Here is a picture of the mag before anything was done to it:
There are two spots where the cartridge binds - in the front and in the rim areas. We had lots of thoughts of how to best modify the mag to allow it to hold the larger 45-70 cartridges. Our inclination had been towards cutting the sides off completely and welding on custom-formed sides from 4130 sheet metal. However before that could be attempted, another solution appeared on the internet (I cannot claim the original author with certainty) - whereby cuts would be made into the side of the mag to allow the rounds to fit. This solution is a lot more straightforward and much easier to implement. Therefore, it is exactly what we did.
Here is a picture of the mag with the removed sides. MAKE SURE YOU SCRIBE A LINE MARKING HOW DEEP THE MAG SITS IN THE RECEIVER. If you cut too deep you will have holes showing from below the receiver - functionally fine, but aesthetically displeasing.
I wish I had a picture to show you here with 3 rounds in the mag - unfortunately it was not taken, so you will just have to take my word for it that 3 rounds fit in very nicely. Notice how on the right side of the mag the curvy portion was not taken out like on the left side of the mag (rim area cut). This is the preferred way of doing it (leave curvy part intact on both sides), though it still works just fine as shown above.
Moving on, now comes the critical mod - magwell relief. This is not to be taken lightly, as a ruined receiver is a very costly mistake. I have to admit I was quite nervous myself as we were doing the following work.
With the first cut, the magwell was opened up just wide enough to fit the rim of the cartridge. Cut carefully, symmetrically and check fit often. Amount to cut on each side = (magwell width - rim diameter)/2. Don't remove all the material in one cut. Work up to the final dimension in several cuts while checking and measuring often.
After opening up the magwell to fit the cartridge rim, the cut was extended to allow the nose of the round to fit as well.
BE VERY CAREFUL NOT TO REMOVE TOO MUCH MATERIAL FOR THE FEEDRAMP. Stop early, reassemble and check. It will seem like it's binding too much, but with the mag in place things will work much better.
The above method of magwell cutting destroys the dimple that is present on the right side of the original receiver. We did one receiver each way - one has the dimple and one does not. Their feeding behavior is the same and the top round is held in the receiver (such as when you turn the rifle upside down) in both cases. If you wish to keep the dimple, carefully machine around it when enlarging the magwell. You will also have to enlarge the rim channel in the receiver with a dremel stone. Here's the dimple that I'm talking about:
With the no-dimple method, I did not modify the ejector nor the interruptor.
After being held in the mill vise, the receiver gets constricted just a tiny bit in the tail section and begins to bind the bolt in approximately this area:
Initially, I used a file to remove a tiny bit of material to allow the bolt to slide freely, but this was not necessary with a little trick that I'll show you below. Here's some pics of material removal. Dykem to mark, cycle to see where binding occurs, file.
But here's how you can avoid filing altogether. (Of course I realized this trick only as I had filed most of the material away already, heh.) Put the bolt forward so that the handle is located in the binding area and hit the bolt handle with a rubber mallet from both sides. Just firm, sharp taps - don't go too crazy. After a couple of these the binding disappears.
The last thing that needs to be done before we can try cycling a mag is to increase the angle of the follower slightly such that the nose of the cartridge aims higher when being fed. This is done by griding away a TINY BIT of metal from the follower extension, as shown in the picture below:
If you go too far and the follower aims to the sky line an anti-aircraft gun you can partially reverse the change by hammering here:
Remember, you just want the follower to aim higher by maybe 15 degrees or so - just so that it protrudes from the magwell a little. Nothing drastic.
Let's take a look at how everything looks once assembled. In the second picture below you can see the angle for the follower that I referred to earlier.
Here is a video of the mag-feed in action.
The next thing to take care of was screwing down the sights. We only ended up doing the rear sight as the hole depth had to be at least .100" in order to allow half the drilled depth to be threaded with our 6-48 bottoming tap from Midway. In the front that would have drilled away more than half the barrel thickness, so we opted to keep the front simply soldered on. Here is that process in pictures:
First align the barrel horizontally in the mill (use a level for this) and drill a hole to 0.100" depth:
Since we didn't have a tapping setup for the mill, it was done by hand. VERY CAREFULLY. You must take extra care to ensure that the tap is going in as close to vertical as possible.
Then the bolts had to be ground to size. This is kind of tricky, as it is not clear whether the reason that the bolt will not screw in further happens because it has reached the end of the hole (is too long) or the bolt's shoulder is mating with the hole shoulder (correct). To check this, take a sacrificial bolt and cut it really short - so that it won't even screw in, but rather will simply sit on top of the hole shoulder. This your guide to how far the real bolt should screw in so that it touches the hole shoulder.
To grind the bolts down, they were held in a vise-grip and ground down to approximately the size shown below:
This is how it looked after setting the screws with some red Loctite.
Ok, now to blast and park:
And finally - after through oiling and reassembly, you can see the finished product (well, not exactly finished - the stock still needs some work):
Here is a picture of both builds, or as I call them - the twins:
Stay tuned, as we still have some stock repair to tell you about - next time.