as well, and it shot pretty well in the TZ. What really surprised me was that it was by far the best velocity in the EMP, but was short of the best in the AR by a few FPS. Close, but no cigar. (Note: It's "Hi" was tied for the highest "Hi")
In my quest to find a good, fast, accurate, 9MM 125 Gr load for the AR, I found a good option for a short barreled 9MM powder. One that still gave very good velocities in the AR. Go figure.
