Start at 16 and work up to 18.5 with 4759 and 2 1/2 Rem primers.

Frank

19 grains of 4198 is good   art

I started at 16 grains IMR 4227 under a 300 grain Hoch tapered bullet. Raised the charge until it started burning clean.  Best charge weight is 19 grains.  Same charge for fixed and breech seated. Rifle is a Ballard. 

             Joe. 

Around 17.1 grains of 300MP and LRMag. primers will get you to sub minute of angle breach seated. My barrel was an 18 twist Douglas.

For years my standard 38/55 load was with 4759.  Club matches I load 6 60 round MTM boxes at a time, in order to keep consistent sight settings match to match, short sight in period no sighter target like in Schuetzen. Has to be consistent month to month.

  4759 I had to weigh shifted to 4227 which throws from a measure fine no need to weigh and gives performance just as good as my old 4759 load

I see guys at our matches using trail boss all the time. Never saw anyone win a match with trail boss.  Why they would use Cowboy action powder in a 200 yard rifle match puzzles me.  4227 or 4759 are the standard Schuetzen 200 yard powders.

I would use a heavier bullet than that 250. 250's feed in lever actions you can use a longer bullet in single shots & get better wind performance. If you want  Sub sonic light bullets are good for gallery matches and fast pistol powders like Unique are going to work best, outdoors best bump things up.

Boats

Rich in my very light weight Stevens 44 1/2 38-55 I use the 250gr bullets ahead of 21grs 4198, either brand of powder seems to work very well, this load gives hunting accuracy at 100yds (less than 2" groups open sights, when I do my part) and reasonable recoil.
Mike