Looking for .316

[ulav8r]

Oh, you meant groove. If a .312 bullet falls through and a .314 can be pushed through easily, then your bore diameter is about .313 +/- and the groove diameter should be about .321. Look for a bullet diameter of .323, then size down to .001 under measured throat diameter.

[Wilderness]

The barrel is octagon and is stamped "Marlin Firearms Co New Haven USA" + patent dates and in another spot "Special Smokeless Steel" so it is definitely a Marlin barrel.

There's your answer - "Special Smokeless Steel" will be .32-40. The other version was "for Black Powder Only". This distinction was made for .32-40 and .38-55 barrels for the M1893. Never heard of it on M1894 (.32-20 etc).

My Marlin .32-40 (Black Powder barrel, about 1916) will just engage a .314" bullet, and shoots well with .321" and 323".

Edit: In the days of paper catalogs, Numrich were specialling off Marlin 1893 barrels in .32-40 and .38-55. Evidently they sold the "Special Smokeless Steel" ones first, then the black powder ones. The one I got was black powder. I'd be punting that your barrel is one of the Numrich specials. Turning it into a martini barrel would require cutting off all the threaded portion (square threads) and rethreading (V threads), then rechambering. The original caliber marking on the top of the barrel will have disappeared under the new threads.

[Larry Gibson]

Your barrel apparently has an odd number of lands and grooves. Cut a strip from a soda can 1/2" wide by 2 - 3" long. Measure the thickness, should be close to .004(?). Wrap it around the slug you drove through the bore and pinch it tight. Measure the outside diameter of that and subtract twice the thickness of the pop can strip. That will give a more accurate measurement.

Also, are you saying you have a Marlin barrel that was originally a 32-40 but has been set back, installed on the Martini action and then chambered in 32-20?