Making 308 Norma Magnum from 300 Win Mag cases

[98src]

I have 100 once fired 300 Winchester Magnum cases that I would like to use to make 308 Norma Magnum cases. I have a set of 308 Norma Magnum RCBS loading dies and have heard that all I have to do is use the sizing die to move the 300 Win Mag shoulder back approximately 0.100". Then, trim to 308 Norma Magnum length. Will I have to fire form , and if so, what will a fire form load consist of?

[p246]

Can’t help much on that conversion. When I fire form 260 Remington to 260 Ackley Improved I use a max charge for the 260 Remington load out of my manual. Blows the case out very nicely and is almost as accurate as the AI load used after forming.

[Sunray]

It is alleged that you only need to full length size them in a .308 Norma Mag die, trim them to length then load them with data for either the .30-.338 Win. Mag or .308 Norma Mag and shoot them.
A net search for "308 Norma Magnum cases out of 300 Winchester Magnum cases" turns up over 1 million results. Like this one.
https://thefiringline.com/forums/showthread.php?t=559062

[fguffey]

I have 100 once fired 300 Winchester Magnum cases that I would like to use to make 308 Norma Magnum cases. I have a set of 308 Norma Magnum RCBS loading dies and have heard that all I have to do is use the sizing die to move the 300 Win Mag shoulder back approximately 0.100".

"all you have to do is etc. etc." Is the worst answer. If you knew the length of the chamber from the shoulder to the bolt face you would not be required to fire form.

I have short forming dies; short forming dies are my favorite. For example the 308 W forming die is a short forming die, same for the 243 W forming die. When using short forming dies I must keep up with the length of the case from the shoulder to the case head.

After forming the 300 Win Mag cases I size the case to 308 Norma mag; again I off set the length of the chamber with the length of the case from the shoulder to the case head. The 308 Norma case head spaces off of the belt. There is nothing a reloader can do about the thickness of the belt because manufacturers do not manufacturer cases that changes clearance.

I have formed 8MM Remington cases to 338 Mag and 308 Norma mag. The problem with using a sizing die to reduce the length of the case from the shoulder to the case head is folds and creases.

F. Guffey