Marc - is that a #4 with the receiver sight base milled off and an open sight mounted ahead of the action?

If wanting to use a paper patched slug, I would 'slug' the bore and measure it carefully to ascertain the actual groove diameter, then get a mould that cast a lead slug approximately .300" in diameter, then paper patch it to fit the grooves - or .001" to .002" larger than the groove diameter.

With a hard alloy paper patched bullet, there would be no velocity limit as-with grooved lubricated bullets.

You would be able to load them to 2,500fps if you wanted to, depending on the weight.