In the late eighties as I began to set up or build more 375 H&H's for hunters and finding suitable actions/rifles to do so could be a challenge. By this time I had used quite a few original Pre-64 Model 70's that had left the factory as 300 H&H and 375 H&H rifles and while they all mechanically worked except-ably there were a few things that seemed to crop up over and over again with these factory long-magnum receivers that could use some attention to detail and revision.
I began to consider using the standard 270/30-06 receivers as they came from the factory and modifying them to remove the factory idiosyncrasies that I felt I could correct. Do they need correcting ? Yes, No, Maybe, so let's see.
The one area of contention is usually the 300/375 magazine cavity that holds the magazine box in place just under the feed well is usually cut with a gross amount of clearance. In short most mag boxes rattle in place. Not always a great feature to have a full magazine migrating in recoil as it will begin to distort the mag box over time and use. Then there's the amount of material removed from the lower recoil lug seat as the actions underside is by design opened up forward to allow the 3.600 length cartridges to be used. Yet another major issue is the way the factory then changed the bullet ramp angle and usually left a razor sharp edge at the top edge of the bullet ramp. There had to be a better way.
A radial notch was then cut into the rear edge of the front ring and another large radial cut was added to the front of the rear bridge to allow loading the rifle quickly from the top side of the action with these longer rounds. All was great when those rifles were fit with the Lyman or Redfield receiver sights or even kitted out with the shallow V's found on many older Model 70's. You had an opening in the top of the receiver you could fill the magazine as you back peddled away from danger.
Scope use changed this paradigm radically. Once the 1 piece scope mount became vogue you then had to really practice to load that rifle quickly. A fault ???? not in the original design idea but the system still worked day in and day out. Especially by those that had to make a living with these Model 70's.
Thousands of standard length 98 actions have been altered too except the 300/375 H&H and those suffered the same fate. It works, no doubt about it and still does.
Where was I ?
So in an effort to make a "better" mouse trap can we use a a Standard Model 70 chambered for a 270 Win or 30-06 ? You bet.
I going to assume you have already opened up the .473 bolt face to .538 or what ever you deem the right Magnum bolt face size and fit and chambered the 300 H&H barrel. At the same time you've also modified your standard extractor hook to a magnum hook. As you're going to need that H&H chamber to use for feeding trials sooner than you think.
You also need to understand that this is not a one hour conversion, requires a milling machine, carbide tooling and a very secure way of holding the receiver while it is being modified. The barrel should be removed and in this case the rifle receiver was also going to be fit with my scope mounts. So the 1st course of action for me was to true up the underside of the receiver as my fixtures for fitting a set of my mounts are made to work with the underside of the action being machined flat, so we're now killing two birds with one sledge. With the underside of the action leveled off the feed rails I then make a number of passes along the bottom of the receiver with usually with a 4 Flute 1/2" carbide end mill until the bottom side of the feed well and trigger area and tang are of the same level. Then I insert a 1/2" end mill into the collet that has a .030 corner ground onto the end mill. I then cut the recoil flat pocket behind the recoil lug so it is also flat. At the same time the recoil flat pocket is being cut the back face on the contact surface of the recoil lug is also being machined 90 degrees to the the recoil flat. The pic below illustrates why this alone should be done.
While not the same Pre-64 we're going to open up the photo below display's a very common occurrence with all Model 70's and that is the recoil Lug being originally machined into a wedge shape unfortunately being wider or thicker depending on your perspective at the BOTTOM of the lug. All with complements from New Haven.
Below .006 has already been removed from the back of this G -series recoil lug face and were still not cleaned up or perpendicular. Tightly Glass Bed such an action with this integral wedge into a stock and you risk breaking the stock before you get the barreled action back out of your bedding job. This issue is more common than you think with the Model 70 having machined a couple hundred of them I can guarantee this anomaly.
Since we're converting a 270 action to 300 H&H the next step is to carefully begin to open up the feed well originally set up for the 3.400 width and length magazine box. Now we're making room for the longer and more tapered 3.630 length 300/375 OEM mag box.
Since the client supplied and original 300/375 magazine we begin fitting the new box into the feed well cavity. We measure the thickness of the back wall of the magazine and note its .030 thick. The box will rest on the lip at the rear of the mag well. Usually this lip is recessed approx. 200 deep into the feed well just ahead of the middle guard screw. The 270/30-06 magazine box back wall is approx. .050 thick and the 300/375 box is .030 in wall thickness. So we can move the back wall of the feed well even further to the rear by another .015 to .025 usually.
The lip that the rear of the box sits on is machined to the same thickness as the back of the 300/375 mag box. There must be a seamless transition for the base of the belted mag cases to slide up the magazine and into the feed well. As the 300/375 magazine is wider at the back of the feed well the width on our 270 action must also be modified to reflect this new width. Measure the width at rear of the 300/375 box and cut away for the center-line of the action accordingly on either side at the rear.
Note the 2 red arrows as they show other areas I will need to machine to allow for the proper width in the feed well with the new 300/375 magnum box.
But first the forward end of the receiver must be cut away to the overall extend length of the longer mag box. The cavity being cut will also serve as a Depth Stop for the forward end of the mag box like the shelf in the rear of the feed well. So it will also be cut to the same depth as the rear shelf at .200.
This OP is best done with a 1/2" end mill to match the radius on the front of the 300/375 magazine box. Cutting away from either side of the actions centerline establishes the front cavity shape. Again care must be taken to the cut this overall length to the numbers, now is not the time to be sloppy
The area inked in red will also have to go but not just yet.
If I have done my math correctly the new 300/375 length box should now PRESS into place. I have controlled the amount of steel removed to a minimum to try and arrest box migration in recoil.
Sweet!!!!!! you say, but the job is far from over. Now we have to remove steel from the feed well that is currently still set up to facilitate the 270/30-06 box geometry and over hanging the the newly installed longer box. To do this OP we need to be able to rotate the action as well as off set the taper to make these cuts. As the fixture I use will allow me to pivot the action this it's not at all difficult. The taper is generated by running an indicator done the inside wall of the feed well with pivoted offset until it reeds as close to as you can get to ZERO front to back. Being gang milled in the factory nothing on a 70 is every super precise from one action to the next, frankly they didn't need to be.
So the action is rotated on its axis and then locked down with which ever side of the action you chose to start on at an 8 degree angle. It is best not to guess.
Then the new magazine box is place back into the feed well and you need to determine of much additional steel to remove that is overhanging the new magazine box. This transition BEHIND the feed rib needs to be flush with the top or opening of the new box. This is done one side at a time and in this case with a 1/4" ball nose carbide cutter to match the original radius as cut into the action at the factory. The steel was removed .005 at a time until the top of the box and feed well blended seamlessly up to the rib in the box. DO NOT remove any material above the rib or there will be hell to pay.
With a smooth transition at the junction of the mag box and the feed well to the rear of feed rib we can now work on area ahead of the mag box rib. This feed well width in this area should allow clearance for just the loaded necks of the 300 H&H rounds and no more. Again a 1/4" ball end mill ties the 8 degree side wall cuts into the 1/2" ball mill cuts I will now make to finish up at the front end of the feed well. Confused yet ???
Remember the front of the 300/375 mag box has a 1/4" radius and feed well must reflect the same radius at the not only the front of the magazine on the level but also on the 8 degree taper in the feed well itself. Here a picture is worth a 1000 words. Now that we have one side done we rotated the action and re-adjust taper to cut the opposite side of the feed well. If you've gotten this far, take break for lunch you've earned it.
So far so good