9mm PAK Blank Firing Gun
So firstly the gun I'm going to make is a single shot blank firing gun that ejects the shell and also simulates recoil. I'm currently in the UK and as of the current time of writing this (Feb 2016) a blank firing gun without requiring a certificate must have atleast a 50% bright colour, it must vent from the top of the gun and must not be capable of firing live ammunition or be easily convertible. So the bullet of choice is the PAK 9mm which is pretty much the standard choice for blank firing weapons.The difference between a live Parabellum and a blank is the fact that the blank is only the shell and produces less pressure whereas the live bullet is longer and produces far more pressure (the 9mm Parabellum is the most commonly used bullet in existence). The 9mm PAK blank has a total length of 22mm and has the same rim dimensions as the Parabellum.
The smallest length 9mm live firing bullet is the Makarov which is 25mm long, the chamber in a blank firing must not allow live ammunition, thankfully this is really easy and I will also get an inert round to prove it will not fit. Simply due to the bullet being too long it will not allow the slide to completely return to the firing position and thus not allowing the firing mechanism to work. Now when a real gun fires the barrel is pressurised as the bullet passes down it, this pressure also pushes in the opposite direction forcing the shell and therefore the slide backwards which ejects the shell. If a blank bullet was to be placed in a real gun then it could not cycle as there would not be enough pressure in the barrel. A blank gun does not have a barrel and therefore this pressure can build up and blow back the slide. A vent must still be present as to ensure that not too much pressure will be present, this can also be the ejection port.
The slide will have a specific weight to it to ensure that the recoil will simulate the real thing, now unfortunately in the UK it is impossible to own a handgun without special permissions and then it must be held at a local shooting range. I have however shot a couple of handguns at a range before so I do know what to expect, I can also get some recoil measurements online. The gun will only chamber one blank as I'm first getting the gun to eject properly, the second project will consider a magazine. This gun is not based off anything either as it is first just a bench test, it may not even resemble a gun, I just want to test my own design.
17/02/2016 - February
I first bought some 9mm PAK blanks to get some accurate measurements, I then proceeded with a quick design sketch of what I thought the gun would require. It is quite self explanatory, note that there will not be a hammer, I will hit the firing pin manually with a hammer. Two things missing are the springs for the firing pin and one for the slide return rail. Note that there is no vent in front of the blank but it will still vent through the top of the slide when the blank is ejected. There is only one bolt hold the whole thing together, there must be no blank present in order for the slide to be retracted completely for access to the bolt. I doubt the shell will eject at this stage of design but I will add the mechanism into the design later.
The picture shows what would happen if the blank was to be just 2mm longer, not even the length of the Makarov. The slide prevents the hammer from hitting the firing pin and therefore it's unable to fire anything above it's design of 22mm. Nearly all pistols have this kind of design to prevent them from chambering the wrong ammo, if the bullet was to be smaller then the firing pin would push it forwards and still not fire.
The construction should be fairly easy, it is intended only for a bench test and doesn't require a trigger, magazine or handgrip.
18/02/2016 - February
I had a look over my design again and realised that there probably wouldn't be enough for the slide return spring, the slide and barrel were extended by 22mm. The back of the slide was also extended for when I come to make the magazine system for the Mk2.
I first started off with the slide as this was the most difficult part to make as I had to drill and bore it to a depth of 129mm, I chose to use mild steel as it was cheap and machined fairly easily. It doesn't matter too much about the finish either as it will be painted on the outside to comply with legislation and the inside won't be seen. I encountered a problem straight away, I could not drill or bore the slide to the correct diameter, it was just too difficult with the tooling I had. I chose to go about it a different way, but first had to buy a drill and a long series reamer. I wanted to create the slide in one part to make it structurally stronger but I had to design it a different way as shown below, the insert will either be welded or held in place via grub screws.
I think I will probably TIG weld it in place, it's not under a great deal of pressure plus there is a grub screw that holds the firing pin in place. I realised that I would have to wait a few days for the drill and the reamer to come through the post but it wasn't stopping me from making any progress on the rest of the gun or even the slide. I didn't have any time in the day to do any machining work but later on I set to more of the design. So far I have designed most of the Mk2 which has a some differences such as design for the magazine, I can also use this to aid the Mk1 as I don't want to be making huge leaps between the designs. I have altered the back of the slide to hide the firing pin, I have also modified the spring guide for the slide return.
19/02/2016 - February
I was pretty happy with the design so far, so while waiting for the drills, reamers and the slide spring I got on with machining. I chose to machine the outside diameter of the slide, I had to turn it between centres to keep it rigid. Mild steel machines really bad and it's hard to get a good finish, the finish wasn't so great around where the live centre was so I machined in some shallow grooves.
I put the collet chuck in the lathe to do some more precision work, I chose to work on the side of the slide that I had previously tried to drill. Instead I chose to use this side to house the insert, using a series of drills and then a boring bar it was machined to the correct depth and width, the insert tested inside of it.
The insert as I mentioned earlier was machined right after the slide while the chuck was still in, the bore in the slide above was machined to suit this insert. The rest of the insert was then machined in the collet chuck, first centre drilling and then drilled to suit the reamer.
The insert was reamed, it was then drilled further with a smaller drill and reamer. This insert will be housing the firing pin which is the reason for using reamers instead of drills. Once this was done it was set aside as I could not weld it into the slide until the slide was progressed further.
The firing pin was a simple task and was machined from a piece of sliver steel, again I used the collect chuck to ensure it was concentric. It was tested inside of the insert to make sure it fit. It was great to use the collet chuck as I could safely run the chuck at 2000rpm to get a great finish.
Even though I did not have the slide complete I chose to make the barrel that goes inside of it, I used a piece of 16mm 316L stainless which turned out to be 15.98mm and perfectly round with an excellent finish. I would normally have turned this from some steel to suit the reamed bore of the slide but I figure the reamer should finish the bore anything between 16 and 16.1mm, and 0.02mm gap would be perfect, a 0.12mm would be worst case but still adequate. I first started out with drilling and boring the chamber, all of the blanks measured no more than 9.5mm with a deviation of -0.02mm, so I chose to bore it with a 9.5mm drill at a depth of 17mm, it turned out to be 9.6mm which I think will be ok. A 90 degree counter sink was used to create a taper in the front of the recess, it is not necessary in this design but will aid in chambering the blanks in future designs.
It was then placed in the miller and clocked flat, I used a dowel and the DRO's to locate the correct position to drill a hole.
The hole was drilled to a specific diameter and depth using a two flute slot drill, a two flute is used because it doesn't wander when drilling and keeps the diameter true to the cutter. The hole that was drilled earlier was then tapped to suit a bolt, the last picture shows the finished recess
I chose to have another look over my design and came up with further design changes. I wasn't quite happy with the way the barrel was connected to the slide and also the body of the gun, the previous design could have been prone to fatigue failure. This time the spring guide moved with the barrel and not the slide, like a conventional gun. I also came up with some way of fixing the whole of this assembly to the rest of a gun when I get onto Mk2, it also allows me to hold this in a vice.
20/02/2016 - February
I was looking back over my design again and realised I may have problems with the blank chambering properly, due to the barrel having a chamfer all the way around it may be a too steep angle. Below is the new design, I'm not sure if I will keep it like this for future designs but seeing as the chamfer needs to be milled in If I were to make a mistake then it wouldn't be too costly as one solid piece.
I chose to put the barrel back in the lathe to take away the chamfer, drilled it out further and then I made another insert which was pressed into the barrel, I chose to TIG weld a little section to ensure it wouldn't come loose.
I clamped it in the vice and milled a slot to aid in chambering the the blank, so far it seems to work better than the original.