I have a W70 in 7mm mag that was to be used for a possible conversion to 458 Lott. When I asked Brown Precision they quoted me this would have to be done:

"To do this properly, it would have to be machined on the back side of the loading port. This will eliminate one of the screw holes, so a new hole will need to be drilled and tapped. It is a heck of a lot easier and cheaper to start out with the right action. So, new magazine box, magazine spring and follower, new (or modified) bolt stop, new (or modified)ejector, modified loading port, scope mount screw. Yes, you can pretty much get where you want to be, but this would not be the "right"
way".

I ask this question because in the past forum members here said nothing of the action requiring machining. Is Brown correct?

they are thinking pre-64 M70, not the classic

As I understand, the straight sided cases like the 458 win and lott may need a bit of tinkering to make them feed, but a post 64 mag action is long enough.

see this thread, Chuck Nelson posted some info on mag boxes on std mag to long mag conversions

https://forums.accuratereloading.com/eve/forums/a/tpc/f/9411043/m/286102883

Snowwolfe,

Yes, Brown is correct, and in addition to the operations they suggest, you also have to machine the rear portion of the feed lips rearward to allow the cartridge to come all of the way up so the bolt will catch the cartridge for feeding.

It would be much easier to sell that receiver and buy one that is/was built for one of the long magnum calibers such as 7mmSTW, 300Wby, 300RUM, 375H&H or 416RM.

Headache

I forgot to mention my conversation with Brown was about a LH action. These were not made pre 64.
Headache, that is what Brown suggested also. I have a LH W70 375 Safari. I am going to have to get off my lazy butt and pull it out of the safe and compare them side by side.

Free Miner, thats a URl for another subject.

Converted a classic standard to a long action with a new mag box (spring, follower) and ejector plus some rail work.

Took Jim Kobe to make it feed well with some mag box work a few years later...