I have a CRF Mod 70 in 300 win mag. I want to seat my bullets out to 3.56”. I believe factory mag well is 3.4”.

Some of the internet is telling me to use a 375 H&H mag well and a 7 STW follower and new/modified bolt stop. This makes perfect sense, but some disagree.

First, will this work? What issues am I going to run into? What bolt stop should I use? Anyone have good sources for these or do I go to Winchester for them?

I've done this 50 times at least; all Model 70 actions are the same length and all you need to do it remove the spacer from the rear of the mag well; it is spot welded in. Alter the bolt stop so the bolt will travel farther to the rear. As in grind it off. No new follower is needed, but some machining on the receiver bridge is required for best ejection pattern.
Oh, I just read that you do not want a 375; in that case; just do the first two steps above; no new parts needed.

I guess you just drill the 4 spot welds out? Since the spacer doesn’t touch the seam, I can TIG the seam together first, if needed.

Wendell,

I've just done two of these to make them feed the .300 Win Mag Match MK 248 MOD 1 load with Sierra match king 220 grain.

It's a bit more than just mod the mag box.

I took out the spacer on the mag box and tig weld the seam. Recontour and fit and polish so there's no hangup in the back of the action mag opening to box transition.

Then, shorten the bolt stop and ejector to allow the bolt to go fully to the rear to pick up the round off the lengthened box. Next, the rails need opened out nearly a half inch to the rear to allow it to feed correctly. Last, I had to recontour the feed ramp slightly and polish so the bullet tips wouldn't hang up.

After all this is done, of course you have to do the usual tweaking and polishing of the rails to make sure it feeds reliably.

I took some pictures as I went but haven't had time to put together and post out a tutorial.

I forgot about the ejector; I usually alter push feeds; they operate much smoother.
Sometimes I TIG up the back of the box; sometimes not; it is held in place by the receiver and bottom metal anyway.
I don't alter any rails or feed ramps; they are already machined.
Come on over and I'll show you some.

Ok, I see what you are talking about with the ejector.

As it is, my bolt comes as far back as I would think would be necessary. The bolt face is flush with the back of the action cut out. I can’t see any benefit in going farther back. As it is, it passes the back of the magazine box by at least .040” after the spacer is removed.

This gun was a 338 Win Mag in its previous life, I got it as a donor action.

I guess I’ll remove the spacer and see how it feeds.

Might work as is. Try it and see if it ejects brass and loaded rounds.

Sounds like you're lucky enough to not need much work. Like DPCD said, try it and see.

I altered a Laredo 7mm Mag for the long new heavy low drag bullets so they could be seated way out. Had to remove the spacer, shorten the bolt stop and the ejector, but didn't have to open the rails or touch the feed ramp. Then, did two Laredo 300 win mags, had to open up the rails at the back on both of them.

Will pick up a round, eject live round, but, having trouble feeding from the left side and occasionally it vomits up the ammo in the mag well.

I think the ejector is blocking the rounds from loading on the left side. If I shorten the ejector so it no longer sticks into the mag well and shorten the bolt stop the same amount, will it still eject properly?

I built a MK248 mod.1 on a 70 classic but went with the Wyatt Outdoors mag box and ejector.
This conversion does require machine work to the receiver to fit the mag. box.
The rifle processes ammo flawlessly and runs 230gr Bergers into 1/4 MOA all day long at 2970 FPS. It's doing exactly what the 248 was designed to do.

He is not going to extend the mag box beyond the existing footprint so no receiver alteration will be required for that.
Yes, if you shorten the ejector and bolt stop it will work.
For extended mags, you also have to machine out some of the bridge to allow correct ejection.

After shortening the bolt stop and ejector, the rails held the belt down just enough that the bolt would ride over and not pick up a round. I had to open the rails up slightly to the rear, and I also had to smooth and polish them to make it smooth and reliable. Here's a before and after picture showing the amount the left rail had to be opened up to the rearwards. The left rail took a bit more work, but the right was similar.

Good picture; yes if the rails do not let the rear of the cartridge sit up high enough for the bolt to engage, they need to be brought back. Just make them straight.

I think if I shorten the bolt stop and ejector, it will pick up a round. If not, what did you use to cut your rails to keep them straight? Could you use a dremmel and clamp a guide on the action, or just an old fashioned file and elbow grease?

Milling machine. But anything will work; that little area in the rear just needs to be cut straight back and it is not critical what it looks like. It can be under cut too. So, use a Dremel, file, hacksaw, whatever. You don't need a guide. The 1/4 inch of rail at the very back does nothing as long as it does not prevent cases from rising into the bolt way.
Bring it over; it's a 4 minute job. I do have a minimum charge of $100 though.

Like Tom said, anything will work. If I have a mill set up, I use it. Easier to get precise. But, these two I just used a dremel tool freehand with a small diameter grinding point. Then, beveled the rails and worked them as needed with a cratex wheel driven by the dremel. Faster to do it with the dremel than to walk out to the shop to use better equipment.

Cool. Thanks.

I’m good with tools and stuff like this, but when it comes to removing metal from a gun, I’m a bit cautious.

Yesterday, I ran 4 rounds through it with no issue, but I’m sure it’s not going to do that consistently.