Carcano Rifles

[Bob Prudhomme]

In the late 1800's, European gunmakers began the transition from large bore, low velocity rifles shooting un-jacketed lead bullets with black powder to small bore, high velocity rifles that used smokeless powder. As the un-jacketed lead bullets quickly fouled the riflings of the smaller bores, the copper alloy bullet jacket was introduced to avoid this.

As a hold over from the black powder days, and even further back to the muzzle loaded Minie ball (see diagram),



the groove diameter of rifles was made slightly larger than the bullet itself. As the base of the Minie ball was conical, pressure from burning gases caused the diameter of the bullet to "slug up" to the diameter of the rifling grooves, making a tight fit and guaranteeing greater accuracy.

The original European bullet jackets in the 1890's and 1900's were much lighter than the later bullet jackets, making them pliable enough to allow them to swell under pressure to fill up the rifling grooves of early 20th Century rifles. As a consequence, CIP tolerances for the Mannlicher-Schoenauer groove diameters were, in the early 1900's, a few thousandths of an inch greater than the bullet diameters. I was unaware these tolerances were still on the books, as today's thicker jacketed (and harder) are definitely not likely to swell up to a larger diameter, and no manufacturer is making rifles with over sized rifling grooves.

Just because the CIP tolerances allow a manufacturer to make a groove diameter a great deal larger than the bullet diameter, does not mean they will actually do this. Mr. Emary found the 20 Carcanos he slugged to all be within .2680"and .2690"; just right for a bullet .268" in diameter (.2677" actually). Many gunsmiths have slugged Carcano barrels and found almost identical results as Mr. Emary. This fact alone is amazing, considering the conditions many of these rifles would have been made under during WWII.

The only exception Mr. Emary found was an M91/24 carbine which slugged out at .2710". As the M91/24 was the unfortunate carbine made by cutting down the 31 inch barrel of a worn out M91 long rifle to 17 inches, the large groove diameter is understandable, as the barrel has obviously been heavily worn by the tremendous number of bullets fired through it, as well as the corrosive nature of the early types of smokeless powder employed.

P.S. If you read some of my early threads closely enough, you will find that I pointed out, on several occasions, that the Carcano was not the only 6.5mm calibre rifle to shoot a uniquely oversized bullet. I have pointed out, on several occasions, that a bullet .266" in diameter is often used in the Mannlicher-Schoenauer instead of a standard .264" bullet. I stopped mentioning this fact simply because people were having enough trouble understanding what I was saying about the Carcano, without complicating the issue by adding more numbers. However, the Carcano, with its required .268" bullet, and the M-S, are the only exceptions to the 6.5mm rifles shooting a .264" bullet.