Help zeroing my Sten

[5 Commando]

All. I have acquired a Sten MKII that is quite innacurate. At 25 yards poi is about 15 inches high and to the right of poa, but with tight grouping. I have rotated the barrel (original in extremely good condition with sharp rifling) but at 90° rotation points there is no improvement. The rest of the weapon is in nice condition and functions perfectly in all other respects. What would cause my Sten to be so off? I've fired a few others and never had such deviation. Is there anything that was done with these guns at the armourer level to correct such a deviation?

Information

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[Peter Laidler]

Shoot it with UK/NATO Mk2z ammunition and then come back and tell us how it shoots. 15" high at 25m is so far removed from reality to he true

[5 Commando]

You're right, that should have read 15cm. I'm shooting Fiocchi 123gn and it cycles fine with that...

[5 Commando]

Further to the issue which may help with the trouble shooting. I took out the innards of the weapon and boresighted it at a point 25 yds away. The barrel does indeed point high and to the right of the sights. So it now seems as if the barrel is on skew. Beginning to wonder if, by a process of illumination, the trunnion is the problem here.

[tiriaq]

A higher front sight would deal with the elevation. Shift the sight to the right to move the point of impact to the left.
Is the front sight welded in?

[5 Commando]

It is welded, and quite convincingly so. Not sure what is an acceptable level of accuracy for the Sten, but 15-20cm at 25m seems excessive to me.

[browningautorifle]

If you go out to the range and adopt a standing, waist shooting position...using short bursts of three or four...you'll soon find out what the acceptable accuracy is. Also you'll get pretty good at instinctive shooting. Hard to say what's happened to the gun over the 70 odd years... Put the ruler away, it's an SMG. Wartime production...

[5 Commando]

Yes Browningauto/Jim. Good advice. Amazing how accurate instinctive shooting is. I ran 3 mags dry using short controlled bursts. Wanted to check that the mags were good, but also see if it cycled properly and using a chrono, what the cyclic rate was. All came in exactly on spec with zero malfunctions. My target results were much the same as firing supported in semi mode - high and to the right, but with obviously greater dispersion. You're exactly right, it is hard to know what happened to the gun the past 70 odd years, but she is in remarkable good condition with an excellent barrel. So remarkable that I wonder if her innacuracy spared her useful life...

[Peter Laidler]

Could the problem be an eccentrically bored barrel? This was quite common and I had a chapped and machined Bren barrel showing the end elevation about 6" rear of the muzzle showing how eccentric it was. In armourers terms, we marked the BEST radial position of the barrel with regards to 'accuracy - such as it is - with a chisel mark at 12o'clock. The Sten barrel seating is soft soldered in position and rivetted. They did work loose but it was an easy fix.

Worth pointing out to you 5Commando that it's not a case of whether your Sten works and functions on commercial ammo. It may well do. But it cannot be zeroed using it. It was made to operate etc etc on UK Mk2z ammo. Try this

[5 Commando]

Yes sir. Note your remark on the ammo. I cannot easily source Mk2z so am going to work with what I can get.

Regarding eccentricity. Not sure if this is what you mean, but I ran a crude test on the barrel by rotating it in the trunnion and it appears to rotate evenly when sighting down the bore from above. By evenly I mean it didn't appear to the naked eye to deviate around the long. axis.

I do observe the following however: when fully seated, the base of the barrel appears to be visibly higher in the trunnion on the right side. This would - in my mind - support why it is hitting to the right. The vertical deviation I suspect is normal if the weapon is sighted in at 100 yds. Here a photo...

Attachment 81503

Could the problem be an eccentrically bored barrel? This was quite common and I had a chapped and machined Bren barrel showing the end elevation about 6" rear of the muzzle showing how eccentric it was. In armourers terms, we marked the BEST radial position of the barrel with regards to 'accuracy - such as it is - with a chisel mark at 12o'clock. The Sten barrel seating is soft soldered in position and rivetted. They did work loose but it was an easy fix.

Worth pointing out to you 5Commando that it's not a case of whether your Sten works and functions on commercial ammo. It may well do. But it cannot be zeroed using it. It was made to operate etc etc on UK Mk2z ammo. Try this