It's not surprising that cases give up quickly after Ireload2's explanation...things I found out too. When you FLR cases there is almost 1/8th" of material that needs to come off. When you neck size and leave from the shoulder back alone there's nothing. It's because of the rolling shoulder and the difference between drawings and actual case shape after firing. I just recently sold my last .303 and have quit with them because of the issues...there's no magic gauge that can save them. Lee used to make a gauge that you could check case overall with a quick application. I've had one and not used it either for years. I just use a trim die now. Set to length and during reloading process the excess gets trimmed off. Yes, I still have a rotary trimmer too...
.303 was my first cartridge I started reloading back in 1980...back when it was available in quantity as factory ball.