I'm pretty sure that the H&R Protector/Pardner shotguns are a Remington 870 clone. In that case, an 870 barrel should fit, but NOT a Mossberg barrel as the Remington and Mossberg barrel attachment systems are totally different and incompatible.
I've never owned an H&R Protector/Pardner, so my input is based on a quick Google search (and what I remember reading in gun magazines), but I'm fairly (99.98%0 sure I'm right.
Papershotshells