Safe load for Vetterli-Vitali 1870/87/15 in 6.5 carcano

[Lebel]

I bought a Vetterli-Vitali converted to 6.5, (conversion date 1921), because it has an excellent bore, cycles flawlessly from clips and groups with in a Toonie @ 100 yards. This was all proven to me by the original owner using 6.5 loaded to Carcano rifle specs. I am well aware that these loads are far too hot for its original black powder action to hold safely.
Black powder is out of the question because I’d like to take her out hunting and fowling would make it impractical. I found a smokeless load in Cartridges of the World for its original 10.4mm load using a 250gr. lead bullet backed by 27gr. of IMR 4198 but it didn’t give cup/psi units. I surfed the web with contradictory and frustrating info, with one guy confusing cup/psi with energy, (or perhaps I am), quoting the same above loads and referring to the energy as the chamber pressure.
If I were to use the above load of 27gr. IMR 4198 in 6.5 with a lighter grained lead bullet and a smaller case would I still be in the safe pressure range or would it increase and if so how. I’m not a big re-loader but my interest and use of odd ball milsurps has kind of forced me to.

 

[Andy]

I have one and have shot it quite a bit. You need to choose a cast bullet load that does not exceed 25K psi. If the gun is in good condition that will be perfectly safe. The 10.4mm round for which the action was designed has a much larger casehead, so backthrust is actually reduced with the smaller 6.5X52. Your logic about transferring loads has some merit, but falls short in some important ways: the much smaller bore and much smaller case capacity of the 6.5X52 raise pressure significantly, and the much lighter bullet does not quite compensate for that as you suggested.

I assume your bore is 6.5mm and will slug out at 0.264-0.266". Here's a few loads to try for a 140gr bullet:

Unique - 8.0-10.0 grs
2400 - 14.0-18.0 grs
H4227 - 17.0-20.0 grs
H4198 - 18.0-22.0 grs
H4895 - 22.0-26.0 grs

Don't exceed the Unique loads, and for the others, keep the MV at 1500 fps or less - it will seem like a "pop-gun" with these loads, as it's a heavy gun and these loads are more than 500 fps slower than "standard". My favourite powder with this gun is 22.0 grs of H4198, but the 10.0 grs Unique load is a good one too.

 

[donor]

I'm using 145 Gr NOE mold,alu GC,11.3 Gr of 2400,Win primer and all this goes into PPU case neck sized with Lee 6.5 Swedish die.
Only about 2/3 of the neck gets sized but it seems to be enough to hold boolit in case all the way from mag to chamber.

One thing-I'm using above load in 1918 made M91 with gain twist barrel and to get any decent results (very decent 2-3 inches on a good day ) I had to use very soft lead.