I had a friend about 30 years ago had a Savage 99 in Savage .303. He was complaining about the cost of shells then. I happened to have a thousand 7.62 Nato machine gun brass at the time. I know it is still cheap as dirt! I found that the 7.62 brass full length sized in .303 Savage RCBS dies and then trimmed to length worked just fine. I got his rifle and started out on the low side with IMR 3031 and worked up till I started getting sticky extraction, then backed off a grain and a half. Wouldn't want anyone to try and fire any of those in .308 Winchester rifle though. I have always been bothered by sizing headstamped brass into something else and then loading them for anything different than the headstamp. I consolled myself with knowing that the brass never said it was .308.
If it's incoming - DUCK!