First off, are you using new screws? Torque them 35-50 inch-lbs and stake the key to the screws.
I have a proper MOACKS staking tool and I do them for $5 plus $5 return shipping.
Randall Rausch www.ar15barrels.com Specializing in barrel threading, shortening, lightening and any other machine work. If you want what does not exist, I can probably make it. <img src=/images/smilies/icon_smile_wink.gif border=0 align=middle>