The Side Mounted Scope on the 6.5 mm Carcano

[Bob Prudhomme]

I hope by now I have you wondering how one bullet fired from a 6.5mm Carcano can have deeper rifling grooves left in it than a bullet fired from another 6.5mm Carcano. Actually, we could use the same rifle and get different depths of grooves.

One thing that confuses many novices is that the calibre of a rifle (ie. 6.5 mm) is not necessarily the diameter of that rifle's bullet. In the case of the 6.5mm Carcano, the actual bullet diameter is about 6.8 mm, or .268". Just to confuse things, the .308 calibre rifle actually shoots a bullet .308" in diameter, while its calibre or "bore" is .30 calibre (.300").

While you are getting a headache working all that over, let's take a look at the inside of a rifle barrel and try to explain the rifling grooves; and how they determine things like calibre (bore) diameter, groove diameter and bullet diameter.







When a rifle barrel is made, a piece of roundstock is drilled or "bored" from one end to the other, creating a hollow tube. The diameter of the hole bored becomes the "bore diameter" or calibre of the rifle; ie. 6.5 mm in the Carcano or "A" in the last diagram. In the next step, a special cutting tool is run through the barrel, twisting as it goes, and cutting spiral "grooves" in the inside of the barrel. The depth they are cut too becomes the "groove diameter" of the rifle; ie. 6.8 mm in the Carcano or "B" in the last diagram. The groove diameter is also the diameter of the bullet; this is very important to remember.

When a bullet travels down a barrel, the raised portion of the riflings, known as "lands" ("D") actually cut into the sides of the bullet, leaving distinctive grooves in a bullet. The bottom diagram (left side) is a fairly close depiction of the four lands and four grooves in a 6.5 Carcano barrel, as the Carcano did not have very wide lands. The diagram above it shows a barrel with much wider lands and narrower grooves.

The purpose of the riflings is to impart a spin to the bullet and stabilize it gyroscopically. Rifles are often referred to as having "fast twist" (1:8 or faster) or "slow twist" (1:10 or slower). The numbers mean the number of complete turns a bullet will make in how many inches. For instance, a 1:8 pitch means the bullet will make one complete spin in the space of 8 inches; 1:10 means one complete turn in 10 inches, etc. Heavier bullets tend to require a faster pitch while lighter bullets require a slower pitch.

The early Carcanos (pre-1938) had a type of rifling known as "gain" or "progressive" twist rifling. This was a difficult thing to machine in a rifle barrel but, it is still felt by some that it extended barrel life. There was another reason it was done but, we will look at that later. This type of rifling began with a very slow 1:19 twist at the breech, where the bullet begins its journey, and progressively got tighter as it travelled through the barrel; exiting at the muzzle where the twist ended up at a fast 1:8 twist.

The 6.5 mm is a popular European calibre, and has been chambered for many hunting as well as military rifles. What makes the Carcano unique is that while it shares the 6.5 mm calibre with these other rifles, it and it alone shoots a bullet greater in diameter than the other 6.5 mm rifles. The designers of the Carcano chose to give it extra deep rifling grooves; requiring a wider bullet. All other 6.5 mm calibre rifles shoot a bullet that is 6.7 mm (.264") in diameter while the Carcano shoots a bullet 6.8 mm (.268") in diameter. It is believed the Carcano lands were made narrow because, with their added height, they were already displacing a lot of the bullet jacket material. It is also believed, because the lands were taking such a deep bite, that the progressive twist rifling made the initial forward movement of the bullet easier.

While rifling grooves that are only .002" deeper, on each side of the barrel, than other 6.5mm rifles may not seem capable of having much effect on accuracy, it is a well established fact that shooting a bullet .264" in diameter from a 6.5 Carcano will redefine the expression "can't hit the broad side of a barn".

I'm getting a little tired here. Think over what I have written, and I will continue tomorrow.