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MKII not reliably firing in auto


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Remove the dust cover from the trigger area and cycle the bolt while you hold the trigger to the rear (unloaded, of course). If the bolt locks back (slips into semi mode), continue to hold the trigger back while you roll the gun over and take a look at what is going in in the trigger assembly. The disconnector could be bent, one of the springs could be bad, or the selector could be missing the little spring-loaded detent that is supposed to keep it either in "full-auto" or "semi" and it's flopping back and forth.

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Regarding the 3rd idea, it was a problem on my first STEN (Mk III). The selector's ball detent spring was on the weak side. And the receiver hole into which the selector fit was on the wide side of spec. Individually, neither would have caused a problem, but together they resulted in a semi full auto gun. (I think Pelosi used that term once.) The selector would jerk side to side in an amount and frequency dependent on the ammo type and who was shooting it. My uncle (who loves anything that drives, flies, or shoots) noticed the pattern. So an easy swap with a better condition selector (thanks Apex!) and the problem was solved.

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Remove the dust cover from the trigger area and cycle the bolt while you hold the trigger to the rear (unloaded, of course). If the bolt locks back (slips into semi mode), continue to hold the trigger back while you roll the gun over and take a look at what is going in in the trigger assembly. The disconnector could be bent, one of the springs could be bad, or the selector could be missing the little spring-loaded detent that is supposed to keep it either in "full-auto" or "semi" and it's flopping back and forth.

I took the whole gun apart, the lever in auto is a good few cenimeters or so from the bolt, and i tried to wiggle it back and forth to see if it had play and it wouldn't budge. The selector has the detent and little ball baring, it has audible clicks back and forth from repetition to auto.

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Regarding the 3rd idea, it was a problem on my first STEN (Mk III). The selector's ball detent spring was on the weak side. And the receiver hole into which the selector fit was on the wide side of spec. Individually, neither would have caused a problem, but together they resulted in a semi full auto gun. (I think Pelosi used that term once.) The selector would jerk side to side in an amount and frequency dependent on the ammo type and who was shooting it. My uncle (who loves anything that drives, flies, or shoots) noticed the pattern. So an easy swap with a better condition selector (thanks Apex!) and the problem was solved.

I might just swap out all the fire control parts and see what happens.

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After viewing your video, I can offer a few comments:

 

1. Don't hold on to the magazine while firing. Sten snobs will laugh at you and make unkind comments and you can cause misfeeds if you put torque on the magazine while firing. The suggested position for your hand is with your wrist under the magazine, holding on to the front of the receiver and the ventilated handguard. FWIW

 

2. Your Sten seems a bit jumpy when recoiling. They are usually very smooth shooters, so I would try a new recoil spring to see if that slows things down a bit.

 

3. Rummage around the internet a bit and find a computer-generated video of the internals of the Sten while firing. Make sure you installed all the parts, especially the springs, in the correct orientation. Something is allowing the sear to pop back up and engage the bolt when it should be held down and out of the way. I wish I could be more specific, but without a look at the guts of the gun it's hard to diagnose a problem remotely.

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Dink has it right.

 

For the jumpiness, the bolt may be hitting the rear of the receiver. Old recoil springs can allow that to happen. That is a BAD THING as it can cause permanent damage to the receiver. Since yours is a post sample, the cost factor is not as important.

 

In addition to causing misfeeds, another reason for holding the gun by the barrel shroud is that using the magazine well as a grip causes lateral dispersion of your burst. That's why some folks think the STEn is inaccurate.

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Every few rounds it will reset the sear as if it was in semi. Pulling the trigger will resume, few more rounds it will do the same.

Replace the trigger,sear spring.The same thing happened to me.You know the spring under the dust cover.Spring is weak,swap it out.

Edited by MGMAN45
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  • 2 weeks later...

 

Every few rounds it will reset the sear as if it was in semi. Pulling the trigger will resume, few more rounds it will do the same.

Replace the trigger,sear spring.The same thing happened to me.You know the spring under the dust cover.Spring is weak,swap it out.

It was the Selector, the space that the sear trip sits in had too much space so there was a lot of side to side play. I pushed it in auto to where it was like a centimeter away from the lobe on the bottom of the bolt. So i figure when firing it was enough to move the selector enough further to the left to trip it. And as to the roughness when firing, I was using a brand new recoil spring. So maybe it was from a bad batch? Thirdly, I know you arent supposed to hold it from the magazine, but I opted out of welding the ejector and put in a MK I magwell ejector. Since its a post sample I'm not really worried about accuracy and etc since the value is very low.

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  • 2 years later...

A little late to the party here, but I had the same issue too. I manufactured a post-sample MKIII that would reset after each round in the full auto position. I increased the spring tension against the sear lever with a stronger trigger spring and that got me up to 3-5 round bursts followed by a reset. I finally fixed my issued by drilling and tapping a bolt into the trigger housing that supports the sear lever in full auto position. Since this is a post-sample with surplus guts and not an original, I have no qualms about drilling and tapping. Now it will run through a whole magazine with no stoppages. 

Sten Bolt.jpg

Sten Full auto.jpg

Sten Semi auto.jpg

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