I haven't tested my flat base gas checks yet sadly not enough time. The only flat base 30 cal bullet I have is NOE HTC 153 and shoots very good without a gas check around 2000 fps. I am getting pretty good results with traditional gas checks and powder coated 134gr bullet (NOE 130) velocity 2330fps, cartridge 7.62x40 with 23.5gr of 1680. Both of these shoot 1" or better at 50 yards. The thick .012 flashing would deform the base of bullet quite a bit. If you don't want concave, size it with base up or use lubrisizer, the best results are with later.