To answer the basic question, I know of no one who knows more about gas checks than the designer and builder of the Freechex Gas Check Tools, Charles Darnall. We have spoken about this subject countless times and Charlie will tell you PC does what a gas check does, it seals & protects. When you get into the accuracy issue, a PC bullet with a GC base will function without the check but it is not going to be as accurate as it would be if it had a properly seated GC. A PC cast bullet with a plain base can be driven in excess of 3500 fps, I have done it. So as far as I am concerned the polymer jacket has no limit. A PC full power load should be more accurate with a properly seated gas check because the base of the bullet will be perpendicular and flat. However with full power loads, whether you PC or not or use a check or not the inherent issue of a non-concentric cast bullet remains. The only ways I know to overcome the lopsided bullet problem is swagging, using a bore rider design bullet or shoot at short range, less than 50 yards.