First off here is setup
oly 9mm upper with c-more scout red dot and m3 light mounted in front of vertical grip socom mag block of course. Have it mounted on bushy upper I have had for 3 years.
So any way had 5 sten mags loaded only 1 kept jamming the other 4 ran great not one problem.
Examining mags the one that jamed the moon shaped notch in back of mag is not cut deep enough.
Here is were I get stumped every once in a while I would get a 3 shot burst? This never
happened with my .223 upper on this lower?
ever happen to any one.

ow well I am extremly happy with the performance.

never had a problem with my Olympic 9mm upper  double'n on any lower I've used. You might check and make sure your firing pin spring is in Bolt assembly.

My Oly upper has been doubling and tripling as well.  The firing pin spring is in place but it may need to be replaced by a stronger spring.

The firing pin hole in the bolt has been peened out into a raised ridge, and I scraped it down thinking it may help the problem.

I know the disconnector works properly because the hammer never follows the trigger when manually cycling the action, or when a .223 upper is on it.  I suspect it is slamfiring somehow.  Check your bolt face for any raised protrusions that could activate the primer.

Well guys I went to the range today.
The night before I flipped my firing pin spring
around and put her back together.
After this I shoot 5 mags and not one double or triple tap?

Oh well we will see how it does next  weekend.