Reshimming and POI Shift

Pr589

Pr589

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So a few weeks back my M305 started having its groups open significantly and I started to see significant stringing. Thought it was just a barrel tension issue (the receiver is in a Blackfeather stock and I've been playing with the barrel tension). So after much ammo downrange and no improvement I tore the rifle down to see if I could detect an issue. To my surprise, the gas system shim had disintegrated.

The shim on the left is what was left, the shim on the right is a new one.



After re-shimming and reassembling the rifle I took it to the range for a test shoot. The POI shifted up by about 12" and the groups have stopped stringing but I'm seeing groups with much more variance than usual (1.0" to 2.5") and the the POI seems to wander. While these are pretty good groups, they are larger than I'm used to.



Any ideas?

I'm thinking that barrel harmonics may have changed and I need to confirm my loads but this doesn't seem right to me.
 
Is there a difference in ammunition and ambient temperature from when you did your load development to when you are shooting now? Just a shot in the dark
 
canadapanda said:
Is there a difference in ammunition and ambient temperature from when you did your load development to when you are shooting now? Just a shot in the dark
Click to expand...

The ammo is the same but temperatures have changed. The thing is that this ammo was shooting well through the end of last summer up until say the end of February. That's probably a 30 to 40 degree range. I'm using Varget which is supposed to be relatively temperature insensitive.
 
My thoughts,,,check your gas piston deep inside for carbon buildup, do you have drills for cleaning it,,,, and give your barrel a good cleaning.

Do you use a muzzle bore guide when cleaning,,,thinking crown damage??

I rarely, maybe once a year disassemble my rifle.... M14's hate that...

I only remove the gas plug for cleaning and use a cylinder wrench and never move the cylinder lock.

Check flash hider, front and rear sights for tightness,,,I've had the flash hider loosen and that really throws shots,,

Hope this helps,,,good luck
 
In the early days of my compulsive disorder with the M14 I had a similar issue with shifting pOI and shims
I went to the US gurus for info and found many did not use shims at all. Instead, the gas band shoulder was gently roll peened on an anvil of some kind.
This peening pushed metal forward and allowed for a solid and adjustable shoulder to reseat to. Negating the need for shims.
The solution is permanent, meaning do it once and forget about it.
Unitizing also helps prevent shim deterioration as it stops the gas band shifting that happens every shot (no matter how tight gas lock is)
Shims deteriorate and need to be replaced from time to time. It's an unfortunate fact I'm afraid

I'd also agree with a few other posts here regarding possible piston/plug carbon build up.
I'd also put it in a normal stock for a test to rule out the chassis
 
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Thanks all for the ideas.

I wonder if one can use some kind of metal rod/dowel as the anvil to roll the shoulder on the front band. I'd think that if one over-did it, one could sand the peened surface back to the proper thickness.

BTW, I've cleaned carbon and copper from the barrel since I've started to experience issues but it doesn't seem to cured the "problem" - no bore guide but I'm pretty careful and I like to think that the flash hider helps. I also clean the gas plug and the inside of the cylinder every 200 rounds or so using some drill bits. While I haven't tried H4895, IMR4895 was OK but Varget and 3031 was better. I think Varget is OK as long as the load is mild - I'll change my mind if I bend my Op Rod.

Anyone know someone in or near Toronto who can unitize the gas system for me? Does Casey at TacOrd do this kind of work?
 
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AG welding/boiler/steamfitting services in BC
Most experienced in the country at unitizing.
Not a gun shop at all .... But a bunch of true gun nuts own the company hehe
If yer stuck, I can pass on contact info.
 
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