Well, I'll go against popular wisdom on this one. A smooth .50 will throw and excellent shot pattern for squirrels. The small bores throw a "string" of shot as apposed to a "ball" of shot. I had a 28 bore that shot an incredible tight pattern, I used to win pattern counts with 1 once of shot all the time with it. These small bores make excellent shot guns for things sitting still. !@*%&@ hard to hit skeet with.
Now, you can shoot RB through a jug choke, but not a modern choke. You will not need a choke for a .50 or .54, they shoot just fine as a cylinder bore.
You're going to have to have a custom barrel man make the barrel you have in mind, they are not a standard run type of thing. I'd also go long, really helps the pattern of small bores. I'd go at least 48". With the right barrel profile you can go 5 1/2 to 6 lbs for a finished gun with a 48" or more barrel. Go with something like a 15/16" to an 1" breech, rapid taper for the first 9" or so, and a barrel wall at the muzzle of no more than 1/16" , possibly a little less. You're barrel should weigh around 3 to 3 1/4 lbs or so.