Pietta chambers are tighter than Ubertis. The .375 balls work just fine, cut a good ring, and seal up. The .380s are the right balls for Ubertis and 2nd Gens. I've had .375s roll out of the chamber on a couple of my Ubertis. The .380s also work just fine in a Pietta, it just requires a little more pressure on the loading lever to seat them.