Bore size on a 50-70

[Old Iron Sights]

Have a model 68 Springfield with a very nice bore. I mic'd the muzzle and bore slugged it twice and am pretty convinced that the bore is 508. My lee mold drops at 515 like it's supposed to and these chamber fine. I'm concerned about shooting such an oversized boolit. I know many here recommend fitting to the chamber and not the muzzle but this is a little much. I have a 510 sizerso I can run them down first but then I'll have to find a way to work my brass down smaller. This one had me a little perplexed.

Just for giggles here's a gratuitous bore shot.
100393

[missionary5155]

Greetings
Have a couple of those also. You do have a nice looking bore there.
Do a throat slug. One of mine will not shoot the best unless it gets fed a .517 40-1 slug 450 grainers. It will chamber a .518 which will shoot a bit better. With 2F though the follow on rounds are just to snug. The others will shoot .515 fine cast with 40-1. I am happy to get 3 inch groups but 3.5 - 4 inches at 100 yards are about my average with my caliber .50 muskets. But they are alot of fun at 100 yards wacking bowling pins with utter violence.
I bought one of the HB caliber 50 Group buys that was here and hope that will be my "works in all" caliber .50 muskets.
Mike in Peru

[13Echo]

The bore might be 0.508" but the groove diameter is typically about 0.515" or larger. Measuring the actual diameter of a three groove rifle isn't that easy. At any rate my 1868 does best with a 0.515" bullet and won't keep on the target with one cast at 0.511". The Lee bullet cast at 0.515" out of 30:1 alloy is quite accurate in my rifle. Try it as cast.

Jerry Liles

[Old Iron Sights]

Yeah, it was tricky getting a measurement across the grooves but I'm pretty sure I was touching both grooves and at the widest I could only get 508. The lands are shallower yet. I may try some cerrosafe. It can be tricky to get it out of some older bores.

[13Echo]

The lands and grooves are of a width that precludes a true measurement of groove diameter or bore diameter for that matter with just calipers or micrometer. You will get a diameter that includes the depth of one groove only It takes a special V shaped block and a slug from the barrel to get an accurate measurement.

Jerry Liles

[johnson1942]

i would love a gun like that. i would paperpatch the bullet and use blackhorn 209 powder in it, the right bullet size patched and 209 powder could make that a minute of angle gun.

[bigted]

how I measure the diameters of odd number lands/grooves is with a little math;

first I shove three slugs thru the bore to have an aggregate.
then I measure the depth of the rifling on each land on each slug.
next I measure the diameter from the top of a groove to the bottom of the corresponding bore on the exact opposite side of the slug.
do this with each groove/bore on every slug for the arrival of an average.
now I take the depth of the grooves and add or subtract to arrive at the base line for a groove and same for the bore diameters.

add to your sum the depth of the rifling for the groove diameter ... subtract the sum of the rifling depth for the bore diameter.

[Old Iron Sights]

That actually makes sense. I am really hoping I'm wrong on the bore size which is looking possible.

[mhb]

and measured a number of original .50-70 rifles, Springfield, Springfield/Sharps and Rolling Block (Remington and Springfield). As a barrel maker, I have the means of accurately measuring the 3-grooved barrels - and the groove diameter is the significant dimension, since you need a bullet of at least that diameter if you use smokeless (and I don't believe in bumping-up undersized bullets with BP).
The majority of these barrels have bore diameters of ca. .500" - .502" - this dimension was easier to hold close to the nominal because it is achieved by reaming, and properly shaped and sharpened reamers do not cut much, if any, oversize.
The groove diameters, however, are a different story, most running considerably larger than the nominal .515" - some as large as .525", with an average around .517" or a bit larger.
I have found 1 (one) .50-70 (a 2nd Allin conversion) with a good bore and groove diameter of .5125" - all the others in my experience were over the nominal groove diameter, and required oversized bullets to work properly. This can become a problem, because the bullet of full groove diameter, seated in the case, may not chamber in a given rifle - not to mention the difficulties of finding a mold/alloy combination which will produce a bullet of the proper diameter for any given rifle.
All-in-all, the original rifles in .50-70, while interesting and fun, can be a PITA to make proper ammunition for.
And, FWIW, the original .45-70 rifles are no better or more consistent in measured groove diameters.
Sigh.

mhb - Mike

[Old Iron Sights]

Worked up a few more rounds. 65 gr 2f. Compressed slightly under an oxy-oke wad. Shot about an 8" group at 100 yds. I think I need to just get better at my technique and work the powder level up slowly.