If you check in the catalogues, on the screw pages there is a Screw Information chart which shows the conversion of Guage (used by Quicksilver screws) to Metric (used by Gold & Turbo Gold).
http://www.screwfix.com/search?search=6ga+x1"+woodscrews But watch out, because contrary to what chippe244 says, 'normal' screws are single-countersink, and you would have to go looking for double-countersink. Seems the only listing for these screws are ALL double-countersunk. Shows how much he knows. Mr. HandyAndy - Really