Saturday, January 29, 2011

A work in progress (x25 build)

Here's something that was done 2 years ago -- keep in mind that it is still a work in progress though. What I would like to show here is an issue that is rarely addressed in people's 7.62x25 AK conversions -- excessive recoil in blowback designs. This is not the last chapter in this build by any means, however I would like to put it out there for new builders attempting their own project, as I believe it to be pertinent information. Think of this as a progress report.

The bolt:

As you can see in the picture, the bolt was held in the forward position and a pin was drilled for and installed in the rear part of the bolt/bolt carrier. The pin was non-hardened 1/8" dowel from Enco. (The drill bit in the picture is not the one that was used for drilling). I wanted to still be able to take apart the bolt for cleaning and for that reason opted for the pin rather than welding it in place. The forward position was chosen in order to avoid laborious modification of both the carrier and the bolt. Furthermore, I was not thrilled to have the unsupported end of the bolt driving the hammer cocking process. Don't get me wrong - you can have the bolt in the rear position as well - you just have to be aware of the consequences of that design. But this is the way I did it.

The barrel, which was responsible for about half of the problems with this build, was turned from a .311 blank (yes, I have read all about the 308 vs 311 controversy and you can obviously tell where my opinion is). The turning process was begun without doing the homework on lathe operation and consequently (what can I say -- I really really wanted to get started) produced a barrel with a ridiculous amount of runout with respect to the bore. I trued it the best I could after we got our own lathe, but the damage was already done. Anyhow, here is what the barrel started it's life as (as you can surely tell, I'm using a barrel-less AMD kit acquired during The Great Scare for this build. The bolt and carrier were purchased separately). It was initially a little bit longer than 16 inches (the finished product, however, was always going to be a pistol and never had a stock fitted or installed) but was later cut down to about 11 inches.

The receiver, which constitues the source for the other half of this build's problems (more on this later) was made from a bent blank, which was modified by opening up the magwell (more on this later) and tweaking the rails. The ejector rail was moved a little bit forward, so as to place the ejector over the mag. For that purpose, I welded a sort-of extension on it, in order to facilitate spot welding it in place.

(Oh, the welder used in this build is the Harbor Freight 165A Tig. It's a pretty good machine (so far) - the lack of a foot pedal is detrimental but is not a deal breaker - you can still work without it. It is magnitudes of order above their 90A so-called MIG).

The rail took a little bit of a beating during the installation process - while trimming the top rails, I accidentally cut into the lower ejector rail with the mill. For that reason, I removed the tail part as it was far too thin to be useful and, since the bolt is pinned to the carrier, serves no practical purpose anyway. Be careful with that mill.

The front trunnion had to be modified as well. The locking lugs were trimmed from the bottom to fit the PPS43 magwell. The bullet guide was removed so as to allow the barrel to be pressed in much further in than in the original AKM. This modification was done to facilitate more reliable feeding - the nose of the round needed to be part way into the chamber before the rest of the cartridge exited the mag.

The magwell. The PPS43 magwell was trimmed and cleaned to the point as it appears in the picture. The back tab, where it used to attach to the PPS receiver is left intact however. In the front of the magwell a piece of steel was welded on to serve as the anchor in front of the trunnion. The rear tab was (after careful measuring) was welded to the safety selector stop.


Recoil spring. After some initial test-firing (not shown) it was apparent that the blowback force was very strong (more on this later). I tried to find a stronger spring but after checking all the usual sources McMaster, Enco, Wolff - nothing satisfied me. Wolff had a 10% stronger spring, but I was looking for a little more stiffness than that. I ended up ordering several short springs from McMaster to use in addition to the existing spring. I had calculated the spring constant for the original spring and the new springs but this information has unfortunately been lost. In any case, this approach did not work as the shorter springs invariably ended up jamming inside the bolt carrier after a few shots. So, this is currently an open question - where to obtain a (much) stronger return spring that would fit an AK. At the same time, it may not even be possible to tame the recoil in this manner as the spring strength required would have to be incredibly stiff. I need to do more measurements and calculations before these questions can be answered conclusively.

Test-fire #1. Here is a picture of the assembled pistol as it appears in the first video. What you will see in the video is a few successful shots, followed by a jam caused by the extra springs. The trigger slap is brutal - my index finger has never experienced such pain when shooting a gun. The trigger literally slapped the shooting finger with incredible force, so much so, that I ended up using just the finger tip to pull the trigger in order to minimize the pain. Note, that the trigger, hammer and rear trunnion holes were all heat treated. The egging of the rear trunnion holes was evident after initial testing (not shown) and the heat treatment was done in response.

The first autopsy. The first thing that became apparent were the spent cases. The picture below shows what they looked like with the 16" and 11" barrels - there's not much difference between the two. The case necks are severely bulged, although there is some variation to the degree of severity. This tells me that the the pressure in the barrel is still very high when ejection begins. As the spent case is being extracted the neck loses support of the chamber walls, and due to the high pressure in the barrel expands. This observation led to the decision to cut down the barrel, in this way shortening the time that the barrel spends under pressure. However, as you can see from the picture, the case deformation did not change, indicating that the premature extraction is still just as much of a problem, even with the short barrel.

Further damage to the weapon became apparent when it was taken apart for a thorough cleaning. The trigger had a bent area that was a result of the disconnector violently slamming into it. The interface between the bolt and bolt carrier was also damaged from the recoil.

At this point, the extra recoil springs were taken out and further firing of the weapon was attempted. This, however, did not go well. After firing about one and a half mags, the receiver bowed outward. As a result, the safety selector came out of its mounting hole, at which point the weapon was no longer shootable. I fixed this problem by bending the bow back in only to have the same thing happen again less than a mag later. The bent blank receiver simply cannot stand up to the violent recoil. (The trigger slap was still a problem too).

The second autopsy. In addition to the bowing described above, the rear trunnion was literally ripping itself free from the receiver (recall that the mounting holes were heat treated).

So, this is where things currently stand. I will replace the bent blank receiver with a fully heat-treated NoDakSpud one. Furthermore, I will redo the barrel using a Green Mountain .311 barrel blank. I am still thinking whether shortening the barrel further will have any effect on the spent case deformation. Perhaps I will use this (bad) barrel as a test for the latter hypothesis.

Advice to new x25 builders/Final thoughts. Definately go with a spud receiver. You may have to figure out a way to move the ejector rail forward for reliable ejections. Consider a gas-op build, however that comes with its own set of problems. The way I feel right now however, is that 7.62x25 may be too much blowback to tame without a locking mechanism.

Hope you enjoyed reading about this build. In the next post on this subject, I hope to present you a working version of this x25 conversion.

Sunday, January 23, 2011

Mosin .45-70 Build Part 2 - Extractor Relief, Sights, Stock Inletting

So, this is a little bit delayed but it's a good update so it's worth the wait. As you may recall, the last build session ended with a barreled (but not tightened) receiver. This time we will continue the process with:

1. Extractor relief cut
2. Sights installation
3. Stock fitting

Here's the obligaotry "before" pic:

I made two barrel timing marks, one for a hand-tight fit and one for the final fit.
Here's the hand tight:

And here it is after tightening with a home-made action wrench and barrel blocks:

Looks like it went from about 0 to 2 o'clock, which I guess is kind of far (0 to 1 o'clock is desired). I suspect that the reason behind it is that the barrel shoulder wasn't sharp enough (90 degree ideal). No matter, it just had to be tightened a little further. At this point, another mark is made on the barrel to indicate how far to twitst to full-tight.

After marking the wrench-tight degree of twist we unscrew the barrel and mark the area to be removed for extractor relief. What should the extractor relief look like? Here's what an 91/30 izzy barrel barrel looks like. The 6 o'clock in the picture below corresponds to the bottom of the receiver. I am using a piece of steel wire to indicate the vertical center line. From here you can get an idea of the angular extent to which the extractor relief thould be cut (~135 degrees out of 360).

I then transferred the markings as faithfully as I could to the barrel blank. The extractor cut will be made in the mill. We do not have an indexing head and so this is the trick we used instead. The following picture shows an aluminum bar that I used to practice the cut before performing it on the barrel.

Notice the workstop in the back - this is what allows us to turn the bar in the mill vise without disrupting its position along the mill table. I will not show my practice work, but will instead move on to what I did to the barrel. First, the setup:

Here's where you want to zero the cutter by touching the top and face of the barrel. We will work by moving the cutter to and from ourselves. I started by making all sorts of fancy measurements on the factory barrel, but ultimately ended up simply taking the barrel out of the vise and checking it against the bolt until the extractor was cleared. Here are some pictures of the progress. You can also get an idea of how I was making the cuts from these.

This was supposed to clear the extractor according to my measurements, but alas. The pointy extractor tip was coming up against the surface cut in the last step above. I ended up having to remove most of that material in order to allow for sufficient extractor clearance. The removal was done in the same way as before. The "lip" went from this:

to this:

The lip is indeed pretty thin. Perhaps using a larger diameter endmill will allow more material to remain. I used a 1/8" ball-nose end mill for all the above operations.

The crown was done with Midway's .30 cal crowning tool (here) fitted with a custom-made (from an old Harbor Freight brass punch) pilot bushing. At this point, the barrel was screwed back in to the receiver to the wrench-tight mark made previously. It was now time to take care of the sights. The sights were ordered from Numrich - it was a cat-in-a-bag purchase, but ended up being discounted Williams front+rear sight combo (also available from Midway and Brownells). Make sure they are steel. Voland ordered his rear sight elsewhere and it turned out to be made from aluminum. As a result, we weren't able to solder it to the barrel. The curvature on the bottom had to be tediously fitted to the actual barrel diameter by running them over some sandpaper, which in turn was wrapped around the barrel. (I got some really bad blisters while doing this).

After a quick sanding of the surfaces to be mated, they were degreased with mineral spirits (not the best solvent for this - use acetone if you have it). For the solder we used soft solder from Midway (here) and this is how much I ended up putting on the bottom of the front sight. In retrospect I would say use about twice as much.

The sights were clamped onto the barrel with a C-clamp and heated with a mapp-gas torch. It took quite a bit of heat to get the solder to melt - it made me wish for an oxygen setup to speed things up. Embarassing note: even though I took the time to make sure the sights were "straight" they still ended up being "crooked". Here's a pic of the crooked front sight after soldering.

The fix? Take 'em off and do it again. Here are some pics of the solder after the sights were taken off.

The front sight has enough solder, but the rear is definately insufficient. The old solder was then sanded off and the soldering process was repeated, this time with much better results. Actually, I am skipping a step here, as chronologically - the stock was fitted before the sights were redone. I strongly recommend doing future builds in this order - that is - first stock then sights, as being able to put the action in the stock makes sight alignment much easier. Anyhow, here's wha the redone sights looked like:

Here is the stock fitting step that preceded the final sights installation. The stock was originally slightly longer than the end of the barrel, and for that reason it was chopped right in front of the front sling slot. The new barrel diameter was much larger than the original barrel channel and in order to make it fit material had to be removed. There are fancy tools out there for doing this, however, for doing only one build I chose to rely on the ol' sandpaper-aluminum pipe method. Pick a pipe with a diameter that is similar to the new barrel, wrap some coarse sandpaper around it and get to work. I also used a dremel with a large diameter stone for difficult to get areas.

This is what the rifle looked like after chopping the stock. Kind of ugly and bubba-esque.

The fitting process.

An intermediate stage in the inletting process.

After trimming to size, the front piece of the stock was re-attached as follows. Two holes were drilled into the main stock, into which two pieces of thick copper wire were inserted (with glue). I used a non-specialty glue that we had on hand - you on the other hand should spend some time and get the proper wood glue for this operation. To mark the location of the holes on the front piece, it was aligned and hit gently with a rubber mallet so that the copper wires make a mark on the wood. Holes were then drilled and the two pieces of stock were joined as shown below. A generous amount of glue was added between the two stock pieces.

(You can see the glue we used in the above picture)

After the glue had dried, this is what the joint looked like up close. The front piece then needed to be inletted in the same manner as the rest of the stock.

And here is a picture of the assembled rifle!

Now, it was time for a test fire!

The weather was very cold and beautiful, so I couldn't resist taking some artistic shots ;) Without any sights adjustments the rifle shot pretty close to center right out of the shop. After taking a few more shots I had the rifle dialed in almost perfectly. Later on, while transporting it to the range, the rear sight did come loose as the holding screws on it are very flimsy. I will stake it in place at a later date.

Finally, I will leave you with some action shots of the 45-70 Mosin vs a bowling pin. This was from a little less than 25 yards. Hope you enjoyed this long-overdue writeup. I'm looking forward to the next part of this build where we will work on the magazine and possibly stock refinishing and restoration.