Hi Bart,
If you chose caliber 0.40 , you could probably get Rice to make 13/16 swamped rifle barrel.
Look at the A weight, Reading profile listed on Rice barrels website
http://www.ricebarrels.com/chart.htmlIt's caliber 40 and the waist is 0.606.
13/16" = 0.8125"
0.8125" breech - 0.606 waist = 0.2065" reduction. This is about same swamping used on the J.C.Beck profile on the Rice chart(1.0"-0.812= 0.188").
By "tweeking" the Beck profile, you should be able to make a safe 42" long, 0.40 caliber barrel with this profile:
Breech 0.8125"|--------------27"-------------|Waist 0.609"|------9"------|waist 0.609"|-----6"------|Muzzle 0.700"
This would give you a little bit more swamping than the Beck profile. As for locks, you could use a small Siler or a L&R Bailes lock.
I'm building a long barreled pistol , caliber 0.40, breech 3/4", swamped as much as safely possible. The barrels 15" long and custom made by Ed Rayl. I'm using a small Siler lock.
Best regards
Rolf