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Old June 9, 2013, 01:14 PM   #34
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
My 'smith dialed in barrels held in a 4-jaw chuck with a gauge pin of exact bore diameter to get the breech end of the rifled blank turning on center. Then he started in the piloted reamer that well aligned it with the hole in the barrel. Just like gun drilling a solid blank; the cutting tool tends to center about the rotation axis and makes holes very straight.

Muzzles were faced off a similar way after dialing in the muzzle end with a similar gauging pin. A small amount of outside barrel diameter was turned off so it would put the bore centered in a chuck that held the muzzle turning on bore center while the 11 degree recessed muzzle face was made then a fine abrasive charged lapping ball was used to put the angled crown on the rifling out to a few thousandths past groove diameter.

Some 'smiths have checked barrel chambers for roundness that when used, shot sub 1/4" 100 yard groups all day long with good bullets. Even those chambers were, just like sized case bodys, were not perfectly round.

I think it's the barrel's metalurgy makup that causes all these dimensional irregularities. As the tool's cutting edge removes metal, the resistance it has to being machined causes the tool to move a bit. Another example is rifling twists taking a jump in twist rate in button rifled barrels; the button didn't maintain the same point and it twisted too much or too little to keep the rifling angle exactly the same from breech to muzzle. There's folks that'll measure your barrel for rifling twist irregularaties.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master

Last edited by Bart B.; June 9, 2013 at 01:25 PM.
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