I just got my first .22 semi-auto (and first Ruger), a Mark III Competition Target model. I'm planning on getting an Ultra-Dot for it, as I'll mostly be using it for Bullseye competitions. I was a little confused though--it seems like the Ultra-Dots come in 26mm and 30mm size. Other than a small difference in price, is there any difference in performance between the two lengths? Will either fit on my gun? Thanks!