I've seen this problem/topic come up numerous times in the short time I've been here. The solution is quite simple.
The Italian reproductions (more often than not) come with nipples that are metric (NOT the thread, but rather the cap end). No matter what you do you won't find a cap that fits them correctly. The only option is to change out the nipples on your cylinder, I Like a small flash hole as it acts like a "funnel" and keeps the main charge of powder headed out the intended end of the barrel. It also reduces "blow-back" causing the spent cap from dislodging and keeping you from advancing to the next cylinder.
Cap sizes do vary a bit from one manufacturer to another. After changing the nipples, find the one that your gun likes. DO NOT force a cap on or "pinch" them because they doesn't fit correctly! Many times I just wanted to shoot a new pistol but things weren't right and opted for common sense, safety... waited... and did it right. If I buy a tin of caps and they don't work for my guns, I THROW THEM AWAY. I don't even want them! I don't want any error. There are too many other things that can go wrong as it is!
If you need to, take a nipple out of your cylinder, take it with you and before you buy a tin of caps, open the tin and make sure they fit! Don't do it the other way and try buying nipples that fit your caps...
When the nipple and cap size is correct, chain-fires are eliminated as this is the main cause of a chain-fire. 99 times out of 100 this happens at the cap/nipple end of the cylinder, NOT because you didn't use the correct grease, wad, or other.
Many times we complicate the simple! I want my pistols to fire perfectly every single time and not wonder if the stars will all line up and I will walk away with both hands and eyes.
My only hope is this helps and saves you aggravation, expense and possible injury.
Shoot 'em up!