Uzi with post sample bolt acting...oddly - full auto on semi, ruanaway gun on auto!

[dawg180]

We just got a post sample bolt with the bolt face opening up to clear the restrictor ring and the slot for the blocking bar.

Built up an Uzi with a McKay semiauto receiver, standard DES marked Uzi lower that bolted right on after relieving the inner edges of the sear slightly so it didn't bind on the restrictor ring. trimmed the red buffer on the spring so it clears the blocking bar. Installed wood detacheable stock.

Dropped the bolt in, to a function test-

1. pull bolt back, leave on safe, pull trigger, stays in place.

Switch to semi, pull trigger, drops bolt, pull bolt back and stays locked back, release trigger, pull trigger, drops bolt again.

Switch to full auto, pull bolt back, locks back, pull trigger, drops bolt, keep pulling bolt back, continues to drop forward.

All looks A-OK.

Take to range- Safe works just fine.

Pull trigger on semi and...it fires in full auto until you release the trigger, and when you release the bolt locks back ready to fire another burst. Basically full auto function on semi. Huh?

Put on Full Auto, pull trigger, runaway gun- you can release trigger and it just keeps going, trigger is basically dead and not resetting. Huh???


If it just ran away on full auto, I would tend to think it was not cycling the bolt fully, but the literally perfect function of full auto fire on semi really is throwing me here.

Top cover is tight and appears to be holding bolt in position just fine. Grip frame is nice and snug. Feeding and ejection is perfect from what I can tell.

Tried three different types of ammo including blazer, PMC, and one other that slips my mind.


So any of you full auto gurus know what is going on here? I figure there is something obvious I am missing.

Any help is appreciated!

Sven

 

[dawg180]

Did some further searching and will check the bolt to topcover gap tomorrow and adjust if necessary- it seems to be a tight fit and a hair long on the mckay receiver and might have to relieve the rear edge just a hair to prevent it from bowing upwards when smacked into place.

If that doesnt solve it a complete new German grip frame and top cover from RobertRTG seem to be the next step.

 

[mike]

It’s possible the sear holes on the lower are not opened up enough to let the sear move freely and getting hung up on the holes?

 

[mmi]

could it be a Connecticut gun non select fire gun ?

 

[dawg180]

It’s possible the sear holes on the lower are not opened up enough to let the sear move freely and getting hung up on the holes?

I actually opened them up slightly to prevent some binding on one side that I was having- I will check to make sure it allows the sear to go fully up, good idea.

 

[mike]

One more thing with the bolt and top cover off looking at the hole clearance and pulling the trigger and the sear is just clearing the holes but under recoil if the lower has any movement the sear will rub on the holes and will bind on the holes holding it down.The holes must be opened up wide enough to allow for that movement of the lower if there is any. Good luck.

 

[dawg180]

Top cover was the issue- the bolt to top cover gap was a good 0.030" !!! :O

Couple of whacks with a deadblow hammer on the top cover to adjust it brought that down to 0.009", tested on semi and full auto and everything seems to run fine now.

Looks like the bolt was able to override the sear but now is fixed, thanks for the advice!

 

[dawg180]

Took it out to the range today and it ran away again. Bolt to top cover gap is fine, so it has to be a sear to bolt interface issue.

I noticed the sear notches in the registered bolt are a bit deeper and differently shaped than on the SMG bolt, where registered bolts typically cut to work with a semiauto sear?

Not familiar with all the different sear designs and geometry and thinking this might be the issue.

 

[glockman]

I have a bolt gun that was running away on me, And it turned out to be the interface between the bolt and the sear. I replaced the top cover with a surplus one, and it had no effect really. It was speculated that it may possibly be the ammunition that I was running was underpowered, as I was running some of the Remington UMC green box that is notoriously underpowered for subguns, and that was not it either. I was lucky enough to have a member here hook me up with a SMG surplus lower that was drilled out to accept the bigger receiver pin. With the SMG sear that was in the lower, the holes in the bottom of the receiver had to be opened up a bit to allow the ears to get all the way through to engage the sear with no obstruction. Gun runs 100% now with any kind of ammunition that I throw at it, and most importantly, starts and stops on demand!!!!!!!

 

[jurist]

A post sample bolt most likely is just a full auto bolt modified to clear the destructor ring and slotted it should work with your full auto lower as long as your receiver holes are opened up for the sear