Mini uzi open bolt running away- possible solutions?

[dawg180]

I have a post sample Mini Uzi I built, used one of the new old stock bolts from McKay, and although it initially worked in testing, the bolt is now running away.

About half the vertical surface of the feet on the bolt have the angled cut on them, so what appears to be happening is in dry cycling the sear feet will catch *just barely* and lock it open properly, but in actual firing the vibration and cycling of the gun will overrun the sear pads as it is hitting the angled ramps on the front of the feet, not the flat surface.

There doesn't seem to be any slop/room with the top cover, although I could try tapping/bending it down a little bit to press the bolt down and make sure it isn't over-riding the sear pads.

I figure my options are:

1. weld up the front of the feet, recut the bottom chamfer angle closer to that on the BarrelXchange bolts (my concern here is heat treating them- I could hit it with a MAPP torch and then dip in water or oil to harden, but don't want the feet to crack/shatter and end up with a runaway gun)

or

2. Shave a bit off the front of the feet, maybe 1/16" which would still keep the bolt behind the magazine and cartridge and locked open, and move back the face of the foot so the start of the angle cut ends up being lower.


Any advice is appreciated!

Dawg180

 

[slimshady]

I'd try re-angling the feet first, if it doesn't work you can then move on to welding them up. If welding, you need to know what type of steel is being used and then the hardening process will be known. Steel type will determine if you use water or oil, as well as how high to re-heat them after hardening to achieve the proper hardness. If you just quench from red hot once, the steel is at it's hardest and most brittle. Tempering at a lower temp "draws back" the hardness down to an acceptable level.

 

[trilogymac]

You might look at the sear holes in the receiver. Make sure the holes are big enough and not preventing the sear from coming up all the way.

 

[dawg180]

The sear holes are plenty big and have no interference with the sear feet.

Re-angling isn't an option as I can't lower highest point of the angle at the front of the feet, which is the issue I am trying to solve.

I need to add material in a way that lowers that point (As I said, I can remove material from the front of the feet which will cause the remaining angled portion to be lower, but is essentially slwoly removing the feet, the whole reason I got this bolt in the first place), removing more is just going to make it worse.

 

[delta-10]

Are you using a sear (mini & micro) that has just one foot (which is all they need because they are closed bolt or two? If your sear has 2 feet is it a semi auto sear or full auto? The geometry is different between the two. I have a registered sear and brought up the geometry question with Troy-Barrel Exchange and he didn't think it would be a problem with mine. I have yet to try it out though in my micro. I still need to modify the trunion to allow the feet to clear it.

 

[delta-10]

Also, make sure your feet can come all the way up. In playing around with various grip frames, I noticed the the ring on the semi-auto ramp would not allow a full auto sear to come up all the way but a semi-auto sear would because there is a wider distance between the feet. I only noticed because I had several guns apart at the same time and was playing "mix & match" and seeing all of the ways parts could & could not fit. I have a registered receiver full size gun with the ramp but ring cut away and saw that they had to modify the bottom of the semi-ramp to allow a full auto sear to come all the way up. Hopefully what I am saying makes sense...............

 

[amphibian]

I have a post sample Mini Uzi I built, used one of the new old stock bolts from McKay, and although it initially worked in testing, the bolt is now running away.

About half the vertical surface of the feet on the bolt have the angled cut on them, so what appears to be happening is in dry cycling the sear feet will catch *just barely* and lock it open properly, but in actual firing the vibration and cycling of the gun will overrun the sear pads as it is hitting the angled ramps on the front of the feet, not the flat surface.

There doesn't seem to be any slop/room with the top cover, although I could try tapping/bending it down a little bit to press the bolt down and make sure it isn't over-riding the sear pads.

I figure my options are:

1. weld up the front of the feet, recut the bottom chamfer angle closer to that on the BarrelXchange bolts (my concern here is heat treating them- I could hit it with a MAPP torch and then dip in water or oil to harden, but don't want the feet to crack/shatter and end up with a runaway gun)

or

2. Shave a bit off the front of the feet, maybe 1/16" which would still keep the bolt behind the magazine and cartridge and locked open, and move back the face of the foot so the start of the angle cut ends up being lower.


Any advice is appreciated!

Dawg180

I'm not familiar with the 'new old stock bolts from McKay'. Were those factory IMI Mini bolts? If so, I wouldn't mess with the feet or the bolt at all.
I would first try messing with the topcover. Get a feeler gauge and get in the lower end of the tolerance. Typically gap should be around .010".

On a somewhat related note, I've been having poor ejection on my Mini UZI with the BarrelXchange heavy bolt that has been reliable for over a decade. Been changing to various extractors and the ejector seems very solidly riveted to the receiver. I think I have it fixed now and may do another thread about it.

Regardless, I used my high speed camera to try to see what is happening. Of course when I tried capturing the jam it wouldn't jam.
The video is interesting to see how much the topcover flexes during firing. Make sure it is in spec in the area where the feet of the bolt engage the sear.

 

[mattnh]

@amphibian - looking forward to your future article!
The video was quite informative - thanks for sharing!!!

 

[dawg180]

Amphibian - I will check the top cover with a feeler gauge, thanks for reminding me of the measurement! Yes, the McKay bolts are new old stock factory Israeli full auto Mini uzi bolts.

delta-10 - Good thought on something underneath like the feedramp preventing the sear from moving all the way up- I have a full auto feedramp in the build so I don't think that is the issue, but I will definitely check. I am using a full auto german lower with dual sear feet.


Thanks guys for all the thoughts and feedback, always great to have knowledgeable people working through troubleshooting an issue!

Dawg180

 

[K2]

Hi All,

Ammo hot enough? Ammo too hot? (You mentioned some significant vibration in the gun.)

Other than that my limited experience with the mini indicated top cover clearance should be kept as tight as possible w/o binding. I put a layer of Scotch 810 tap on the top of the bolt. Without the recoil spring, I set the top cover to where it just starts to drag. Pull off the tape and verify things no longer drag w/o the tap.

Enjoy,

K2

 

[dawg180]

I am running PMC 124 grain 9mm, which has had no issues in my Micro uzi and full size uzi. I will try some other ammo to see if it is a fluke that the mini does not like it!

 

[dawg180]

I ended up welding up the chamfer on the front of the mini uzi bolt feet toa squared profile, cleaned up with a mill and a file, and dropped in the gun to test. Would run fine on full auto, but bolt would not drop on semi (which I kind of expected) I cut a small chamfer at the bottom edge to let the sear feet clear on semi (as they do not drop down quite as far as on full), and bingo! Problem solved, runs like a champ.

I heated up the feet one at a time to just about cherry red with a MAPP torch, then dunked in water to re-harden, and all is good! Might give the bolt a refinish when I paint the gun, but other wise she is done and dumped a couple of mags on both semi and full auto with zero problems.

I had a post sample full auto bolt that would fire full auto on semi and semi on safe, I am wondering if that is a very similar issue of the sear and how far it drops down in each position- just a little in Safe, more in Semi, and completely out of the bottom of the receiver on full. Going to do some testing to see if I just need to weld up and recontour (or just recut) the sear engagement cuts in that one too and get it running properly!

Dawg180

 

[dawg180]

BTW, Amphibian, I am running the BarrelXchange bolt in my Micro Uzi (with a homemade steel top plate with a notch cut in it for the micro uzi top cover charging handle setup) and the ejection clears the port, but it definitely is not "with gusto" as with most guns. The extractor doesn't seem stuck or wedged or anything on mine, at least not anymore than on a standard uzi bolt.

Have you tried pulling the weights to see if it runs faster if that makes the ejection more pronounced? (which it should)