Sunday, May 7, 2017

A Little GB-22 Update

Let me lead with the conclusion: I'm not done yet.   

On the other hand, I'm very close to making the last piece, the frame itself.  All of the other pieces are cut and ready, mistakes have been left in place, and recovered from as best as can be.  The frame is to be cut from a piece of 3/16" thick steel and looks like the brownish piece here:
The turquoise colored piece is there to take up room, so that when I tell my CAM program to make tool paths to cut that frame out, it doesn't waste time cutting away that area.  My plan is to cut the main outline out with a 3/8 square nose cutter (End Mill) and leave a bit of a skin on it - about 25mils thick.  This cuts away the majority of the frame, cuts away most of the metal inside the trigger guard and that large open area in the top of the frame, where the slide goes.  This gets followed by another machining operation with a smaller cutter, 1/4", that gets everything except for the area around the trigger and that small gap extending to the left.  The small slot between the trigger and the frame will get cut with a special cutter bought just for that.  

I hold this on the machine to cut with those screw holes in the handle: 10-32 screws threaded into a plate that holds it for the cutting.  Up in the frame picture, around where the barrel mounts, you'll see four 1/8" holes.  Those line up with matching holes in the barrel holder and 1/8" spring pins make the mechanical connection.  Since 1/8 is in that awkward range that's too large for a #4 and too small for a #6 screw, I'm undecided on how to firmly attach those holes to my tool plate.  Today, I did a test cut to ensure the system could hold a piece of sheet metal down, withstand a generous cut (in aluminum), and come out the right size.  Because the piece of 1/16" aluminum I had was too small to cut the entire outline, only one of those four was available, so I added two special tooling holes to hold the plate in place and used the spring pins as fasteners.  It was all rock solid.  Those extra holes are visible in this picture on the left side of the frame and just in from the right edge of the frame, below and to the right of the real pin hole. 
This is the result of the test cut and came out as expected.  My next move is to go to on to the steel plate. 

I showed the barrel and holder a month ago; here are the three parts of the slide, the sides and the shorter middle piece.  Note the fancy cuts on the left edge of the short piece.  That had to be done by the CNC and tested out to make sure it worked.  The two long pieces, BTW, are not supposed to be beveled on the end facing the camera.  Oops.  Purely cosmetic, so "no harm, no foul".  Since I don't have metal white-out, these will stay that way. 
I'm pretty bad at estimating how long it's going to take to finish this, but I'd hope to get the first rounds through it this week.  I had no idea what to use for the spring that drops the firing pin onto the round (the firing pin is just visible as a nub above the centerline on the right hand piece).  I found a few bad ballpoint pens around here and grabbed the strongest spring to try.  I need some grips.  In the CAD world, I took a pair of 1911 grips and modified the hole spacing to fit the handle, since 1911 drawings are widely available.  Scaling the whole grip to make up for the hole spacing being different made them too narrow, though, so I need to start over.

How do you test a pistol you've made?  How do you fire the first round?  I'd like to go to my club range when nobody's around and try a few different things.  Maybe the typical "long string"?  I have some subsonic .22, and that might be quiet enough to use inside the shop.  Just to make sure the spring is strong enough.  A subsonic 22 shouldn't be any louder than a nail gun or a hammer, right? 

Some of you may be unaware that Mark came up with a closed bolt version of the GB-22.  I haven't heard anything about plans being available, but I signed up for the mailing list, so I'd hope they'd let those of us who bought the originals know about that.  Along another interesting path, the ECCO Machine video channel upgraded the pistol to a .380 centerfire, the GB-380.  People are having fun with the basic idea of the GB-22; this guy answers my basic "how do you test it?" question by firing a blank in his shop. 


  1. "A subsonic 22 shouldn't be any louder than a nail gun or a hammer, right?"

    Until you get nipped by a ricochet. ;-)

    Make sure you have a trap.

    Agree that the quiet, subsonic .22 will test primer strikes.

  2. Metal white-out exists, you just don't yet have it.

    You could work up to a live round test.

    Pull the bullet, dump the powder and fire the primer.
    Or use .22 blank for a nail gun for the test.

    For the live fire.

    Pistol in a vise pointed straight down and aimed at a bucket of dirt or soil. That will be the trap as Joe Mama said. (kitty litter might be OK, but soil or sand will be fine)
    Step away and activate the remote trigger device. (that sounds cooler than saying pull the string)
    Check on the results. If satisfactory then repeat live fire while filming.

    Are you planning on bluing, or browning, or maybe powder coating? Electroplating? CNC engine turning? This is the perfect time to try out processes.

    1. The subsonic ammo is a natural for this because it doesn't matter that it has too little gas to cycle a semiauto.

      Are you planning on bluing, or browning, or maybe powder coating? Electroplating? CNC engine turning? The best answer is yes. One of those, I'm just not sure which. Something to protect the metal, so probably not the engine turning. I think some of the small pieces have picked up dots of rust - and that's in an air conditioned room. (There's rust, I'm just not sure it wasn't there when I got them.

      I hadn't thought of pulling bullet and powder from a round, though. I have an RCBS bullet puller but didn't consider using it. It was a little unnerving to try on a centerfire round and rimfire seems more dangerous.

  3. Aguila Colibri or Super Colibri would be excellent. In a rifle, the firing pin falling is louder than the report from a Colibri. They recommend against using those in rifle length barrels, but I only use them in one rifle, a single shot, that has never had one stick in the barrel yet.

  4. Why build such a piddly little toy gun when you could go 12 gauge with far less work:

    1. Violates my First Law of firearms: a gun needs to be less lethal to the guy behind it than to the one in front of it.

  5. Ah, but with that gun you get a "two-fer". If you're surrounded, you can get them on BOTH sides with the same shot!

    And besides, it's not like you don't know what's going to happen with the breech. Stay clear of it when you fire and it's no more dangerous to you than if you were dumb enough to put your finger in front of the barrel of a short pistol when you fire it...