Reloading .303 British : 7.62x39 in 303

Did a lot of reading in the last few days on 303 reloading topic. Then, came across flashover issue with which I never heard of until now.  It is sill a big puzzle to me.  I have been using Unique powder since mid 70's on 12 gauge shotshe!!, 38 special and 357 magnum, 9mm, 38 S&W.  In 38/357, the case fill is always around 45 to 55% at best from the book and never came across or heard of flashover. I did hear and read about W296 should not be lower than recommended minimum charge when I start reloading 300 BLK.  I stopped reloading Win 308 and 223 (5.56) because of low ammo cost in recent years.

I do like to hear from you and other members regarding 303 british using ~12 grains Unique with light cast and plated bullets, especially the 123 Gr Berry plated for plinking; and cotton ball fill if necessary. I don't plan on reloading jacketed rounds as I find PPU factory round is quite reasonable at about 70 cents per.

Thanks,