Welcome,
There is a FAQs sticky post at the beginning of the H&R forum which can answer a lot of your questions. Use control "F" to search for terms. I will try and answer what I can of your questions.
You can have H&R fit accessory rifle barrels to a rifle (SB2) frame. Depending on year of manufacture, there are SB2 frames that the factory will not fit barrels too. Also, I believe the barrel acc. program is backed up a bit right now, would need to call H&R to verify this.
If you go to the
www.hr1871.com and look at the their barrel accessory info. it will tell you what barrels are potentially available.
If you buy a used barrel to fit an existing rifle it may or may not fit to your frame. The FAQs provide details on fitting a barrel to a frame or a good gunsmith can do it. It is very important to read all the safety precautions and warnings if you decide to fit a barrel yourself. There can be real problems with an improperly fitted barrel.
One needs to know the difference between a shotgun (SB1) frame and rifle (SB2) frame as the pressure ratings are quite different. In general centerfire barrels do not go on SB1 frames, with exception of a couple of pistol calibers. FAQs will provide the info. to determine which frame you have. This is important when buying used Handis because there are cases of people buying rifle barrels that have been fitted to shotgun frames. Shooting this combination get be very hazardous.
Good luck & welcome again. These are fun guns and there are lots of good folks at GBO that will be willing to help answer your questions.
BB