I'm making a 12 ga adapter for my M203. Due to obvious length contraints, i.e. the 40mm barrel only opens up to about 4.9", the shotshell barrel length will be a little under 7" long.  I'm wondering if I should have a choke installing in the barrel or just say fuck it and keep it cylinder bore.  I'm only planning on shooting the Federal 1-7/8" buckshot and slugs.

You could make your adapter any length really… you’d just have to hit the barrel release lever when you wanted to remove it or load it into the launcher.
I found that, if using a 12ga adapter (and even the 20ga I tested with the explosive trainers) that I like to mill a part out of the rim so they don’t eject when I open the launcher. I want the 12ga casing to come out easy though… the ones with the finger grooves on both sides of the 12ga case work best.
As far as choke goes… not for slugs or buck. Go straight barrel. Chokes are good if you are hunting turkeys or pheasants… not so much with slugs and buck. GROG

Only thing I can think of with the title of this thread
https://imgb.ifunny.co/images/155bf1be3c459ef367788b08537b32cf604b61730cebf113a2c208a509797ff3_1.jpg