No4 mk2 going awol.

So I went shooting today, practicing for the Dick Travis VC shoot next weekend. I have been chasing an issue with the No4 for a while. It shoots the first 3~5 shots very well, vertical dispersion is under well Under 2MOA, horizontal about 2 MOA. On the next shot it throws 6MOA high and then shoots the next 4 or so up there (but a little more dispersed). The next 4 are not grouped terribly well at all.

So what I have done so far is I found the stock had bananna'd a wee bit and there was a gap under the receiver. So placed a 30thou wood shim the length of the bottom of the receiver and epoxyed it into the well. Now when the main screw is done up tight you can see the imprint of the receiver the length of the wood and even maybe imprinted 5 thou into the wood. The draws seem tight, you have to lightly hammer the end of the forestock into the wedge so that seems tight. The re-inforce of the barrel bed seems tight, front bed OK.

So frankly I am stumped.

Does this symptom suggest what I should to look at to you?

Is the front handguard firmly fixed? If it moves forward under recoil and contacts the front sight band it may be contributing to the shot dispersion.

No its well clear.

If I back off the front main screw and re-tighten it it seems to go back to shooting OK. I keep looking at the draws but for the life of me it seems tight. To get it to wedge in I have to fairly lightly tap it "up". So I could make it tighter but It would then need a lot of "tapping" force.

Stumped...totally stumped.

I have added 5thou shim to the draws and 2thou to the back of the forestock where it comes up against the wrist, its now incredibly tight, almost abuse of the wood to assemble it.

:/

I had a No4 that did a similar thing. On a deliberate at 300 I could get the first 5 in the v ring if the marker was quick enough then the shots started to wander up and right to be into the 3 ring. No method of bedding fixed the problem. An oldtimer after looking at the rifle told me the barrel had been bent and straightened at some time in its life. He explained that as the barrel heated up it would warp under stress. I ended up selling the rifle as there seemed to be no cure.

Well Ive shimmed the draws 5though and the wrist 2thou, lets see if that helps.

Criterion are going to be making new barrels really soon, maybe I have to replace it.