I have a range report. I took the Thompson to the range along with the Glock 21 to see what the longer barrel would give me. The Glock is the standard 5" barrel, the Thompson has a 16.5" one. The load was the standard 5.0gr of Bullseye under a 230gr RN bullet.

From the Glock, it developed the standard 800fps. Nothing earth shattering here! The exact same load in the longer barrel developed 1000fps. Now, you should be able to plug those numbers into your ballistic software and see what kind of energy you can expect at what ranges.

And with those numbers you can see that Bullseye develops 17fps for each inch of barrel with this load.
Slower powders will give higher velocities. Maybe I will have to play around with that in the future.