The bases seem to be cheap Weaver aluminum products. Are there any holes drilled in the barrel for front target bases? If so, I would guess that the rear holes in the GB pictures may be suitable for rear target bases. I have had Gebby Varminters and other pre factory 22-250 guns and have shot commercial 22-250 ammunition in all of them. I have not fired my present Gebby Varminter built by John Oberlies, but plan to fire it this year. It has had its target bases removed by some idiot and it now has a nice Leupold on it. I would like to restore it to original with a Unertl target scope but have too many irons heating up.
