Premature ignition closed-bolt FA Uzi

[Garrett]

I also posted this over on Subguns.com

I've been tinkering off and on with a closed-bolt setup for my Uzi for the past couple of years now. I mess with it for a while, get frustrated and put it away, and then pick it up again several months later.

The ultimate goal is to get the ROF slowed down to something manageable, and at the same time get it to run reliably. I'd like to use it to shoot the closed-bolt match at the Creek. I've got the ROF down to where I can pull doubles all day, and sometimes I can pull singles as well. In standard "factory" closed-bolt setup, the best I can do is a 4 or 5 round burst.

However, it's never been terribly reliable in this setup, whether in the slow-fire mode, or in the "factory" high-speed mode. The most common issue is with the empty brass getting caught in the ejection port. Obviously, the bolt velocity is slower, having to push against the extra striker spring (or so I presumed). Low-speed subsonic ammo won't hardly run at all in this setup. So I started loading some fairly hot ammo, and this seemed to help somewhat. The hotter ammo cycles the bolt harder, causing the case to hit the ejector harder, and kick it out of the ejection port more positively.

Additionally, it sometimes does a weird thing where the gun will fire, but the bolt will stay forward, basically jammed shut. You have to hold the charging handle and slam the butt of the gun against the ground to break the bolt loose and eject the round that's stuck in the chamber. I hadn't figured this one out until today.

So I was at the range today, trying out a different loading of "hot" ammo, and noticed something. It appears the low bolt velocity is because the gun will occasionally ignite the round too soon, before it is completely chambered. I know about "advanced primer ignition" on open-bolt subguns, which reduces felt recoil. The closed-bolt Uzi also operates in this manner after the first round in a burst is fired. However, it seems to be igniting some of the rounds a little too early, as the rear of the case swells, and then is forced into the chamber. This would cause the slow bolt velocity, as it has to overcome the sticking, oversized case. It also explains how the bolt sometimes will simply stay shut, with the oversized case not releasing from the chamber.

My gun is an original Group Uzi (pre-Vector), and I've been using the factory Group barrel. You can still see light machining marks in the chamber. I tried it with a couple of surplus SMG barrels, which have smooth chambers, and it seemed to run better. But I still found a few cases that looked like they were bulging a little.

I also ran some Winchester factory ammo, and while I didn't have any failures (only ran one mag) some of the brass still looked like it might be doing the same early-ignition thing.

You can also see the difference if you watch the cases ejecting as I fire a burst. The first case (chambered by hand) flings out of the gun, while the following cases just roll out the side. So there is obviously still something going on there.

I just checked and the firing pin on my open bolt protrudes .027" up from the breech face. The pin on my closed bolt protrudes .050". So this might be the issue. I'll have to try cutting the pin back to see if this takes care of the issue.

Anything else that I could try checking? Anyone ever run into this issue before?

I'd post a picture of the brass, but Photobucket is having issues not wanting to upload at the moment.

 

[JWH 223]

Try thinking about Baseball.....




I'm sorry, I couldn't resist after reading the title....don't hate me

 

[medphys]

I just checked and the firing pin on my open bolt protrudes .027" up from the breech face. The pin on my closed bolt protrudes .050". So this might be the issue. I'll have to try cutting the pin back to see if this takes care of the issue.

Since I don't own one of these, this question may seem silly, but I enjoy troubleshooting things so I will ask it anyway.

Does the firing pin protrude from the bolt face at all before the bolt closes?

I always assumed that the pin would be entirely within the bolt upon closing and then released shortly after, but maybe the CB Uzi doesn't work that way? I read c3junkie's site and the UT bolt section and couldn't really determine it's operation.

If it does stick out and it's not supposed to, I think your idea to shorten the pin is logical. Kind of like timing a lightning link in an AR. If it doesn't stick out, I suppose it still could be shortend just to try it out.

Do you have an extra pin to compare/modify?

 

[root]

After the 1st shot it's all open bolt slam until you let off the trigger!

the only thing holding the firing pin back is your finger other than that it's gonna operate as a open bolt slam fire after the 1st shot. It;s faster 'cause it's lighter.

Later

 

[Garrett]

After the 1st shot it's all open bolt slam until you let off the trigger!

the only thing holding the firing pin back is your finger other than that it's gonna operate as a open bolt slam fire after the 1st shot. It;s faster 'cause it's lighter.

Correct on all but one point. The CB setup does operate the same as an OB once the first shot in a burst is fired, with the firing pin protruding through the breech face the entire time.

However, it's not faster because the bolt is lighter. It's marginally faster from the added spring pressure from the striker spring. But it is mostly faster because the rear of the striker assembly and the buffer take around 2" out of the bolt's travel. On a subgun, that's pretty substantial. It's like a MACJack on an M11, but more so.

Here's a picture of the two so you can see what I mean.

This is the stock CB setup anyway. I've modified another set so it slows the thing down to at least manageable levels.

 

[root]

Garret
It makes sense.
only NFA's I have Xperience with are a B&G cut from the front to make a mini bolt and a closed bolt slotted.

'I always figured the material taken from the bolt made it faster never thought of the spring for the striker.

Lesson learned and will keep it as "now I know why"

 

[BWE Firearms]

What is your extractor tension like? If you increase the tension slightly it will keep the round from snapping under the extractor (and firing) longer letting the round chamber further. Get it too tight and it wont close and fire.

 

[az_gun_nut]

Sounds like soft primers to me. You mentioned you were reloading, so I'd look at switching out the primers to CCI's, they are harder than most and should eliminate that problem.

 

[Garrett]

Sounds like soft primers to me. You mentioned you were reloading, so I'd look at switching out the primers to CCI's, they are harder than most and should eliminate that problem.

The ammo I've run through it most recently used Winchester small rifle primers. I don't think primer hardness is the issue.

 

[Garrett]

What is your extractor tension like? If you increase the tension slightly it will keep the round from snapping under the extractor (and firing) longer letting the round chamber further. Get it too tight and it wont close and fire.

There might be something to that suggestion. In a past attempt at diagnosing the issue, I did experiment with swapping extractors. I believe I used three different extractors, with the other two coming out of working SMG (open) bolts.

It's something to try though. The more I think about it, the more I'm thinking the firing pin is the next place to try. I'll shorten one by .020" or so and hopefully get a chance to try it out this weekend. Or possibly try a short one and then a long one and see if I can see any difference.

 

[Garrett]

Getting better - but still not all the way there.

I shortened the firing pin by .020" and rounded and polished the tip. My reloads were running better, with the ejection more consistent. Not like before when the first one that would kick out and land 5 feet away, and the rest of the brass in the burst just rolling out the side.

I still had one empty that got stuck in the chamber and locked the bolt closed. Still scratching my head over that one.

I had a handful of light primer strike failures, but I believe they were where the bolt had not closed 100% and the striker pushed it the rest of the way closed. This is on the slow-fire conversion, and I'm using a cut-down SMG spring to run the bolt. I may have to go a little longer on that one, to make sure I've got enough to strip a round and close the bolt reliably. Then again, these were all during a burst, so I've got the M16 buffer spring pushing the striker as well. Should have no problem closing the bolt. Hmmm.

I ran a couple of mags full of Win White Box and it ran really well. None of the brass (WWB or reloads) showed signs of the pregnant bulge, so hopefully shortening the firing pin took care of that.

I switched back to the standard (high-speed) closed bolt setup, and the gun started jaming left and right. It was actually running pretty well in this setup the other day. Not sure what happened there either. It may have just been getting dirty. It's probably had at least 2000 rounds through it since it was last cleaned. But then, it's an Uzi. Cleaning is usually optional.

 

[Showboat]

Can sombody that has a SA Model A Uzi, 45acp, that works properly, put a feeler gauge to the extractor/bolt and let us know what you get for gap.

I really would appreciate this!

 

[Surly]

I had a handful of light primer strike failures, but I believe they were where the bolt had not closed 100% and the striker pushed it the rest of the way closed.

If you shortened your firing pin, that's where your light strikes are coming from. You might look into buying a new one.

I know there is a difference between semi and FA extractors...have you tried a semi extractor ion your closed bolt? Is there a lip at the base of the bolt for stripping the rounds (like a FA bolt) or is it smooth like a semi?

You say WWB runs fine...maybe your reloads are a little too hot and they are causing the bolt to open prematurely due to the lower mass and possible decreased spring pressure.

I'd stick to lighter loads, get a fresh spring set, replace the firing pin, and try a semi extractor. After everything is up to spec., try a shorter buffer to increase the time the bolt is cycling. This may help if your firing pin plate is not quite catching the sear feet and following the bolt home...providing it has the lip like a FA bolt.

 

[az_gun_nut]

If there is no bottom lip on the closed bolt it will not run due to the rounds hanging up on the exposed firing pin. Which could be why you're getting light primer strikes. On a Fullauto UZI, whether it's open or closed bolt, you need that bottom support lip to keep the rounds from riding up on the bolt face and hanging up on the firing pin.

As far as the extractor goes... If you increase the tension you are going to slow the bolt down as it forces the extractor over the rim. If anything I would lighten up the tension not increase it. Don't use a Semi Extractor, that shouldn't make any difference. My closed bolt setup uses a Fullauto extractor.

You might want to try a stiffer striker spring perhaps? You're might be too weak to keep tension against the bolt. Just a thought.

 

[Surly]

If there is no bottom lip on the closed bolt it will not run due to the rounds hanging up on the exposed firing pin. Which could be why you're getting light primer strikes. On a Fullauto UZI, whether it's open or closed bolt, you need that bottom support lip to keep the rounds from riding up on the bolt face and hanging up on the firing pin.

As far as the extractor goes... If you increase the tension you are going to slow the bolt down as it forces the extractor over the rim. If anything I would lighten up the tension not increase it. Don't use a Semi Extractor, that shouldn't make any difference. My closed bolt setup uses a Fullauto extractor.

You might want to try a stiffer striker spring perhaps? You're might be too weak to keep tension against the bolt. Just a thought.

If the firing pin is protruding all of the time it's going to be an open bolt when the bolt drops because it will set off the primer. The bolt will close and the firing pin will either be retained rearward in semi or fall with a delay in full. I may be completely wrong, but that's how all of the other closed bolt autos function that I've worked on and handled. Thje sear causes the delay. What does the Uzi use for a sear?

 

[az_gun_nut]

If the firing pin is protruding all of the time it's going to be an open bolt when the bolt drops because it will set off the primer. The bolt will close and the firing pin will either be retained rearward in semi or fall with a delay in full. I may be completely wrong, but that's how all of the other closed bolt autos function that I've worked on and handled. Thje sear causes the delay. What does the Uzi use for a sear?

Here's how a closed bolt UZI works. The sear holds back the striker unit not the bolt, so when firing in fullauto the striker travels with the bolt as one unit and fires the same as an open bolt which is why you need to have a supporting lower lip on the bolt face. When firing in semi mode the striker locks to the rear but the bolt returns to closed, same as on the first round for fullauto, (when you cock the gun the striker locks back and the bolt closes. Otherwise anyone could make a semiauto into a fullauto very easily. The cartridge does not slide up under the extractor, the extractor snaps over the rim just the same an an open bolt does. Trust me... I've run thousands of rounds through mine and that's how it works.

 

[Surly]

Here's how a closed bolt UZI works. The sear holds back the striker unit not the bolt, so when firing in fullauto the striker travels with the bolt as one unit and fires the same as an open bolt which is why you need to have a supporting lower lip on the bolt face. When firing in semi mode the striker locks to the rear but the bolt returns to closed, same as on the first round for fullauto, (when you cock the gun the striker locks back and the bolt closes. Otherwise anyone could make a semiauto into a fullauto very easily. The cartridge does not slide up under the extractor, the extractor snaps over the rim just the same an an open bolt does. Trust me... I've run thousands of rounds through mine and that's how it works.

I stand corrected. It sounds like he'll need a new pin and stronger spring to get it to quit misfiring in that case. :thumbs_up

On a slightly off-topic note...would I be legal to convert a semi bolt by adding the lower lip for my SMG? I'd like to mess with a closed bolt system to change things up a little.

 

[Garrett]

I stand corrected. It sounds like he'll need a new pin and stronger spring to get it to quit misfiring in that case. :thumbs_up

I don't think that's it. If you read my original post, you will see that even with the pin shortened, it still protrudes out of the breech face farther than the pin on the standard open bolt. It could be the spring, but I doubt it, just based on what I've got in it for a striker spring currently. I'm still thinking it's happening when the bolt isn't closing completely. I think I need to go to a stronger recoil spring to ensure the bolt is closing completely.

On a slightly off-topic note...would I be legal to convert a semi bolt by adding the lower lip for my SMG? I'd like to mess with a closed bolt system to change things up a little.

You would need to permanently "plug" the blocking bar slot first. Otherwise you would be building an illegal conversion bolt. I'm not sure, but I think rather than modifying a S/A bolt, Vector at one time was converting SMG bolts to CB operation. They may still make the conversions if you ask.

 

[RoverDave]

On a slightly off-topic note...would I be legal to convert a semi bolt by adding the lower lip for my SMG? I'd like to mess with a closed bolt system to change things up a little.

If you add a lower lip to a semi-auto bolt, you also have to fill in the slot cut in the side of the bolt so it can't be used in a semi-auto any more.

 

[Surly]

You would need to permanently "plug" the blocking bar slot first. Otherwise you would be building an illegal conversion bolt. I'm not sure, but I think rather than modifying a S/A bolt, Vector at one time was converting SMG bolts to CB operation. They may still make the conversions if you ask.

If you add a lower lip to a semi-auto bolt, you also have to fill in the slot cut in the side of the bolt so it can't be used in a semi-auto any more.

Sounds great...i figured there would be a catch. :thumbs_up