Hi Steve- I'd start with 3 drams myself. If European, probably has 2 5/8" chambers.

Since early rifled guns were called in calibers by the bore size rather than the groove diameter, it would probably be called a 15 bore as 15 is .677".

I'd not stress the stock with slugs, and think the 54" twist will shoot RB's just fine, especially with lighter charges. Regulation will, of course tell the tail on projectiles.

The chambers might be a perfect fit for a 16 bore brass case, available from Buffalo Arms for around $21.00 for 25. Cheap! They are 2.4" in length and will actually hold just about 5 drams and a round ball with no wads. Wads are used to take up the extra space for lighter charges, of course.

I shoot 3 drams in my 15 bore straight rifled, right hand barrel on my Husky 16 with a 14 1/2 bore ball. This ball (.684") engraves about .003" per side in the .703" groove diameter and shoots quite well but better accuracy is obtained using a cloth patched 16 bore ball loaded above the 3 dram load and wads. Not only is it more accurate than a bare ball, but the lubed (neetsfoot oil) patch keeps fouling soft and shoots quite clean, with no fouling buildup shot to shot.

For your gun, I suggest a 16 bore ball, .662", Lyman mould with about a 10oz denim (.021") patch and 3 drams, probably with a, 1/8" and 1/2" wad. Wads are available from trackofthewolf.com. 14 bore wads fit perfectly in the 16 bore brass cases and help prevent blowby. They also have 15 bore wads, etc. To load, pour charge in the case and appropriate wads, then lay the patch over the case mouth and push the ball down into the case with a short starter or dowel. Then, simply crimp in a 16 bore crimp starter, or seal with a liquid sealer, like water-glass.

I use the crimp starter station in the 16 bore press to 'roll' crimp annealed case necks. The brass 16 bore cases fit chambers that also fit 16 bore plastic, but since they're thin, they take larger wads and allow larger projectiles than do the plastics or paper cases.

Of course, this is all based on the measurement you gave as the bore size. If this was the groove diameter instead, then it is a 16 bore, not a 15 and will require 17 bore balls, etc. A chamber cast and slugging the bore are necessary to get proper sizing.

Good luck.