Ruger Mini-30 bore specs
This rifle is chambered for the 7.62 x 39mm cartridge. This cartridge, traditionally, uses a projectile with a nominal diameter of .311 to .312 inch.
At the time the Mini-30 was introduced very few bullet makers where producing .311/.312 inch bullets for reloading in the light 125-130 grain weight required. Ruger initiated the use of barrels with a groove dimension of .308 inch and a long tapered throat. The throat allowed the use of ammunition with .311/.312 projectiles by gradually squeezing them to the .308 diameter. In addition, ammunition loaded with more commonly available .308 diameter bullets could also be used.
Commencing in 1992 Ruger initiated a change to using .311/.312 nominal groove diameter, 1-10 inch right hand twist barrels in all Mini-30's. It was likely well into 1993 before all rifles coming of the production line incorporated the .311/.312 barrels.