Seems that some guns like the harder to find .380 balls and others like .375" balls. I have a bunch of .375" on hand and wonder why the big difference in guns of the same (.36) caliber????
You bought the .375 why? If I understand right most Pieeta like them and Uberti .36 cal like the .380 but I like to see how much of a lead ring is produced when I seat the ball, if there isn't some resistance when the ball is seated then with full/heavy charge the other balls may walk forward. I have both sizes, my little Uberti 1862 took the .380's as does my .36 cal ASM.
My uberti 62 police has to have 380. With 375 there was hardly any lead shaving at all. Matter of fact the balls even fell out of the cylinder as I was loading it. Ordered some 380 problem solved!! If 375 is all you can get they'll do but from now on all ill use is 380.
As long as there is a lead ring shaved off when you seat the ball then you'll have the right sized ball.
Just go with the .380 it'll shave off whats needed for a tighter fit. May have to use a little more force. I had a bunch of .457 balls left over from the ROA (that I sold like a doughnut head) and I regularly shove them in the '58 sheriff. Shaves a nice ring of lead but they go in tight and seal up good. My philosophy Buy .380's for 36's .454's for 44's. That way you only have to worry about one size of ball for each caliber. And if a loading stand is used no sweat. Just my .02
I recently met up with a fellow that loads his ROA with .451 balls and 40 grains of powder and he said that he hasn't run into any problems. I was surprised because that's not what is recommended, but in actuality it's been working in his gun. He said that he also uses over powder wads and some kind of a seal over the balls. Maybe those also help to prevent ball creep.
You don't say what "gun" you want to use them in. Others have assumed you are talking about C&B revolvers.
If using soft lead either will be just fine as long as you get a little shaving of lead on seating into the chamber.
Using a .451 ball in a ROA might "work" but it is not the best choice. Regardless of wads, etc. you can still get gas blow by in the cylinder and lower velocity. It also might not swage properly in the forcing cone and degreade accuracy. It 'might' be OK and might "work" but always best to with proven methods.
I use 375's in my Pietta with complete satisfaction, good ring of lead and all. 380's are probably a better compromise if you have more than one brand of gun. If you can't find 'em, cast 'em yourself, that's what I do.
Wash your hands good before you eat or get your fingers near your mouth and you'll be ok. You have to get lead a lot hotter than a lead pot will get to have fumes.
I haven't been casting long, but I take necessary precautions. I wear all the PPE , respirator or mask, long sleeves, pants, steel toe boots, and do it outside. And like hawg said wash my hands before I have a nip of skoal.