Catshooter,
To answer your question you will lose the range of your gun that you are use to. Your Pardner has the Modified or Full fixed chokes. (It’s marked of the top of the barrel) These chokes constrict/squeeze the shot column to a smaller diameter to hold the pattern tighter for longer rangers. This is why the shotgun and ammo manufactures recommend the use of a full choke for small game to be harvested at 35-45 yards and 25-35 yards for the modified choke. Your range for a cylinder bore IMHO would be at the maximum 20 yards with standard store bought field/game loads. You may be able to increase this to 27-30 yards with a premium 3” copper plated buffer load, but you cost will go up for these shotgun shells.
What I have done for my Shorty is to purchase the Young Model Pardner Model# SB1-25C (Now called the Compact) with the screw in chokes. I added a Walnut TDC (Topper Deluxe Classic) rear stock and a Walnut TJC (Topper Junior Classic) forearm. The Modified choke comes with the gun and the threads for this style are any that were made for the 20 gauge Winchester/Browning or Mossberg shotguns, so there are a lot of the different size chokes tubes out there for cheap.
I have included a good read about chokes from Tru-lock Chokes and the link to the H&R 1871 site with the info for the Compact.
A link about chokes:
http://www.trulockchokes.com/cgi-bin/eShop/index.cgi?page=basic_choke_info.htm&cart_id=2358679.67612Link to the Screw in Choke Compact Pardner:
http://www.hr1871.com/Firearms/Shotguns/pardnerScrewIn.aspI mainly use my Lil’ Shorty to rabbit hunt behind Beagles. My shots are close. 5-10 yards most of the time and 20 yards is a long one for me. I have a cylinder choke and an Improved Cylinder (IC) choke I use. I reload to the standard 7/8 oz. Field/Game load, but I use magnum shot which is a harder shot. Good Luck and I hope I have helped!
Froghunter