bore slugging question

[jballs918]

just to make sure im doing this right, where do you measure for the bore diameter. i was going off the grooves was this wrong.

[Wayne Smith]

Measuring off the grooves gives you the largest diameter. Measuring between the grooves gives you the bore diameter, the size of the bore prior to rifling. For a rifle bullet you want the nose riding portion just at or a couple thousands less than the bore diameter and the shank portion a few thousands larger than the groove diameter. Of course, this assumes your throat is to spec. For pistols you want your chamber diameter a few thousands larger than your groove diameter.

Hope this is what you are looking for.

[NVcurmudgeon]

jballs, a slug driven through the barrel will give two dimensions. Groove diameter is the distance from the bottom of one rifling groove to the bottom of the groove exactly opposite. Bore diameter is the distance from one land to the opposite land. In a typical thirty caliber barrel, bore diameter will be .300" and groove diameter .308". Groove diameter can be measured with a micrometer. Bore diameter can be measured by a caliper. This works best with barrels having even numbers of grooves. In barrels with odd numbers of grooves, a very specialized and expensive "tri-mike" must be used. What complicates things is that some of us say they slugged their bore, when the dimension they meant is the groove diameter!

[jballs918]

ok i need bore diameter. i went from where there was 2 flat spots on oppsite sides with a set of calibers. i got .305 for my musty nugget. so i think i did this right.

[StarMetal]

Actually you want the bore riding a thou or so over, not less. If it's less you may get leading and definately not centered. Being over is better. As for the groove diameter you should be over, but the bullet should really be fitted for the throat....but if your are a thou or two over groove and dead on or a thou over bore, you should be ok.

Joe

[twotoescharlie]

slug from both ends of the barrel.. drive a slug about 6 inches from the chamber and drive back out. do this from the muzzle end also. most likely you will get two different measurements.

TTC

[Buckshot]

--bore n. 1. A hole or passage made by or as if by use of a drill. 2. A hollow, usually cylindrical chamber or barrel, as of a firearm. 3. The interior diameter of a hole, tube, or cylinder. 4. The caliber of a firearm. 5. A drilling tool. [Middle English boren, from Old English borian.]

1) Bore: Commonly as the entire interior surface(s) of a firearm barrel, irrespective of individual interior features.

2) Bore: That portion of a firearm barrel being the portion initially drilled and reamed to size. Ie: lands, or the bore eventually formed by grooves being cut into the bore surface. The smallest ID of a firearm barrel's interior.

A couple interesting addresses having to do with barrels and barrel making.

http://www.riflebarrels.com/articles/50calibre/uniform_rifling_depth.htm
http://www.firearmsid.com/Feature%20Articles/RifledBarrelManuf/BarrelManufacture.htm

The supplied drawings of the barrel rifling forms (Bore forms) are of obsolete and no longer in vogue types, but serve to illustrate the barrels' 2 basic structures, the bore and groove. Dashed line is the bore. A slug produced by driving a lead slug through or a casting of the barrel produces a positive of the internal form.

Measuring across the widest features gives you the groove diameters. Measuring between 2 opposed engraved (in the slug) features gives you the bore diameter.

..................Buckshot

[jballs918]

thank you buckshot,

that explained alot. so where the oppsite grooves are is 305 and then i dont know what groove diameter is. i used a .312. now to figure out the perfect mold