What is a good/optimum bullet diameter for a 0.3103 barrel? When my current batch of 0.3095D bullets are done, should I get more of the nominal .310, or should I try some .312 FMJs? (I think only Sierra has .311 bullets in the mid-120 grain range, and they're hunting projectiles.) In other words, should land to land diameter match bullet diameter (e.g.; .308 to .308 for a .30 caliber barrel), should the bullet be slightly smaller, or slightly larger than L2L bore?
Some background on how I got to this question:
Any attempt to reload 7.62x39 for a Ruger Mini-30 has to start with the rifle. Rumor has it that Ruger sold two different-sized barrels for the Mini-30 over time; one with a land-to-land diameter of .308, and the other with a .310 or .311 barrel... depending on the writer. Since my Mini-30 is an older edition, I assumed (yeah, I know) the barrel was made for .308 bullets. And that's how I reloaded. I used the smaller diameter sizing button when full-sizing brass, & used .308 bullets. I got OK groups, but the point of impact at 100 yds was about 6-8" lower than Winchester or FC commercial hunting ammo. (Groups with commercial were about as good as the .308 reloads.)
So back in Aught-Nine, I decided I needed to slug the Mini-30's barrel. Starting with a .36 Hornady round ball, I hammer-swaged it down the bore. Land-to-land diameter miked to 0.3103; definitely bigger than .308. And that is where things stopped for a while.
Last month I decided to break the Mini-30 out of her protracted closet stay, and to try reloading with bigger diameter bullets. I found some 123 grain .310 FMJ bullets from Mid-South (Thank you, Jessica!), and loaded 50 of them in once-fired Winchester cases over IMR 4198 powder and Federal LR primers. Cases were full length resized using the larger button. I could not get good neck tension. Without a crimp, the bullets would push into the case the slightest thumb pressure. With a moderate seating die crimp, bullets would spin in the case on the cannelure. Finally had to use a factory crimp die to hold the pieces together against movement. Groups were definitely so-so. I think I was just recycling the brass for the next attempt.
When I got back from the range, I miked both the large button and bullet diameters. The large button averaged out to .3097 (three readings), and the bullets averaged .3095 at the base and .3093 just forward of the cannelure. Think I see the problem.
Next, I measured the smaller button. (My die set came with both.) It miked at .3064 D. For S&G, I attempted a couple of dummy assemblies today using the .3095 bullets; plenty of neck tension and no little ring of gilding metal. I can build cartridges for this gun using the smaller button and the larger bullets.
But looking past the next 100 rounds, is it worthwhile to try reloading with the large button and 312 bullets?
Some background on how I got to this question:
Any attempt to reload 7.62x39 for a Ruger Mini-30 has to start with the rifle. Rumor has it that Ruger sold two different-sized barrels for the Mini-30 over time; one with a land-to-land diameter of .308, and the other with a .310 or .311 barrel... depending on the writer. Since my Mini-30 is an older edition, I assumed (yeah, I know) the barrel was made for .308 bullets. And that's how I reloaded. I used the smaller diameter sizing button when full-sizing brass, & used .308 bullets. I got OK groups, but the point of impact at 100 yds was about 6-8" lower than Winchester or FC commercial hunting ammo. (Groups with commercial were about as good as the .308 reloads.)
So back in Aught-Nine, I decided I needed to slug the Mini-30's barrel. Starting with a .36 Hornady round ball, I hammer-swaged it down the bore. Land-to-land diameter miked to 0.3103; definitely bigger than .308. And that is where things stopped for a while.
Last month I decided to break the Mini-30 out of her protracted closet stay, and to try reloading with bigger diameter bullets. I found some 123 grain .310 FMJ bullets from Mid-South (Thank you, Jessica!), and loaded 50 of them in once-fired Winchester cases over IMR 4198 powder and Federal LR primers. Cases were full length resized using the larger button. I could not get good neck tension. Without a crimp, the bullets would push into the case the slightest thumb pressure. With a moderate seating die crimp, bullets would spin in the case on the cannelure. Finally had to use a factory crimp die to hold the pieces together against movement. Groups were definitely so-so. I think I was just recycling the brass for the next attempt.
When I got back from the range, I miked both the large button and bullet diameters. The large button averaged out to .3097 (three readings), and the bullets averaged .3095 at the base and .3093 just forward of the cannelure. Think I see the problem.
Next, I measured the smaller button. (My die set came with both.) It miked at .3064 D. For S&G, I attempted a couple of dummy assemblies today using the .3095 bullets; plenty of neck tension and no little ring of gilding metal. I can build cartridges for this gun using the smaller button and the larger bullets.
But looking past the next 100 rounds, is it worthwhile to try reloading with the large button and 312 bullets?
