I got some of this ammo from the Lucky Gunner website and it has a "K57" headstamp.

Did you shoot it? How did it group and did you notice any rust after?

I shot a box of it. I didn't shoot it at paper but I was shooting golf balls with it at 50 yards and it was fine for that. I shot it around New Year's and was checking my gun for rust every day for about 2 weeks and never had any, I pulled the rifle out last weekend and checked again and still nothing so what I have is not corrosive.

Awesome thank you, sounds like it is good stuff.

If it is anything like the German.308 with the same packaging. It’s got to be good.

But it is British and 30-06 ammo so......

American Rifleman magazine article April 1969 page 35 states Kynoch (K) ammo loaded with brass primers is generally noncorrosive and nonmercuric. Kynoch ammo loaded with copper primers is corrosive and in all probability mercuric.

Is it safe to shoot this ammo in an M1 Garand or 1903A3?

Yes...why wouldn't it??

"Stored in less than ideal conditions" tells me all I need to know.