Saturday, May 20, 2017

GB-22 Progress is at a Crawl

A couple of GB-22 posts ago, I asked how to go about testing something like this.  The suggestion that made the most sense to me was to pop the bullet out of .22LR round, dump the powder and just see if I can pop the primer.  Sounds like a plan.

But first I had to build it.  Rude mistake/awakening: none of the ballpoint pen springs I could find were long enough. They also wouldn't drop the firing pin all the way.  About the same time, I was going through the prints for the umpteenth time looking for hints on what the spring needs to be.  In the original drawings Mark Serbu put down two numbers with the spring.  I just assumed it was some sort of in-house number for the spring, but then asked myself why there would two numbers.  A web search for the two numbers revealed that one of them was an MSC stock number for a spring.  They came in bag of 12, which I had to order.  I was able to assemble the gun.  All except for one last thing I'll get to.
Clearly not finished, but much more "finished" than the last post that showed it on the milling machine after being cut out.  I went over the frame with a file, cleaning up all the edges, squaring up some features that milling rounded off; generally doing light finish work  Along the way of assembling it, I found a couple of places on the other pieces where I needed additional file work, but nothing major.

There are two real differences between my implementation of the GB and Mark's: the firing pin and the barrel.  Mark used a 1/16" dowel pin for the pin.  Not wanting to play the shipping-costs-more-than-the-hardware game, and having a box of 100 1/8 dowel pins, I figured I'd grind one of them down.  It's pretty symmetrical, and centered in the hole.  

The other difference between my version and Mark's is that he didn't use a separate barrel: he rifled the 2-1/2" long front piece of steel and reamed a chamber.  I am not set up to rifle barrels, so I bought one. That raised the issue of just where to seat the barrel.  Mark's drawing shows the back of his barrel chamber recessed for the rim, so that the round would be flush.  Re-watching his videos a bunch of times, the shell appears to be sitting on that surface, not flush with the back.  I spent a while with some fired .22 brass trying to hold the barrel in varying positions and see where the pin seemed to make a good solid dent.  Not surprisingly, it seemed to be best when the barrel was farther back.  I epoxied the barrel into the rectangular barrel holder so that the back of the chamber is flush with the holder.

Old joke in quality control:  Designer puts a note on drawing "Build in accordance with MIL-TFP-41C".  QC Inspector says, "What Milspec is TFP-41C? I've never heard of that".  Designer replies, "Make It Like The F***ing Plans For Once!"  I didn't follow MIL-TFP-41C.

With the GB-22 now built, it was with some trepidation and excitement that I pulled a .22 bullet, dumped the primer and chambered the brass.  I pulled back the slide, pulled the trigger and ... nothing.  No pop.  Tried again.  Nothing.  Thinking I should verify that I didn't somehow dislodge and dump the primer, I put it in a junky old .22 revolver I have, and it popped with one hammer drop.  No primer issues; the problems are purely with the GB-22.  With no other information, I pulled another .22 round apart and repeated.  On the third hammer drop, the round popped.  So one successful primer pop out of five or six trigger pulls.
The round that fired on the third pull.  It's difficult to see, but the indentation that's closest to the rim isn't the one that fired it.  The second and third indentations push into that outer one.  Do I need a stronger spring? 

Before I'd be comfortable taking the gun on the 40 minute drive to the my range, I'd like to resolve a couple of issues.  The first one is the obvious: it shouldn't take five slide drops to fire a round.  I notice in the YouTube videos that it's not that unusual to need to pull the trigger twice.  That would be a big improvement.  The second things is the general fit of the slide.  If I tighten the 10-32 screws in the slide all the way, the slide won't move.  That sounds like there are some burrs or "something sticking up" on the slide that's keeping it from sliding.  The two of these together sound like general "fit or function" improvements.  I have some troubleshooting to do before it's actually usable.


7 comments:

  1. I have to give you major props for even taking on this little project in the first place and thank you for sharing your progress.

    One thing that came to mind when you mentioned that tightening the screws binds the action up is just how similar that is to the gib screws on the compound slide for my mini lathe. There is a fine line there. That also brings to mind many threads and opinions I have since read about dealing with sticky/ binding metal parts that slide against each other.
    They are myriad to put it mildly but one suggestion I might make would be to oil the hell out of it while running it back and forth twenty or so times. That might give you a "shiny spot" or two to see if it is a high spot giving you trouble. As for the spring, and pin, I'm wondering if the pin is actually traveling far enough?

    I'm sure you will dink with it and figure it out but I am pretty damn impressed with what you have accomplished already!

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    1. Thanks for the kind words. This project is around 75% learning experience. The mill is capable of breaking shit in an "ohnosecond". Things that would stall my Sherline won't even slow this one down, so I'm learning lots of lessons about fixtures and holding the work.

      I figure I'll keep learning as long as I keep trying to make things. I just hope to break fewer tools and ruin fewer parts. I have tremendous respect for manual machinists, tool and die makers, and mechanics.

      The pin's travel is set by the dimensions of the parts. It's glued into the middle piece of the slide, and the length it can move is set by the overall design. It's either losing force due to drag holding it down, or maybe I need a stronger spring. A simple test is to wrap some rubber bands around the slide and the front of the barrel holder. Slam that dude down faster and harder. First, though, I'm going to make sure the frame is free of burrs and crap.

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  2. My boy was into RC (Radio Controlled) cars for a while. The local shop has all kinds of spring/strut assemblies. They come in many sizes. One nice thing about a strut assembly is that the spring is pre-loaded and contained. It won't go squirting all over the place when you try to assemble it.

    You might look around and see if you have any RC shops in your area.

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  3. If you had done the 12 gauge zip gun like I suggested, you wouldn't need no steenkin' springs! And all you need is a solid backing block for the breech and it won't be going nowhere. And it would be far more impressive to your rangemates than a piddly little .22lr...
    }:-]

    Glad to see you're making progress!

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    1. Truth be told, I'm remarkably ignorant about all the moving parts in a modern gun. This is a learning experience.

      It's really the first step in a bunch of expected projects.

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  4. Got my first rifle in 1964. Wasn't long before I took it all apart to figure out what makes it go. Been doing that ever since. Firearms are fascinating in the array of mechanisms you run into.
    Looks to me like your firing pin strikes are both too weak and too far from the rim. The priming compound needs to be crushed inside the rim.

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  5. I have access to bins full of springs of all types. Give me a diameter and length so I can find several with different wire gauge and strength (if I locate what you need, will contact for delivery addy.

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