7x57 Mauser Fire Formed Into 6mm Remington?

Have you ever done this or know someone that has? My reason for asking is, I want to use Lapua brass & they don't make 6mm Remington, but they do make 7x57 Mauser. I checked my Sierra reloading manual & the two rounds are so close I believe it will work. I did it when I couldn't get 6mm Remington brass from Winchester. I bought 100 rounds of Winchester 257 Roberts brass, ran it thru my 6mm Remington full length sizing die, fire formed the cases & bin go, I've been using it for 45 years.
PLEASE reply DIRECTLY to: [email protected], thanks.

NRA Patriot Life Member
SASS Life Member #6683

I have no idea. But just want to ask, is the 7x57 the original parent case for the 6mm Rem.? And the .257 Roberts?

YES to both. Been there done that no problemo !

And so it goes...

Can you get away with just neck sizing?

I have done the exact same transformation from 257R to 6mm R. Here is how I do the neckdown using a standard RCBS type full length sizing die...we know the 257 brass will not fit the 6mm so I take the rifle and empty 257 brass and size the brass down with 6mm die until it fits the chamber with a slight push on the handle, then I can fire form the brass and then use neck die (if desired). You can leave your full length die at this setting(this is what I do and did) or if you like you can reset the FL die ever so slightly down to set back the shoulder for easy chambering. If you only want to use a neck sizer, try it and see if it works, but for the first sizing I would expect the FL sizer to be a better choice. I am assuming this is a bolt gun with some cam action.. good luck