Considering a project. I posted in the "Want to buy Primitive" section.

Concept. Cut down a muzzle loading (MZ) "long gun" to about 12-14" barrel. Picture a side lock cut down an inch or two in front of the stock ending.
Why? - SUPER "handy" gun in the woods or inside a blind, and because I like different.

NFA rules do not apply as MZ are not firearms.
Realize I will be surrendering velocity (probably under 20" is where drag loss/velocity increase intercept in a MZ). Most of my shots are well inside 100 yards - most inside of 50.

Larger bore will give more volume to burn all the powder in a shorter barrel. So .54 and .58 better than .50.
Twist rate will play a role and I probably want a faster twist to get more effective "length-of-rifling". I may be wrong here. Fast twist drives me to heavier projectiles - In my opinion a good thing. Energy is mass time velocity squared - so with less velocity all I have to play with is mass. Same thing as going Sub-sonic with a suppressor.

What am I forgetting?

I had a cousin that did that to his ML. Like you say, loses some velocity, but if you are OK with the reduction in range and knockdown, start sawing.

He put plenty of deer in the freezer and I never heard a single complaint about the rifle. And if he replaced it, he never mentioned it to me.