Vee block for measuring odd numberof lands and grooves

[bruce drake]

speaking of measuring 3-groove barrels. I have to slug an older 50-70 Govt Trapdoor this week to get a proper bore dimensions for a sizing die purchase later on. Anyone know the angle for the v-block for that odd-groove barrel?

[M-Tecs]

speaking of measuring 3-groove barrels. I have to slug an older 50-70 Govt Trapdoor this week to get a proper bore dimensions for a sizing die purchase later on. Anyone know the angle for the v-block for that odd-groove barrel?

120 degree

[bruce drake]

Thanks! Seems almost too simple after you consider it! Thanks for that knowledge!

[gnoahhh]

All good info, and food for the inquiring mind! But I submit, if I may, that it is a useless exercise to measure groove diameters. I quit that long ago and simply rely on throat dimensions when fitting a cast bullet to a gun. I size to .0005" under throat diameter and ignore whatever the grooves are. Life is easier and good, accuracy likewise as a result. Frank Marshall disabused me of measuring groove diameters 30 years ago, and as usual he was right.

[John 242]

I quit that long ago and simply rely on throat dimensions when fitting a cast bullet to a gun. I size to .0005" under throat diameter and ignore whatever the grooves are.

Not arguing, but could you explain the theory behind this, because it seems to run counter to most of the bullet fit theories I'm familiar with.

If the groove diameter in the last, i dunno... say 6-inches, from the muzzle in a nominal .308 barrel, opens up to .311, this doesn't affect accuracy or doesn't lead to leading with cast?

If a revolver throat is .352 and the groove diameter is .357, this isn't going to affect accuracy or cause leading?

Again, not trying to start an argument. I love learning and this is something I haven't heard before. I know of a few rifle builders that will slug a bore and either reject it if it isn't consistent along it's length or will at least make the tight end the muzzle end.

[gnoahhh]

If a revolver's throats were that grossly undersize, I would have them reamed. If the last 6" of a rifle barrel were as egregious as your example, it would indeed be conducive to leading there, not to mention I would be surprised if it were accurate. In an instance like that I would be making tracks toward a re-barreling.

A rifle's throat will be at least groove diameter, and in most instances a skinch bigger. Everybody has their own idea as to what constitutes a perfect throat. There are few bad ideas, really, as long as the bullet is matched to it. What you must remember is the throat (and leade) is what starts the bullet straight into the rifling (assuming straight seating in the case to begin with)- more important with cast bullets than with jacketed, and it's pretty darn important with jacketed too. If the bullet fits the throat (which also in turn means the bullet will also be at least groove diameter, for a good seal in the bore), and better yet with a nose that conforms to the leade of the rifling, it can't help but start down the bore straight- or reasonably so. There are a bunch of other factors to take into consideration such as lead hardness/alloy, velocity, chamber pressure/burning rate of the powder charge, bullet shape, condition of throat/rifling, etc. etc. I merely maintain that if the bullet is correctly fitted to the throat then you're well on your way toward accurate shooting.

My idea of a perfect .30 throat for cast bullet shooting, given a .308 groove diameter: a .310 throat of around 1/4" followed by a 1½º included angle leade. (If the magazine will tolerate a loaded round of the length this would likely dictate. If not, adjust back toward a "standard" throat- if magazine feeding is needed.) I have a throating reamer of those exact dimensions, and have treated a bunch of my .30's with it. I also used the reamer to make a sizing die to taper the noses of bore-riding designs to perfectly match the leade taper. Kind of surprisingly, jacketed ammo seems to shoot a bit better through such an arrangement too, but don't take that as gospel.

I hope I clarified where I'm coming from regarding my statement about knowing throat diameter versus knowing groove diameter.

[jsn]

Pee Dee wires in one of the lands on the slug. No fixtures, excessive time and cost, just center it and don't push the wire into the soft slug.