Make sure the countersink angle matches the screw head angle. If it doesn't, it will keep coming loose no matter what you do.

I've heard of this type of problem on Savage supplied base.

You can get a Weaver 20 moa rail for $27 from opticsplanet if it keeps getting loose.
http://www.opticsplanet.com/weaver-e...lot-bases.html

Have have 3 of the Weaver 20 moa bases (on a 300winmag, 243win and 308win) and can't believe the value of these rails. They even have a machined slot that acts like a recoil lug for the base.