ALL modern H&R rifles are drilled and tapped for a scope rail, even those that come with sights. What Pete said on the bore slugging, see the FAQs for the how to. Both of my 45 Colt barrels slug .4525" in the grooves and .4465" at the lands at the muzzle, slug measured with a digital caliper, so not as precise as a micrometer, but close enough.
Tim