Unfortunately, that cannot be done.
H&R stopped supporting the single-shots about 6 years ago, and went to only importing Turkish shotguns.
Then they died a horrible death in the Remington bankruptcy. The brand is now owned by the umbrella company for Palmetto State Armory, and cannot and will not repair or service rifles built by Remington or H&R 1871.
If yours left the factory as a shotgun, it is probably an SB1 and not suitable for rifle cartridges.