Huge fan of the Custom Air breeches. I'm sure they are the best available for the Crosman/Benjamin guns.
Just in case you don't have a lathe to turn down your barrel the slight amount it needs to fit the breech snugly.
I introduce to you a brand new product with several years in development and painstakingly manufactured to the highest of standards with only the finest quality materials "The Caveman Lathe".
Here is just one example of the caveman lathe. I used to carefully snug my 1/2 inch drill upside down in my vise on one end when I did this many years back. Just spin the drill motor up, hold the file to the breech end of the barrel and make sure the drill turns the correct way so the files will cut. Slide the file/files side to side to keep the diameter of your cut even for the length of your cut. Some masking tape wrapped around the barrel, for a stop on one side, will keep you where you want to be on your cut.