According to the Text Book of Rifles 1909, the Carcano ammunition used a .266" FLATbase, roundnosed bullet of 163 grains ahead of 30.04 grains of their own special powder. This powder was made very like Nobel's Balistite. That made it a true double-base powder in flake form, with the flakes cut from flat sheets of nitrocellulose which had had a coating of nitroglycerine rolled-in during manufacture by using considerable pressure on steam-heated rollers, then the sheets were cut into flakes and tumbled with graphite.
I was experimenting at one time because one of the first rifles I ever bought was a cute little critter which I had given-up on many years previously. A Model 38 6.5mm Cavalry Carbine, the thing was sold in Canada only with Western US military contract ammunition which was loaded with a Ball powder. The little thing had murderous recoil (not helped by the very short butt), made a 3-foot fireball when fired and produced a deafening report. I wanted something to tame the critter down.
My reasoning was that if the Italians wanted to use such a very FAST powder as they did, perhaps they had a reason.
Most modern powders are the IMR type originally designed by Hudson Maxim more than a century ago. They are designed to burn PROGRESSIVELY, maintaining a satisfactory pressure as far down the bore as practical. Problem here is that a powder designed to work well in a 32-inch tube (Model 91 original rifle) WILL make a horrific blast when used in a 17-inch Carbine barrel. The Italian powder, on the other hand, builds its pressure initially close to the chamber and the pressure lowers noticeably as the bullet passes down the bore.
Thing is, the Italian powder is the perfect recipe for ammunition which has to go into a wide variety of bore diameters and still perform.
I considered all of this for some time (not wanting to blow anything up while I was holding onto it!). Another consideration was the fact that my Hornady .268" slugs had been on back-order at the local gun shop for 4 years already..... and I had a bag of Remington 140-grain PSP bullets, although they were .264" diameter. But by using a VERY fast powder, I should be able to kick the .264 slugs on their bases HARD enough to promote their obturation, sealing the bore completely.
Previously, I had attempted to produce ammunition for the Carcano using surplus Norwegian 6.5mm slugs and a .30-'06-type of powder. I got horrible muzzle blasts and slugs hitting the target sideways. Definitely NOT a solution. A faster powder was indicated, very plainly.
I loaded up a small batch using 28 grains of IMR-4198, my nice FLATbase Remington bulk-pack 140-grain slugs and headed for the range.
The little carbine turned from an absolute monster...... into a pussycat.... and started hitting what I was shooting at. Having along a near-new Model 41, I tried a clip in it..... and marvelled at groups of almost exactly 1 inch. These were followed by 2-shot groups of an inch and a quarter from a 1917 Infantry Rifle which had come to me as a refugee from the Pipestone Dump back when the Liberal Registry was in force.
In all rifles tried, this load shot well, produced very little recoil and mild reports. It is actually a relatively-mild loading. It is NOT found in ANY reloading manual but has become my standard Carcano loading. My rifles are quite happy with it.
What happens is that the very quick 4198 powder ignites very fast and kicks the relatively-soft Remington slug on the base HARD ENOUGH to cause it to upset enough to take the rifling properly in a variety of rifles. I have detected NO signs of excessive pressure with this load.
Hope this is some help. Good luck!
