At one time you had two choices: (1) drill & tap (2) S&K.

There is now a third option, at least for US rifles. Someone (Kraghaus?) makes a Picatinny rail which mounts to the rear sight holes and uses a long eye relief pistol scope. This also solves the ejection and safety issues encountered with a conventional scope. Best of all, it is completely non-invasive.

Only downside is that it looks a little dorky, requires hand-guard removal (with the perils of breakage and safe storage of same) and would require some care in handling. That said, for range use, I do not see how one who wished for the best possible sight picture could go wrong.