Yesterday at a local 800 aggregate match I had a serious accuracy problem while shooting rapid sitting.  On both strings, 7 of 10 shots missed the paper completely.  String one 10, 9, and 7 were the  hits,  while string 2 had a 10, 8 and 5.  The pit said the shots were all over the place, high, low, right and left.  The load was the same for standing in which all shots, except the 17th, were on call.  This 17th was also a miss in which my call was a high ten.  thought at the time that I may have cross fired––-I've done that before.  But after what happened on rapids, I'm not so sure.  My rapid sighters were also on call and both 10s.  My hold for the rapid shots should have been near cleans, maybe on or two nines at most, and none should have been out of the black.

The sights were checked after the first string to see if they had come loose, but were nice and tight (WOA spacegun upper). After the second string I pulled the upper and bolt and looked down the barrel.  There was a bit of debris (looked like a bit of unburned powder) about 1 inch from the muzzle.  I though it a bit unusual, since this rifle typically leaves almost no unburned powder.  Before the rapid prone, I ran a bore brush down the barrel back and forth once to remove this bit of debris to see if it were a rock or something embedded into the rifling.  This cleared the debris, but I did not find a rock on the tailgate or anything else like that.  I then shot my rapid prone with near clean sized groups.  (If only the nut behind the buttplate had moved his sights in the right direction for the second string  he would have dropped only one point on that string.)  My prone slow score was the best I've done to date 196 - 7X.

My 200 yard load was slightly different from my normal one in that I ran out of IMR4895 and switched to H4896.  This powder with 24.0 grains propels a Sierra 53 grain Matchking.   My 300 yard load uses Reloder 15 and a 69 grain SMK.  Likewise, my 600 yard load also uses Reloder 15 but with an 80 grain SMK.

The rifle barrel had not been cleaned for about 390 shots before the problem manifested itself.  However, I would think that if the barrel had been too badly fouled the accuracy would have gradually deteriorated––-not suddenly, but I could be wrong.  Maybe there was a piece of debris that was affecting the bullets badly enough to cause this, but again I'm not too sure.  It might also be possible that the bullets were being pushed into the case upon feeding, but again, this does not seem to account that much change of impact nor the wide dispersion of shots.

If anybody has had this kind of experience, or has any other ideas as to what may have caused this, I'd appreciate hearing about it.

That is weird. I've never had that problem. I wouldn't think that some un-burned powder would cause that much problem. Back when I used to shoot highpower, rapid sitting was where I shot the best. I would probably try benching or sandbagging your rifle, and see if it does the same thing, only taking you out of the equation. Try some of your reloads that you were using, and maybe some good factory rounds, and see if it happens again.