Hi, most bubble levels seem to clamp onto the rail, or the mounts. I have alligned my scope with the barrel using the mirror method, so in reality it matters not where the dovetails, or mounts are. I thought about a bubble level that clamped round the scope tube to get perfect alignment, but there dosnt seem to be too many of these in existance, thus do people just shim the rail bubble? I would prefer to clamp the bubble to the scope tube if there is an ideal type? Thanks - Geoff.