Handi Rifles are the easiest firearm to check cast bullet fit. First be aware that bullet fit and alloy selection are what controls leading. Light loads in .357 need no harder lead than the PSI of your load equaling the psi ultimate strength of your bullets. If you can't figure that out, you need to learn how to for cast bullet shooting in any caliber. To check the fit in your Handi:
Make a dummy load with the front driving band of the bullet exposed. Apply machinist indicator ink or magic marker to the exposed band. Chamber the dummy round. You should feel the bullet sliding in on chambering and the ink should be disturbed from sliding. That is the perfect fit. Any less fit increases leading and decreases accuracy potential in cast bullets. Don't ignore this basic principle of cast bullet shooting, it isn't a myth. Following the principle makes cast bullets shoot cleaner than jacketed bullets unless your barrel really stinks and is rough. Handi Rifle barrels are usually pretty good but polishing also helps. If your bore is not shiny it will lead less and clean easier with a shiny barrel. Try my Bore Polishing Method from the Accuracy section of the forum:
It is the easiest on a Handi and works great for cast bullet shooting. Just make no substitutions and take no shortcuts from the method. Read all the comments on the method and you can easily proceed with confidence.