Reloading .303 British : "o" rings for fireforming brass

The ones at Wal Mart for hairdressers are perfect, cheap & disposable! They're in the "cosmetics" section with all the hairsprays & so on. A big packet has about 100 or so black rubber bands about 1/2" in diameter. I believe they're called "hairdressers bands".

Its supposed to be done with a light "forming" load, not a full house load & the case is supposed to be lubricated!

(Some say oil others say grease)

Here's how its (supposed to) work.

The rubber forces the head rear into contact with the bolt face, removing any "slop", called "head freespace", caused by differing rim thickness of ammo brands. Basically a tolerance between headspace & rim manufacture.. When fired the lube allows the case body to expand out & forward filling any "generous" chamber dimensions. It slides & slips rather than grabbing the chamber walls & subjecting the case to differential expansion. (which is supposed to cause the infamous case separation woes of the ,303 Brit.)! This is also why thrust on the locking lugs is increased when doing it. Oh and don't forget to thoroughly degrease the chamber & bore after you're done.

Co incidentally it also forms a new neck/shoulder junction matching the individual chamber closely. This effectively makes a kind of pseudo rimless case which headspaces on the shoulder rather than the case rim.

I've never actually done it as I find a technique called "Partial full length resizing" does the same thing faster & better.