The old Service Rifle competitions were for people like us. The rules were very plain: your rifle was MilSpec, ya run what ya brung and the ammo was free...... and you were disqualified if you were caught using anything other than the particular lot number being used!
As to the rifles, modifications were allowed but you had to be able to return the rifle to "issue" condition with NO TOOL OTHER THAN A SCREWDRIVER.
This led to some very interesting aperture sights by A.G. Parker, A.J.Parker, Alex Martin, Sutherland, BSA, Gibbs and several others, all of which could be attachd and removed with a screwdriver only. NO need to drill holes.
These turn up at gun shows. They are on the expensive side but they WORK wonderfully. With a set of the A.J. Parker sights, for example, I have shot a 3.2" group at 300 yards, so they hold..... and likely the sights are better than I ever was!
Perhaps the best of the lot was the Parker-Hale "Twin Zero" or Model TZ. These were popular with the DCRA crowd back in the '60s, cost $60 for a set but you were buying PRECISION.