My barrel Metford rifling and oversized. It shoots a consistent 8” group with Hornady 150’s. My cast it will do 2 to 3 “ groups. I was going to try and rebarrell it but figured I would try cast. Forearm has been cut. I have since found an original nose piece so a future project is to redo the forearm.
I like your project idea better.
After getting some information from Ganderite, IIRC, I made up some loads with exposed lead bullets over a very compressed black powder charge, for my Martini Metford.
I was quite surprised by how much recoil was generated by this charge, under 220 grain, round nose, cupro nickel bullets, over CCI maganum primers.
I wasn't getting consistent ignition with standard primers.
Upon doing some due diligence, the black powder charge in the original cartridges was compressed to the point of being a porous solid pellet. I also found some very early, pre 1900 catridges that were falling apart and saved the bullets as well as weighed the black powder charges over 30 cartridges.
Even with 4FA, I couldn't get that much powder into the cases.
As mentioned, recoil was very noticeable, even with a reduced charge, when compared to the original.
Velocities of early black powder loaded 303 British cartridge were supposedly 1850fps, with a 215 grain, round nose bullet of .313 dameter. I'm getting the .313 diameter from the records of the bullets pulled from the early BP cartridges. My scale indicated they were closer to 218 grains.
The best velocities I could get with modern 4FA, over CCI magnum primers in surplus Winchester made 1944 date cases was just aroung 1750fps.
One thing I did find was the rifle actually shot better with the BP loads after the first fouling shot.
This came as a surprise to me, because it's not uncommon with patched or cast lead bullets out of other BP firearms.
It's also not unusual with modern rifles shooting jacketed bullets.
For some obscure reason, I had it in my head that the black powder fouling in the Martini Metford would cause accuracy to deteriorate very quickly. Not so. It actually improved after the first couple of shots and the jacket fouling was minimal at best.
I don't shoot BP out of that Martini Enfield anymore. It's messy and very hard on the cases. Still, my personal experiment proved to me that the conversion to ''small bore'' firearms/ammo, with heavy for caliber, heavy/soft jackets was a very reasonable alternative to the older (no diss) rifle/BP cartridge systems.