OK now lets tie all this together. The first thing to do is file the different flat widths we've cut into that wasp waist shape between each feed rail. Had we been using a CNC we could have programed this step hit the RUN button and watched these mirrored contours take shape.
Instead we must make the modification blend by hand. This is best done with 6" to 8" medium cut mill files followed up with a # 4 cut file blending our stepped cuts together in an uninterrupted transition from front to rear. Since we made these mill cuts into the rails with use of the digital readout, both the depth and length these two apposing sides should blend out in almost a mirror image. The upper and lower edge of each rail must also be filed with a slight corner breaking radius. The transition along both rails should feel smooth and interrupted. The plan is to just blend these milled flats not hog out more rail width than is necessary.
Then starting with 180 grit W&D paper both rails should be polished top and bottom, front to rear. This sand paper work should be continued up through 320 grit. Keep the radius theme on both sides of the rails
Next you need to file an angular ejection bevel on the underside right leading edge of the rear bridge as we have left a very sharp corner that will catch on every cases being ejected. In addition any sharp corners generated by and end mill need to be chamfered with a file and then paper.
You need to address the side walls and radius of the feed well. This operation is best done with mold making stones and stoning fluid. These rapid break-down stones will take on the contoured surface you're working with and starting with approximately 180 grit and progressing up to a 300 then a 400 grit stone the sides of the feed well should soon shine like a new dime.
You must remove the sharp corner at the rear of the feed well wall that we've generated by widening the feed well. 1st with a pillar file, then a stone and finally with paper we need to turn that sharp corner into parabola shape. Failure to do so will cause the belts on the case to hang up on that sharp corner.
Next we begin blending the 90 degree shelf above the leading edge of the magazine box ID we have established by cutting the feed well forward with the existing angle of the 30/06 bullet ramp. Some would suggest making this new angle straight from the top of the mag box to the rear edge of the flat behind the lower recoil lug seat, you could. However if you blend the standing material into the established bullet ramp angle with again a parabola shape you will in fact be leaving much more steel in the ramp area than Winchester ever left behind on factory 300 H&H receiver. This leaves the lower lug area with significantly more material and removes the knife edge effect at the top of the bullet ramp found all too often on a factory 300 or 375 H&H receiver. Another advantage of this extended parabola shaped ramp also puts the bullet nose in contact with the ramp sooner allowing the bullet nose to begin its rise towards the chamber as well as rolling the base of the case into the bolt face and under the extractor sooner. Remember the beaten to death term "controlled round feed" in my experience the sooner this control starts as the bolt is pushed forward the better the entire concept design becomes.
When converting any Standard action to a Magnum you need to make up a complement of dummy rounds to cycle through the receiver to fine tune the effort. I choose to use round nose bullets on this project as if you can get these to feed perfectly every other semi or Spitzer bullets will feed but not necessarily vice versa.
At this stage I choose whatever brass is available or the brass the client wants to use to properly fit the extractor hook to the those cases.
As an example the extractor groove diameter on a cross section of six different manufactured 300 Winchester cases I measure a while ago showed a + and - .020 variation in the six brands of brass.
Without plowing up too much old ground I have mentioned this in other post in the past. The fit of the claw extractor is critical to actually give you controlled round feed. Go back and re-read that last sentence. Now read it again.
A milling machine vise and a simple indicator is the best fixture to use for determining the proper extractor tension
Depending on the extractor, zero, some or a lot of material will need to be removed from the extractor for this operation. Starting with a Standard bolt face extractor will take more time to fit than an extractor already converted for a a magnum bolt face. But starting with a standard allows more control over the final results. The actual shape of the finished extractor hook depends largely on the gunsmith and the number of these he has taken the time to actually look at the contours and geometry of one that really works well verses a dud.
The fit of the extractor hook to the chosen manufacture case should allow the extractor spring away from the bolt body .004 to .006 when the case is rolled and centered into the bolt face in the chambered position in my opinion.
Note: This is a pic of a Left Hand G series bolt and a properly fitted extractor
In short if the extractor is too tight it will inhibit and retard the process as the case tries to roll into bolt face coming out of the magazine box. To loose a fit and the case will not be captured and controlled by the extractor hook and fall away from the bolt face if the bolts forward or rearward movement is stopped for any manner. This loose round or spent case will drop into the open gap between the rails. Can you say "Jam" "I knew you could boys and girls"
A finished right hand extractor, filed, polished, blued and installed. Note the shape
So with the now extractor fit we need to alter the length of the Bolt Stop. The amount to remove will vary. I like to have the leading edge of the extractor hook buried just behind the back end of the magazine box opening. I alter the bolt stops length with a carbide cutter in the mill making sure the face of the shortened stop is perpendicular to centerline of the action, this part was designed to fit flush or evenly on the back of the left recoil lug when the two surfaces come in contact.
The ejector blade is done in a similar manner BUT the angle of the ejector sticking out of the fully retracted bolt face needs to be considered. I want a minimum .110 of ejector blade sticking out of the bolt face with the blade parallel to the bolt face when fully when retracted. That's why we alter the Bolt Stop 1st.
Now we address the follower, the 30-06 follower is not going to work as it's now to narrow to use in the 300/375 magazine box. A Pre-64 300/375 H&H OEM follower might work but much of this depends on the shape and width of the parabola you cut into your rails or if you chickened out and elected to leave the rails straight per the factory Winchester theme. Likely the OEM 300/375 follower is going to slip up through the new rail contour with the wasp waist and not hold the last round in the magazine under the rail properly. So what follower do we use ? That is a damn good question.
NECG offers a 98 375 H&H length and width follower, Swift/Blackburn sometimes has them in stock, if Jim Wisner has any on hand I'd recommend his hands down. A 375 G Series Model 70 follower might work but frankly I've never tried one.
No luck finding one ? If you've gotten this far then making a follower shouldn't be beyond your capabilities and given the choice I'd make it with similar geometry to a Mauser 98 follower instead of the Pre-64 Winchester follower. Having made more than my share of these over the years just plan on most of a day making and polishing the follower from a block of steel. When I've read online that someone felt scalped by paying more than $50 for a follower I know this cry baby has never had to make one.
Note: The follower on the left and is chrome-moly 300/375 Echols Classic/Legend follower. The center follower again made in this shop is for a 300/375 magazine and the material is 7075 Aluminum that will be hard anodized when completed. The other follower on the right is a steel 300/375 follower made by Winchester.
7075 Aluminum what the f_____?
The original 300/375 OEM Winchester magazine box is very thin and subject to a lot of abuse resulting in major dents appearing in the both the front and rear walls of the magazine box in recoil if the rifle is fired a lot. The owner of this rifle shoots quite a bit so the 7075 was chosen as the follower material. It will also have a 3/16" Delrin bumper placed in the leading and rear edge surface of the follower to further negate any mag box deformation caused in recoil. Slight modifications can be made to the follower if necessary to tweak the functional movement of last round coming out of the magazine. Holding the last round under the feed rail and then releasing the last round in the magazine is one of two functions of the follower, so you better get it right
. You will need to have the fixtures or a means to hold the follower to make as well as tweak one, kind of chicken before the egg situation.
With the barrel installed we now begin checking the progress of what we've done by cycling one, then two, then three and then all four of the dummy rounds through the receiver. What we're doing now is likely removing steel from the area in the newly established bullet ramp to allow the rounds to slide easily up the bullet ramp. I want to monitor the contact point on the bullet ramp with Red or Black Sharpie and remove with a Crossing or Pippin file the required amount to do so. Great" you say "How much is required ?". The simple answer is, you file and blend the outer edges of the bullet ramp until the bullets contact the ramp on either side of the ramp when driven forward with the bolt. Each side of the ramp should require the same amount material removed when the final shape is established.
The larger the bullet diameter or the further away the bullet nose is from the centerline of the action the more material will need to be removed. This can take 5 minutes to a number of hours depending on the case and cartridge design. When you feel you have the best shape working for you then you can use a Foredom hand tool with the proper sized cartridge rolls and or the rapid break down stones to go over and remove the file marks and then I paper polish entire bullet ramp. You want that bullet ramp smooth as a Beagle's ear.
You need to also make sure that the rear of the receiver above the magazine box has a smooth transition above the box opening with no, zero, nada lip over hanging the box for the case rim to catch on. These two surfaces should be flush. The rear of the feed rails should also have a radius that is filed and then polished so as you load the magazine the cartridge rims won't catch on a sharp corner anywhere at the rear of the mag box and receiver. We good ?
Strip all the parts and clean all bits of steel and polishing grit out of the receiver, ejector and stop bolt area with a good solvent and compressed air. Wipe it all dry but do not oil it.
Now is the time to cycle the bolt at normal speed and then very aggressively with a dummy round in the chamber and a full magazine to look for any problems. There will be a few and most will stem from sharp edged corners you neglected. Now lightly oil the rails, bolt body, raceway, etc and try it again.
When this conversion is 100% complete you will have a Pre-64 300 H&H that Winchester would have been proud of and possibly would have done if given a little more slack in their leash.
This is not the time to call your favorite smith and ask for the exact same conversion without being willing to go the distance to pay for the effort. The smith will have the better part of two solid days in this endeavor and we haven't even re-addressed the one original scope base hole in the rear bridge being removed. That's for another Blog post I'm afraid.
The 1st of these conversions will seem a bit daunting, after the 3rd you'll be wondering why you hadn't done this long ago.