Enfield No 4 in 6.5x57r or 6x57r

Andy, I think you will spend a lot more than what a newer rifle platform would cost. The #4 action is not especially strong, and you would have to either rebore the existing barrel or replace it, and to keep a #4mk1 profile will be expensive. I don't know offhand the chamber pressures of the two calibers listed comparing to the .303, but that is a major concern. But, it's your rifle and you can certainly do as you please. I just hate the loss af even a fair condition #4mk1. They are getting harder to find.

Given that you are in Australia, you should be able to find a smith that can handle a Lee Enfield as far as rebarreling, etc. Given the pressures involved, the 6.5x57r as long as you stay within the 48 k psi/42 CUP limits, then you should be okay as the .303 is rated a bit higher when considering just pressure limits.

I suspect given the vintage of the 6.5x57r, it would probably be okay but I would want a smith's opinion before I did that. Feeding and other issues can sometimes be a pain but at least with the LE rifle, you can practice on old battered mags and followers until you get them to feed dummy rounds regularly.

a friend in Africa has one made into a 6mm-303, I have fired it and I think the head markings were 6mm mauser. I brought a case home and gave it to a friend who collects cartiges. I,ll see if I can get it for a picture.

I'm doing a No.4 in 6mm Musgrave, which is an Australian cartridge. Musgrave necked down a .303 to 6mm with no other changes. Lee sells die sets for the Musgrave. I'm sure there are Musgrave dies in Australia, but they're US$35 here; it might be worth your while to pay international shipping.

If you want a 6.5, you can use a 6.5x54 Mannlicher reamer and die set to make a 6.5/303. You just run the .303 case into the 6.5 die. The Mannlicher is a rimless cartridge, but the case length and taper are very close to .303, which means it will feed through almost any .303 magazine and ramps without modification. And a Mannlicher reamer would probably be cheaper than a custom-made wildcat reamer... note you want the "modern" rimless 6.5x54, not the old rimmed 6.5x54, which is a whole different cartridge. Dies and reamers are unobtainium for those.

Don't forget that for best accuracy, the bullet weight and the twist need to match. Just because various weights and twists are cataloged, doesn't mean the manufacturer actually has any in inventory. I've been burned by that one before... larger manufacturers tend to do things in batches, and when they're sold out, you have to wait until they decide to make more.

the rifle my friend in Africa has is a number 4, I thought the head stamp was 6mm mauser, I was wrong. it,s head stamp is PMP 6mm mus and I think is the 6mm Musgrave.

Wow, I had no idea anyone had loaded 6mm Musgrave commercially, much less with its own headstamp!

The web says PMP is Denel's ammunition brand. Their web site shows 6mm Musgrave, 100 grain bullets, 2756fps, 1,687 ft-lb at the muzzle vs. 1,818 for the .243 Winchester with an 80 grain bullet at 3199fps. Not bad at all.