I have a MK II in 300 win mag. I cant seat bullets out far enough to get to the lands , and fit in the magazine.
I'd like to extend the mag box .070 any suggestions as to how ? Looking at welding a .125 piece of sheet metal to the front of the box and then Milling the interior of the box to fit. Any thoughts on this ?

Yes, that will work just fine. Alternatively, you can solder on a piece of sheet metal or just cut out the front end and use the stock as the front of your mag box, but the way you describe is probably best.

And mill out your receiver to match. Piece of cake.

Thanks for the support ! Will go a head as planned!

Hope it feeds

I would lengthen the backside but.....

How does it shoot now with ammo of the shorter length.

All rounds do not have to be real close to shoot well

the action will be HARD

I could cut it

P dog.... Rifle shoots crummy with ammo that fits in the magazine. 1" to 2" groups. It has a Bartlien barrel on it. When loaded to just off the lands it puts shot after shot in to 3/8, to 7/16 one after another. ...
Looking at the rear of the magazine as well, but there isent as much space as there ,as there is to the front. Got a good 1/8 from the inside face of the mag box to the receiver. In studying it I think it will feed. Hope it will !!!

No, those actions are not hard and are easy to cut with standard HSS tooling. I just carved a sliver off the front ramp with a pocket knife to make sure.
There is the lip for the mag box up there so you can't take off too much.