Current production barrels made by Marlin have the same bores for both 22Lr and 22mag, except the mag has more grooves to deal with the higher pressure.
22 LR
Bore .219"-.221"
Groove .2225"-.2240"
Twist 1:16"
16 grooves
22 WMR
Same except 20 grooves
The problem will be with older barrels that were made by different barrel makers that use the tighter bores in the 22Lr, a rechamber of one of those barrels may result in cartridge case head failure.
So, no matter what the specs are, it would be very advisable to slug the bore as Stimpy said, before wasting time and money on a rechamber, H&R has used different barrel sources over the years, it's hard telling what the bore is.
Tim