Bore diameter versus bullet diameter for doubles

Rick Hill

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I was wondering, is there a general rule or thoughts on bore diameter vs bullet diameter for double rifles? Seems like you might like to limit barrel distortion a keep your barrels together longer? I have an "I" bore 8x75R double rifle that slugs to .306/.316" with a 1:9.5" twist rate. I am wondering if people would select .316 or .318 for load development and regulation?

Rick Hill
 

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By "I" bore do you mean 2 groove? Not knowing the age or how well made or type of steel, hard to say exactly but I would certainly lean toward the .316 diameter bullet. Even in a modern single barrel, the only thing the .318 jacketed bullet would do in a .316 groove diameter bore would cause an increase in pressure and increase the jacket fouling.
 

Rick Hill

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By "I" bore do you mean 2 groove? Not knowing the age or how well made or type of steel, hard to say exactly but I would certainly lean toward the .316 diameter bullet. Even in a modern single barrel, the only thing the .318 jacketed bullet would do in a .316 groove diameter bore would cause an increase in pressure and increase the jacket fouling.

The German/Austrian 8 mm's were made in two bore sizes: the older I bore - the German letter I looks like a J (.318) and S (.323). There was a concern raised about an incomplete seal and gas blow by causing a cutting/erosion effect with a bullet same as the bore diameter. Next time you shoot let a finger or two touch the outside of the barrel on your double. You will be surprised how much the barrels flex with bullet passage. The barrels were top quality for 1912 and are Poldi - Anticorro No.183. They still make barrels to this day. I have a pristine Persian Mauser - I think I will slug the bore and see what they were running relative to an S bore .323
 

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The German/Austrian 8 mm's were made in two bore sizes: the older I bore - the German letter I looks like a J (.318) and S (.323). There was a concern raised about an incomplete seal and gas blow by causing a cutting/erosion effect with a bullet same as the bore diameter. Next time you shoot let a finger or two touch the outside of the barrel on your double. You will be surprised how much the barrels flex with bullet passage. The barrels were top quality for 1912 and are Poldi - Anticorro No.183. They still make barrels to this day. I have a pristine Persian Mauser - I think I will slug the bore and see what they were running relative to an S bore .323
The rifle was certainly originally regulated for a .318 bullet. That is where I would begin my load development. I would also note that you could hardly have chosen rarer project!
 

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OK got it... "J" and "S" 8mm bores. Yes, well known.
As far as blow by- most of that occurs as the bullet transitions between the throat and bore. That's why the throat wears out first... no matter if the bullet is a perfect seal in bore or not. If you were careful and got accurate measurements of the groove diameter of the bore at .316 then, if it were mine, I'd still shoot .316 jacketed bullets. The only question or reservation I'd have about the .316 is bullet choice. There can't be very many .316 options out there, other than specialty lead cast or swaged bullets-- but I think Hawk (and maybe a few others I'm not aware of) makes a good quality 150 gr .316 jacketed hunting bullet. I would also think, given it's age, it has probably been shot with all manner of "8mm" bullets from more or less .318s to even possibly some .323s. And for POI regulation of the barrels- who knows.
 

Rick Hill

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The rifle was certainly originally regulated for a .318 bullet. That is where I would begin my load development. I would also note that you could hardly have chosen rarer project!
Rare but a pristine double rifle that fits perfectly - so worth the effort
 

Rick Hill

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OK got it... "J" and "S" 8mm bores. Yes, well known.
As far as blow by- most of that occurs as the bullet transitions between the throat and bore. That's why the throat wears out first... no matter if the bullet is a perfect seal in bore or not. If you were careful and got accurate measurements of the groove diameter of the bore at .316 then, if it were mine, I'd still shoot .316 jacketed bullets. The only question or reservation I'd have about the .316 is bullet choice. There can't be very many .316 options out there, other than specialty lead cast or swaged bullets-- but I think Hawk (and maybe a few others I'm not aware of) makes a good quality 150 gr .316 jacketed hunting bullet. I would also think, given it's age, it has probably been shot with all manner of "8mm" bullets from more or less .318s to even possibly some .323s. And for POI regulation of the barrels- who knows.

Interesting point on throat erosion. I may swage the first 20 rounds to .316 and then reslug the bore. When you look at the rifling and condition of the gun I am not sure it was shot much after proof. The .316 is the tightest portion of the bore most is slightly larger as you can "feel" this as you are slugging the bore. I had another old double like this. Once the bore was broken in accuracy went to lots of smiles.

Rick
 

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Yes, It's amazing how sensitive such measurements can be. With care, you can feel just a few ten thous difference in diameter. Won't hurt anything to shoot some .316s and see how it acts. If it seems sub par then move up to .318. For practice and targets and fun shooting something like a 180 gr .318 gas-checked cast bullet in a fairly soft alloy (about 12 BHN or so) with a fairly soft lube loaded to maybe 1200-1500 fps would be worth a try.
 

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