Thursday, August 19, 2010

Mosin .45-70 build - part 1

Mosin .45-70 build - part 1

Disclaimer: Neither Chapaev nor Voland approve of bubba-ing military surplus firearms. Nothing breaks Volands innards more than seeing a vintage militray rifle that has been basterized by some idiot with a hacksaw and a welder. The experiemnts conducted in this blog are on firearms wich are not readily restorable to original military configuration, or are beyond historic salvation.

Over the last week, after collecting the required parts and tooling we began our attempt to convert a Mosin-Nagant action to fire .45-70 Govt cartridges. Eventually this will be made into a full functioning rifle, but at the moment the project is split into three (3) parts:

1. Modify and blueprint the action, chamber and install the new barrel.
2. Cut extractor relief, tighten barrel to the receiver, fit stock
3. Modify magwell to feed .45-70 cartridges

What follows is the documentation of step 1.

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Background and components

Chapaev is using a beat-up and rusty (keyholed upon cleaning and test-firing, cracked and otherwise damaged stock) Chinese Type-53 as the donor rifle. Voland is using a 91/30 barreled action that was purchased as such some time ago.

For the barrel blank we are using a Sarco .45-70 Govt ex-Navy Arms project barrel. After trimming and facing it will ultimately be about 16.25" long.

Disassembling the donor actions

As one may be well aware of, the Mosin barrels do not unscrew easily - even with a proper barrel vise and action wrench set-up. So, ultimately it was decided to make a relief cut in the barrel to facilitate its removal from the receiver.

We ended up having to cut a relief in the barrel where it mates with the receiver to relieve the compression.

After making a large enough cut, I was able to remove the barrel without much effort using only a regular vise and a small crescent wrench.


The receiver ended up getting roughed up at the face, but that's ok because it will be faced true later.

Voland did a much better job cutting into his barrel, and left the receiver face untouched:


Also, I should note that by this time we had put together a crude action wrench which came in useful for the barrel removal.

Aside: Differences between Type-53 and 91/30 receivers

As we finished the barrel removal on the 91/30 we observed that there were significant differences between the two receivers. The differences were mainly in the locking lugs and barrel shoulder inside the receiver, the profile of the receiver near the trigger group location, and minor differences in the thread dimensions.

Type-53 barrel thread area:


91/30 barrel thread area:


Type-53 on the left, 91/30 on the right, overall view:


Bluprinting the receiver and bolt

The first step in the process was to turn a receiver mandrel, which you can see here:


Here's a pic with the receiver screwed on:


Using the fixture the receiver face was turned true:


We decided not to try and get inside the receiver to true to lugs as the receiver wasn't going to fit into the headstock (at this point we are using an 11" Logan), and only lapped them instead. Here are some pics:




Now it was time to true to bolt face, and open it up for the new cartridge. We enlarged the bolt face diameter first, it took about .030 in total, with several small cuts and fitting every time. After that we took material off the bolt face (truing it in the process) until the rim of the .45-70 cartridge was even with the bolt face lip.


And here is the tool we used for the work on the bolt head:


Work on the barrel blank

The barrel blank ended up having quite a bit of runout, I would even go as far as to say it was bent, which complicated set-up tremendously. We initially held the blank by (and dialed indicated off of) the highly polished shank which you can see in the picture below, and removed the taper in the thickest section of the barrel in order to hold it in the lathe chuck. We wish we had the components needed for a between-centers setup, but we didn't have them on hand. We only had three days to complete this part of the project and so ordering them was not an option (we'll make sure to have them for next time though ;)However, after dialing in the barrel on the new non-tapered section it was evident by the naked eye that the bore was nowhere close to true. The rescue came from a bushing we made the night before (long story, don't ask) which was fit to the barrel and allowed us to dial indicate the bore true, even if the thick section of the barrel wasn't. Let that be a lesson to  you all: always make a custom-fit bushing for whatever barrel you're working with, so that you can indicate off of the bore, rather than some outside diameter. This is NOT an optional step.

(The barrel is being held by the thick section which was turned flat. The smaller diameter section of the barrel is the highly polished shank, and sticking out of it is the custom-turned bushing)

Initially we explored the option of making a press-on (and weld) adapter that would allow us to use the shank as part of the new barrel, however, on second thought that direction was abandoned and the shank was simply cut off. Like this:


Ok, so now comes the tricky part: calculating the barrel shoulder length. The barrel shoulder length is indicated in the following picture.


On the Type-53 it is pretty straightforward. "a" is the barrel shoulder length, but a more convenient method to measure it is by measuring "b" adding to it the thickness of the cartridge rim (here is a dilema actually: do you use "spec" rim thickness? or one measured off of an actual round? We went with measuring off of a cartridge - Winchester 45-70 Govt - 0.064") and subtracting 0.002" for compression during barrel tightening. Therefore:

Should lenght (Type-53) = "b" + 0.064" - 0.002"

The situation is more complicated on the 91/30. Because it has a distinct shoulder in the receiver there is not much you can do about the barrel shoulder length - it has to be "a". However, what do you do if the bolt sits too far back in the receiver, in other words what if:

"a" - 0.002" < "b" + 0.064" - 0.002"

This was indeed the case in Voland's build. The solution was to add a "lip" to the barrel face like this:


The lip is to extend past the receiver shoulder and thus come into contact with the rim of the cartridge, which would otherwise be impossible.

Lip thickness (91/30) = "b" + 0.064" - 0.002" - ("a" - 0.002") = "b" + 0.064" - "a"

For Voland, Lip thickness ended up = 0.007", and was machined into the barrel face. The diameter of the lip was 0.670".

Ok, back to the build. Here's what the Type-53 45-70 barrel shank ended up looking like after threading. Notice the flat barrel face, with no lip. Notice also that the threading is done up to the shoulder:


And here is 91/30 new barrel shank with the lip (before threading):


Finally, we cut the chamber:


The chamber was cut just deep enough for the cartridge rim to contact the barrel face (no pic, sorry). Once that was done, the new barreled action could be assembled for playing with testing purposes.


Final thoughts on part 1

It is Volands opinion that given the presented findings, the chinese T53 rifle is the best candidate for rebarelling or custamization due to the simplified locking shoulder.

For the purpose of historical perspective we ask that if anyone has additional photos of Mosin nagant locking shoulders, they share them here (email us and we will post them). It would be intereing to know if the locking shoulder variation observed in this article is due to country of origin, or to model differences? i.e. was the simplified locking shoulder used on all M44 type rifle regardless of country of origin?

4 comments:

  1. Firmitas, Utilitas, Venustas! Well done so far gents.

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  2. here is a great resource that shows the receiver variations, my mosin 91/30 is of the more straightforward design as seen in your t53

    http://7.62x54r.net/MosinID/MosinReceiver.htm

    ReplyDelete
  3. there is a god description of the various models etc at 7.62x54r.net

    ReplyDelete
  4. Hello- I'm working on a Mosin heavy barrel project, and was wondering if you had the blueprint or dimensions of the receiver mandrel you used on your project...

    ReplyDelete