Husqvarna Rolling Block
The husqvarna rolling block shotguns are one of the best rolling block actions to build a BPCR on. They are not converted military, but are a swedish modified design originally manufactured for the purpose.
They will feed any cartridge possible for use in a rolling block. It is a lot of work to modify a #1 or #5 to feed a .50-110. (I know, I do this conversion). The Husky will feed a .50-110 with no modification.
I typically have about 20 #1 or #5 actions in my shop to choose from for projects. If this tells you anything, two of my personal use rifles are Husky's: my primary use 45-90 and my long range .50-110. I prefer this action to the #5, which is commonly considered the best of the RB actions.
If the action pins have an "H" stamped on the end of them, you have got the best of the Husqvarna actions. This denotes a late model, smokeless rolling block, intended for and proofed for modern nitro shells. The H stands for Hardened, not Husqvarna. The steel in the receiver is correspondingly hardened.
Only potential problem with the shotgun action is the firing pin is large. Since I only shoot blackpowder in mine, I don't worry about it. But, if you will shoot higher pressure cartridges, it is a good idea to weld up the firing pin hole and redrill it for a smaller diameter pin.