It depends on the barrel and how the lead (the taper at the front of the chamber where the bullet enters the rifling) was cut.  Does it have an original barrel marked "Winchester Repeating Arms?"

If the barrel has no markings look under the forearm to see if another barrel maker made the barrel for this rifle (Pope, Geo Schoyen, etc.).

Yes you can handload fixed ammo for a .38 55.  Have a reputable gunsmith check the rifle over to make sure everything is good condition before you attempt to fire this rifle.  A 255 grain cast lead bullet (dia .376) and 16 grains of IMR-4227 would be a good starting load for this rifle with fixed ammo.  But then only after a gunsmith has inspected it and pronounced it safe to fire.

I would strongly recommend you get a copy of Randy Wright's book, "Loading and Shooting Traditional Schuetzen Rifles."

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