SKS stock fitting and POI shift when warm?

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diegocn

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I have an SKS in Tapco stock (was sold to me this way). Originally the stock to receiver fit was quite loose, and the group was probably 10" or so.

I tried to improve the fitting by filling the rear of the stock with epoxy (devcon 10110), I also created a bedding area around the chamber (where the cross bolt in the original wood stock would sit). This took out all the forward and backward play, and the receiver is actually fitted between the chamber bedding area and the back of the stock, and the front of the stock does not touch the barrel band.

Taking it to range, the group improved to about 5" for the first 5 shots, then it started moving down about 8" once the gun gets hot, producing a 7" group.

I noticed the front of the stock lip, while not touching the barrel band, still have some up and down play. So I took a piece of oiled cork and shimmed that part to remove all plays.

Back to the range again, in big picture nothing has changed, except everything shirnks about 1" (4" initial group, moving 7" down after gun gets warm, and 6" group when warm).

This bothers me as I'm not sure what moves the group down in my setup when the gun is warm. I've studied the original wood stock, it seems I'm putting pressure points on the barrel/action all in the same places. None of my SKSs in original stock develops noticeable POI shift after rifle is warm.

Please any input would be appreciated.

Also personal advise on SKS, keep them in the original stock. I have shot over 10 Russian SKSs all of them are capable of 4", some even 2~3". But as soon as you swap them to an aftermarket stock, accuracy goes down due to poor stock fitment.
 
I find that the best contact points are the cross bolt forward of the action and the rear face of the receiver.
Basically, the firearm should be pinched between these two points.
On a loose wooden stock, I cut a small length of steel hose clamp and shim the rear interface with it.
When trying to engage the barreled action into the stock, you should have to bump the butt onto your work bench to get it to click in.
Then it's tight enough.
This should minimize the worst of your pattern stringing.
 
The barrel to stock fit is very tight, I had to use a mallet to get the action in.

Unfortunately Tapco stock does not have a cross bolt. I was hoping the bedding near the chamber area would serve as the cross bolt, i.e. pulled down in the rear by trigger group and at front with barrel band, and "pivot" on the bedding area.

Maybe I should try to install a cross bolt into my Tapco stock?
 
Some factors to consider:

As the rifle heats up it also heats up the rounds in the magazine, which in turn raises the temperature of the powder in each casing. Heated powder will burn more efficiently and, therefore, hotter. For a rifle like the sks, this can open up groups substantially. The inverse is true in the winter, and with the cold weather groups will tend to tighten up.


But if you're getting that large of a shift in group size so quickly, it could be that the bore is out of spec. As it heats up the the groups will open up. How old is the rifle?

Have you slugged the barrel to get a determination of the extent of wear to the rifling lands? Standard spec for the new production bores is .311"
 
Boris Badinov said:
Some factors to consider:

As the rifle heats up it also heats up the rounds in the magazine, which in turn raises the temperature of the powder in each casing. Heated powder will burn more efficiently and, therefore, hotter. For a rifle like the sks, this can open up groups substantially. The inverse is true in the winter, and with the cold weather groups will tend to tighten up.


But if you're getting that large of a shift in group size so quickly, it could be that the bore is out of spec. As it heats up the the groups will open up. How old is the rifle?

Have you slugged the barrel to get a determination of the extent of wear to the rifling lands? Standard spec for the new production bores is .311"
Click to expand...

You can’t be serious in saying that the ammunition heats up enough in the magazine to cause POI shift.
 
I shimmed a couple sks in wood stocks this year, groups shrunk 50% after shimming. Groups open up a bit when the rifle heats up but not a ton, I was still able to shoot 4.5” 5 shot groups at 200y with irons. With a cold barrel it was 3.75” at 100y, both with bulk Norinco non corrosive ammo.

I’d look and see if the barrel is rubbing in the stock anywhere, once things heat up I’d guess that would make it even worse. I’d want contact outside of the rear and front lugs to be at a minimum to get as much accuracy as possible. It’s an sks though, so you’ll only get so far with it.
 
coleman1495 said:
You can’t be serious in saying that the ammunition heats up enough in the magazine to cause POI shift.
Click to expand...

With a total of ZERO info on the shooting conditions -- is the shooting location exposed to the sun or shaded?, is the bench, Wood or Cement? Was the ammo ideally or poorly stored? Is ammo exposed to sun before loading?etc.-- it's certainly in the realm of possibilities.

However, re-read the second paragraph of my response. Given the how quickly the groups open up, if it's a heat related issue, it is far more likely related to the condition of the barrel bore.
 
diegocn said:
Also personal advise on SKS, keep them in the original stock. I have shot over 10 Russian SKSs all of them are capable of 4", some even 2~3". But as soon as you swap them to an aftermarket stock, accuracy goes down due to poor stock fitment.
Click to expand...

Doubt it. One of my SKS's sits in a Archangel stock, the gun literally slides back and forth in that stock 3/16ths of an inch and yet it will still pattern surplus ammo 3-4 MOA, no POI change outside of norm.

Like others mentioned, u should try putting the rifle in a different stock. If theres no improvement, theres a good chance its the rifles condition itself.
 
diegocn said:
Maybe I should try to install a cross bolt into my Tapco stock?
Click to expand...

This.

Use threaded stock: measure, mark, drill, cut, and install accordingly. It's the only way to replicate the way the groups should be with the original stock.

You can also bed the threaded stock with epoxy or devcon after to snug up the fit.

Mine still shoots 2-3 MOA, but I can hit the 10 x 12 gongs at 300m with ease. Which are exactly what these rifles are designed to do.
 
Bubba Yugga said:
Drop it into a tight wooden stock and see what happens.
Click to expand...
Good idea, I will swap stock with my other SKSs to see if the POI shift still persists. This particular rifle came to me with no original stock.

I’d look and see if the barrel is rubbing in the stock anywhere, once things heat up I’d guess that would make it even worse. I’d want contact outside of the rear and front lugs to be at a minimum to get as much accuracy as possible. It’s an sks though, so you’ll only get so far with it.
Click to expand...
My thought exactly. There's got to be somewhere rubbing for a POI shift like this to happen but I can't seem to figure it out. I mean the barrel is contacting the stock at the barrel band, and the chamber area, but so is the original stock.
 
If that doesn't work you might try removing parts until it stops doing weird things, kind of a process of elimination. If you pull the mag and it stops stringing then that would prove the heat/mag theory.
 
If stock fitting doesn’t help reduce the group size by a fair amount, you could try redoing the muzzle crown. I redid mine with a brass screw in a drill, and I went from 6” groups to 2” at 50 yards.
 
Boris Badinov said:
With a total of ZERO info on the shooting conditions -- is the shooting location exposed to the sun or shaded?, is the bench, Wood or Cement? Was the ammo ideally or poorly stored? Is ammo exposed to sun before loading?etc.-- it's certainly in the realm of possibilities.

However, re-read the second paragraph of my response. Given the how quickly the groups open up, if it's a heat related issue, it is far more likely related to the condition of the barrel bore.
Click to expand...

You will not be able to detect changes in POI if you have the ammunition in the sun vs shade. If you were seeing differences that large that you could tell with an SKS, I would suggest you stop shooting that ammo.

You are simply making stuff up and it isn’t helpful to anyone.
 
coleman1495 said:
You will not be able to detect changes in POI if you have the ammunition in the sun vs shade. If you were seeing differences that large that you could tell with an SKS, I would suggest you stop shooting that ammo.

You are simply making stuff up and it isn’t helpful to anyone.
Click to expand...

Youll have to excuse me, Coleman1495, if I take the word of an expert over your hot air. I take it you don't read much, or shoot very often?

http://bulletin.accurateshooter.com...ffects-pressure-velocity-and-point-of-impact/


In the article the author, Tommy Todd, Chief Ballistician for Sierra Bullets specifically discusses how shade vs sun, ambient temperature, bench temperature and chamber temperature, etc, affect ammo velocity, accuracy, AND Point Of Impact. The rilfe tested was a .308 Class Match rifle with noticeable changes in POI between heated and room temperature ammo. Extrapolate the test results of the match grade .308 for the SKS (a 5-10MOA platform by design) and you get larger groups with even minor fluctuations in ammo temp.
 
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Boris Badinov said:
Youll have to excuse me, Coleman1495, if I take the word of an expert over your hot air. I take it you don't read much, or shoot very often?

http://bulletin.accurateshooter.com...ffects-pressure-velocity-and-point-of-impact/


In the article the author, Tommy Todd, Chief Ballistician for Sierra Bullets specifically discusses how shade vs sun, ambient temperature, bench temperature and chamber temperature, etc, affect ammo velocity, accuracy, AND Point Of Impact. The rilfe tested was a .308 Class Match rifle with noticeable changes in POI between heated and room temperature ammo. Extrapolate the test results of the match grade .308 for the SKS (a 5-10MOA platform by design) and you get larger groups with even minor fluctuations in ammo temp.
Click to expand...

As shown in the article, temperature of ammunition does effect precision and long distance shooters. The effects however are too small to notice with an SKS. The variation in POI at 200 yards is only significant for accurate setups.

The effects do not “extrapolate”. If you notice a difference with an SKS then the ammunition is not safe to use.

It’s becoming painfully obvious you are simply making stuff up.
 
coleman1495 said:
As shown in the article, temperature of ammunition does effect precision and long distance shooters. The effects however are too small to notice with an SKS. The variation in POI at 200 yards is only significant for accurate setups.

The effects do not “extrapolate”. If you notice a difference with an SKS then the ammunition is not safe to use.

It’s becoming painfully obvious you are simply making stuff up.
Click to expand...


Keep the hot air coming, c1495.


Here's the extrapolation in as simplistic terms as I can muster:

Temperature's affect on POI will vary with the accuracy of the rifle. The less accurate the rifle -- either by design or wear-- the greater the affect ammo temperature will have on accuracy, and POI.

Which rifle do you think is the least accureate of the two, Coleman1495? A class match rifle or a 50 -70 year old SKS with an 5-10MOA by design and an indeterminate amount of wear to the the bore?


I can't really dumb it up for you any further.
 
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To the OP- if it truly is heat related issue and it is occurring after so few rounds, it'd be a good idea to slug the barrel to determine the extent of wear to the rifling.

There are literally hundreds maybe thousands ofarticles and videos discussing the effects of ammo and rifle temperature on accuracy. If the bore is severely worn, temperature will be a major limiting factor on its accuracy.

There are brands of ammo (dont recall which ones) with modestly larger diameters that will provide some improvement to accuracy in a worn barrel bore.
 
Boris Badinov said:
To the OP- if it truly is heat related issue and it is occurring after so few rounds, it'd be a good idea to slug the barrel to determine the extent of wear to the rifling.

There are literally hundreds maybe thousands ofarticles and videos discussing the effects of ammo and rifle temperature on accuracy. If the bore is severely worn, temperature will be a major limiting factor on its accuracy.

There are brands of ammo (dont recall which ones) with modestly larger diameters that will provide some improvement to accuracy in a worn barrel bore.
Click to expand...

I don't believe this is a factor in my case because all other SKSs shoots the same ammo consistently during the same range trip. Neither the gun nor the ammo was too hot to touch. None the less it is a interesting read. Thanks for showing us something new.
 
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