My 4 Mk1's were the 22-250, 270, 300 win mag, and a 358 win on the early action I mentioned.
The 300 win was a screamer and I used to shoot 150 gn Barnes X bullets at 3400 fps and never had to trim the brass and cases just seemed to last forever.
Stupidly I sold it and later bought a stainless synthetic Mk2.
The Mk2 was a tack holer straight from the box and I have some video shooting crows a long way out with it.
But, the loads that produced 3400 in my old Mk1 only made 3200 in the Mk2 stainless barrel.
I stoked 'em up a bit but started getting seperations and had to settle for about 3250.
Never did like the feel of that synthetic stock, so I sold the rifle.
Getting back to the Mk1 300 win mag, back in the early 80's I shot buffalo with it using 200 gn Nosler Partitions.
The bullets used to cop a real pounding in the magazine and the tips would get all flattened out and look terrible.
In those days partitions were quite expensive here in OZ and I didn't like the idea of them getting flattened tips, so I made and installed a couple of metal strips into the magazine.
These strips were filed to fit against the case shoulder and prevent the cartridges from getting slammed forward during recoil.
They worked great and with a bit of fine tuning that gun fed cartridges from mag to chamber beautifully.
That 300 was bedded and floated and the only other thing I did was to smooth the sharp corners on the tang safety slide.