You need to know the dimensions of your bore. You get this by slugging your bore and measuring the slug. Then you buy or make bullets that are the same size as your grooves, or .001" larger. You might get away with bullets slightly smaller than the optimum, say .378" for a .379" groove diameter.
If your bore is the same as mine, you'll need .379 or .380" bullets.
If you load these you may have trouble chambering your reloads.
Do a search for 38-55 and be prepared to do a lot of reading.