I have an old Mossberg 340 bolt 22. Had it a while, haven't shot it much. Accuracy is only fair. I decided to give it a good cleaning for the first time. Looking into the bore, the rifling looks awfully thin. It is stamped accro-groove rifling. Is accr=groove a very thin rifling system or do I perhaps have a shot out barrel? Bob

Unless it's had some unusual bore damage, I doubt it would be possible to shoot out a .22 barrel under normal circumstances.

Those old Mossbergs have a reputation for being shooters - most shot 'way above their class', as it were. I have a hunch a good cleaning is about all it'll need.

I have one that literally spent a summer (circa 1963) underwater in a northern Alberta river....from spring breakup, until I found when the water was low just before freeze up. After a couple of weeks in a pail of diesel fuel (just to get the action open!) and another week of fiddling around, it shoots ragged one-holers to this day. While the outside looks like a moonscape, and the crown looks like it was chewed off by rats, the bore remained shiny..possibly due to a waxy coating from bullet lube (?). I'd probably have tossed it, but it was the first rifle I ever bought with my own money - $35, well spent in about 1959..

The Accro-groove rifling is shallow as you describe. Try changing brands of ammo. If you are accuracy testing standard velocity may work better.