I have 30mm medium BKL one piece mount on my .22 Brummie Longbow and they definitely did not need over stretching to fit. On the contrary it was very straight forward to fit. They work perfectly with no creep as of yet (over a thousand shots).
I find that many do not know how to fit BKL's properly (not assuming you don't). If you know how to fit them it is extremely easy.
I use BKL'S (1 & 2 piece) on all my Webleys, which includes Vulcan, Tracker, Longbow, Xocet & a Stingray I owned until recently. All perform perfectly, all have fitted in straightforward fashion, no creep on any of them (the Tracker is gasrammed & that has not affected the scope rails at all) & I don't use any other type of mount.
The other benefit is the auto centering of the mount & scope, it really does make a difference when zeroing, I find the windage is a lot easier to zero in with the BKL's.
TBH they are a lifesaver for me because Sportsmatch were terrible on the Webleys due to no arrestor holes & they are just not anywhere as capable of applying the level of clamping force the BKL's can.
I know I sound like a BKL salesmen but I really have found them to be excellent & I'm by no means well off financially, so paying the quite high price for BKL's is not something I would do without good reason.