You're not wrong, but I'm not making anything yet. I just want to go ahead and get some CF on the way. I already have a couple of different types of stock I'll be using at the ends to make bushings holding the CF. I'm pretty sure the shroud and "barrel"(liner holder) will be the same diameter across calibers, and I'd think it would be pretty hard for someone to get measuring those ODs wrong. Obviously, the most critical parts will be where the barrel runs through the gun, and where the shroud meets the frame. I plan on ordering a couple different OD of CF too. I'll add what I don't use to my collection. I already have a lot of CF tube floating around from other builds, but I'm almost positive I don't have any on hand that is large enough. I think 20mm is the largest I have on hand, and I know the factory shroud is larger than that. The bushings I make to go on each end of the CF will be where the perfect fitment is most important.
The most important measurement I need will be the OD of the shroud, because that will be the only part I'll really need the CF for. I already have the barrel(s) I'm going to use, which I'll likely have to turn down a bit to fit. Come to think of it, I really don't need to know the OD of the barrel right now.
