I just looked at the front sight on my own RAR Rimfire and the mounting screw size is a bit of a mystery. It's a #6 diameter but neither a 6-32 standard machine screw nor a 6-48 scope base screw thread in properly. Both bind after a couple of turns. The head of the factory bolt is a slightly loose fit for a 5/64" allen key but too small for a 3/32" and none of my metric allens are even close. A hardware store 6-32 could be forced in but might damage the threads if you want to reuse the front sight.
Perhaps Ruger could tell you what the correct size really is. Maybe a 6-40???
best to check the screws that came out, measure with a thread gauge, ,if you are certain it is a # 6, then it just maybe a #6-40.........most gun screw thread sizes are NOT at the local hardware store...............
once you find out, you can get "plug" ( capped) screws, from brownells or any gunsmith supply company.
I'm going to bump this thread back up because I just got a set of filler screws for my Ruger Gunsite Scout to replace the Picatinny rail mounting bolts. They are indeed #6-40 x 0.125" and are the same bolt used as a hole filler for the American Rimfire front sight. Ruger sent me a package of 5 of these at no cost when I called their customer service.