I was just going back through the blog looking for anything I've written on the GB-22 and it doesn't look like I posted anything since I finished it - for the first time - at the end of May.
According to the in-process pictures I've got on my drive here, I played with it until mid-July when I gave up and thought I'd let it sit until I could figure out what to do next.
The GB has one overriding problem. It just doesn't work reliably. It doesn't really work at all - the cheapest, crummiest firearm you can find works better. If I had to blame it on something, it looks as though the slide doesn't slide forward fast enough or hard enough to fire a .22 round.
All of my tests have been with the same box of ammo, a box of Winchester White Box 555 that I've had since before the Great 22 Scare a few years ago. I drilled a hole in a piece of 3/4" thick oak that's smaller than the rim, but that fits the brass snugly. Push a round into the oak, pull the bullet with pliers and dump the powder into a plastic bowl so that all that's left is the primer. What I'm doing in the shop is dropping the slide on the primer. It's frequently not as loud as a hammer blow on a nail or piece of metal. I've had rounds go off that I haven't even heard, and only when I pull the brass do I realize it's black inside and smells of the primer having been ignited.
I've had the occasional round that fired on the first drop of the slide, but I've had others that I can drop the slide on as many as 10 times without them going off. I then test those in a .22 revolver. Of the 50 or so rounds I've tested this way, maybe two that didn't fire in the GB also wouldn't fire in the revolver and in one of those the primer had broken into chunks that fell out in my hand.
Work for the last few months has focused on getting the slide to move faster. Concerned that the firing pin wasn't leaving as deep an impression as it should, I spent some time trying to remove the original firing pin and replace it with one that is shaped more like the pin in a 10/22 (by the imprint it leaves in the brass). The pin was bonded into its piece of the slide with red LocTite and nothing that I did would break that free, including putting it in my gas oven at 550 for an hour. I eventually drilled a hole on the opposite side of the chamber and put in a second firing pin.
The prints I bought say that the frame should be ".003 to .010" under thickness" of the part of the slide that holds that firing pin. Both of those pieces are nominally 3/16 thick, which means 0.1875". The mill tolerances, though are such that (I suspect) Serbu's sheet came in thinner than his slide piece, while mine came in thicker. My slide is .188"; the frame was .191 or .192. That meant I needed to reduce the thickness of my frame piece down as much as .014. Short of using a surface grinder, how can one do that?
The accepted answer is to put the metal frame on a sheet of rough sandpaper and sand it down. My first attempt was using a Dremel-like tool with a grinding wheel. It ended up being impossible to keep the area flat, so on to the sand paper. First, I wrapped a 1-2-3 block with 150 grit sandpaper and worked on it for several hours. I successfully thinned the frame. Somewhat.
There was a pause here to try to troubleshoot how fast the slide moves, but I was unable to find a camera with fast enough exposures to make meaningful measurements. This was the end of June. Tests at this point showed the same problems. Some rounds would fire with one or two drops of the slide, others would never fire, for no obvious reason. So I said , "*&!% it" and I'll get to it later.
With the eclipse trip over and no other distractions, I decided it was time to address this again. My plan was to resurrect my fixture, put the frame on the big mill and thin the frame out with a large cutter. I would zero the mill's Z (vertical) axis where the frame is the thickest (I chose the red circle on that drawing), then take off .001 at a time until it was uniform thickness or at least being cut the whole way around. If you look at those numbers, you'll see the most important places to cut are both ends of the travel.
I did a careful setup on my tooling plate, added a couple of threaded
holes for 10-32 screws so that I could use some of the Sherline hold
down clamps I have. This picture is after I did the cutting, but shows the
clamps. If you look closely, you can see machining marks over most of the path of the slide, except for a little spot in the skinny beam that holds the trigger; the area visible just above the bottom clamp.
depth of cut was .007. Originally, the tool path didn't cut the
leftmost portion of the frame so it had a .007" step there. I cut the
left end of the frame to the same thickness as the rest of the area
where the slide goes.
After puzzling over what to do to confirm it, I thought the most
reasonable thing to do would be the acid test: put it together and try to shoot some primers.
Not even the slightest bit better. The first primer
popped on the third slide drop. Out of six rounds not one more primer
would pop. All of them popped in the revolver. Midway through the test, I added some oil to the frame to
see if that helped, but nope.
So I took it apart again and got the micrometer to remeasure it.
If a "good" frame is ".003 to .010 under" the
thickness of the center section of the slide, it should be between .185 and .178. It's virtually all there; but a little too thin in the middle three measurements on the top.
So with the frame the right thickness and the GB-22 still a steaming turd, what's next? Is it time to disassemble everything and put it my scrap box? I found and bought some stronger springs, but that didn't work back in June when I tried them. I could try one again. Otherwise, it seems like a total redesign might be in order. If that's even remotely worth it. Any other suggestions would be appreciated.
My only comment would be, you've come to the conclusion the design sucks...so, since you enjoy designing, why don't you come up with your own cheap gun?ReplyDelete
Is this a open bolt, fixed firing pin design with the trigger releasing the bolt/slide on a loaded chamber?ReplyDelete
If so, the variables.
Bolt throw (length of travel)
bolt to frame friction- this seems to be your main focus. Does it slide easily by hand?
And the biggie- ammo- some .22 is notorious for bad primers, especially in ammo that has been stored for a while. Could be the misfires are a normal condition for the ammo, and the gun you are using as a test control has an exceptionally hard strike.
Try a few different new, quality ammo selections. I like CCI blazer (standard velocity) or minimags.
Is the internal radius cut top and bottom on the frame, stopping the bolt/slide before it makes full travel, limiting the pin strike?
I'm a little handicapped in not being able to show you the prints, but internal radius is per the print and the sliding piece is radiused as well. The drawing calls out 0.1 radius. I posted a picture from my 3D CAD showing what the frame looks like hereDelete
The way I've tried to judge this is by looking at how the impact from the firing pin looks in comparison to a round from a 10/22 or the revolver (those two are night and day different). My second firing pin is a blade about .025 thick by .125 long. It's hardened steel dowel pin that I had to grind to thickness with a diamond lap.
Your point, though, is well taken. It's a stupid simple design and it shouldn't be this hard to get it working. I think everything is made to the prints so bolt throw and weight are right. The spring is the one he specifies in the prints. The pin protrusion is per his prints - if anything, it sticks out a bit too much.
The spring is a bit long, so preloaded a bit, and it's something like 7 pounds/inch. It compresses 1 inch when you pull back the slide. I found a 14 pound/inch spring at MSC but it's shorter. I made a little brass spacer to keep that spring captive so that the full inch of compression is compressing the spring. Like I said, it didn't work before but maybe with the slide fitting better it will.
A suggestion that probably won't be appreciated:ReplyDelete
but my bet is that it would work reliably every time...
As I've said before, I think it's highly impractical.Delete
These, on the other hand, are much more ergonomically designed, although not as much as something like a Mossberg Shockwave.
This was designed to show the absurdity of gun buybacks. It's a junk gun that's easily made with the (almost) sole purpose of being destroyed.ReplyDelete
You're overthinking its design.
You're right, of course. There has never been a buyback in this area; I built it to build a simple .22 pistol. Just because.Delete
In his videos, it's not uncommon for Mark to drop the slide twice on a round, and most go off with one drop. I'd be delighted if I could get it to work that well.
Imagine going to a gun buyback and they say, "we're not going to give you $100 for that thing unless it's really a working gun" and they take it to a bullet trap or bucket of sand. If it never fires is it really a gun?
I'm coming in a little late here. Hope I can help
Rimfire are notorious for difficult ignition. It's always a concern in the 22 stages of Bullseye matches. Scrupulously clean chambers, proper firing pins, and clean chamber faces are required.
I think you might be overlooking the importance of proper chamber fit and 100% contact between the rim and the chamber face. Even the slightest burr or angle can prevent proper firing. Check the breech face and the clearance between breech face and bolt face. That clearance should be small. Factory seems to run about .040 to prevent slam fire. My match pistol is right around .025 between breech face and bolt face.
The other dimension is chamber itself. Any interference that keeps the rim from sitting squarely on the breech face will result in misfires. A little polishing goes a long way to fix this issue.
Spring rates can be adjusted as you like.
Trybto get the clearances proper in the firing chain. Test by bumping your "slide" closed on a primed case with a Babbitt bar. If it fires then you are close. After that, move on to spring rate adjustment.
Don't give up. You can make this run.