I have done some testing over the past few days on the "butter" measurement for cutting the inside segment of a 2" SCh-40 sleeved golfball barrel. I have looked and looked and the only definite, (not vague), answer I could find was on Aturner's site, "...maybe an extra 1/16"...". I decided to do some tests.
All of these ideas were done making a short 3" barrel segment.
The first was a 7/8" strip cut out the inner pipe. This worked, but was freaking DIFFICULT to start sleeving and the fit was loose on the golfball.
The next one was a 1" strip cut out, thinking since Aturner just estimated how much more he cut out, I'd just cut the 1" strip out. This one sleeved VERY easily, yet the golfball couldn't fit. You had to shove and hammer it to get it into the barrel.
Finally, I tried 15/16". This one was moderately difficult to sleeve, but once started it was only a matter of hammering the inner pipe in with a pipe wrench, (which subsequently fell apart and flung the adjustable head at me). This was almost a perfect fit for golfballs! I could probably make a *PERFECT* barrel if I had more time and if I felt it was worth it. This would involve measuring with with an accurate ruler, (1/32" incriments), and cutting with a table saw.
The conclusion? If you cut a 15/16" strip out the inner pipe with perfectly square and parallel edges, while accounting for the width blade of the saw, (mind you), you will get a fine barrel for golfball spudding.
As with all barrel pictures, the supposed "gap" is a composed of a lot of shadow. The fit can be comparable to 1-1/2" SDR-26.
And last, I still agree that the best barrel you could get for golfball launching is a sleeved 1-1/2" SDR-21/26 barrel. Hell, there are a lot of places you can find the stuff. Irrigation places are a good way to start off. They usually use the stuff. As for the SCH-80 to sleeve it in, you can sleeve it in 2" SCH-40 with spacers, but with that idea, if the barrel cracks your screwed and have to start over. With SCH-80, the barrel is held a lot better and won't crack.
Good luck, and happy spudding.