SKS zero moving down after 5 shots?

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NuJudge
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SKS zero moving down after 5 shots?

Postby NuJudge » Fri Jul 03, 2009 3:23 pm

I was wondering if anyone else has had this experience.

I purchased the rifle some years ago and it has sat in the back of my safe. It looks as if it was rebuilt and refinished by the Chinese before shipping to the US. A club I belong to has a 200 yard range and frequently runs theme matches, one recently specified only SKS and AK rifles and the usual National Match course of fire. I sighted in the day before with decent quality 7.62x39 (recent S&B boxer primed stuff), shooting 5 shot groups until it printed where I wanted it to.

During the match, this thing kept happening. Having a Kowa 821 spotting scope, it's no problem to see holes at 200 yards. The rifle grouped well for the first 5 shots, then suddenly the Point of Impact shifted to a very loose group just below the aiming black. The range does not have bunkers into which the targets are pulled after a shot or after a string of shots, one has to walk down and score the target and paste up shot holes before the next stage, so a considerable period elapses between stages. At the beginning of the next stage it would again shoot where it was supposed to, but only for 5 shots, then down they would go.

The last stage is 20 shots, prone, slowfire. After 5 shots the POI shifted down again, and I corrected as best I could by switching from a "6 O'Clock" hold to a "Center of Mass" hold. At about the 12 shot, the POI was again down below the aiming black, so I fired the rest of the shots in the match allowing just a teeny bit of the aiming black to appear on top of the front sight.

During cleaning, the gas tube above the barrel was really tight. I'm wondering if the gas tube somehow was heated a lot during firing, more than the barrel, the gas tube increasing in length more than the barrel constraining it, forcing the muzzle lower? I use a relatively loose sling, so I don't think that was it. I've not noticed the gas tube being particularly hot on other SKS rifles after firing multiple shots, but in addition to staying away from things that may be hot, I stay indoors during rainstorms.

If anyone has any thoughts, I would appreciate hearing them.

CDD
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oldironsights
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Re: SKS zero moving down after 5 shots?

Postby oldironsights » Sun Jul 05, 2009 9:35 am

NuJudge, sounds like you have diagnosed your problem.
There should be a few thousands of an inch of play between the gas block & gas tube when the carbine is cold.
When cold, you should notice some slight sliding movement of the gas tube on the gas block.
Either change the gas tube with one that allows for expansion, or carefully file away the offending amount of metal from the forward end of the gas tube.
I have found those older Chinese SK's to be very accurate.
My 1967 model with aperature sights would shoot 2moa all day long @100YDS.
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A square 10
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Re: SKS zero moving down after 5 shots?

Postby A square 10 » Thu Feb 11, 2010 7:48 pm

i was about to say these are not noted for small groups , but that last statement - 2MOA @ 100 "all day long" has been my experience as well , so i will just second the sugestion :SCO:

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