It absolutely must be kept in the forefront of your mind ........ Cap Guns can be Malevolent. They are Cantankerous. Some of them simply DO NOT like us and/or harbor Poltergeist. They are close relations of MURPHY!!
The OP is/was having problems with a 2d Gen Colt. That makes it a Uberti. It also sounds as if the nipples ave already been replaced by someone who doesn't know what he, or she, (Dragoon ?? Nah) was about. Unfortunately, there are only two choices. 1. Buy one can of every size nipples and keep the ones that fit. Give the others away. 2. Change out the offending nipples, buy one can of every size, and keep the ones that fit. Give the others away.
Because I run light Main Springs, I don't like TRESO. I like SlixShot. Work in everything I have. Everything I have is a Pietta. It is my understanding, at this time, OL #4 and company now offer Slix for the Dragoon/Walker. I'd strongly suggest trying those. With the Slix nipples in my Piettas, I run Remington # 10 caps. After shoving them on what a snail capper, I go back around and "seat" them with a push stick. This has resulted in 100% reliability. Because I don't run full propellant charges, the caps stay on the nipples, but since I shoot Gunfighter in completion and don't have a free hand, I run Cap Rakes as added insurance against sucking caps.
The Foregoing is a result of my experience attempting to make Cap Guns run. Remember, a Malevolent gun just may give you fits!!
There have been times I just wanted to melt the sucker down into a little PAPAER WEIGHT!!
COFFINMAKER