[M14]

It sounds like the stock may be binding against the rifle. I would take a peak down the barrel from both the ends. Shine a light down the bore and angle a small mirror or polished kitchen knife in the reciever so you can peer into bore/throat. If you see lands and rifeling it probably is not the barrel. Field strip the rifle and carefully put back together but keep and eye out parts binding and the stock binding against the barrel. Also check the surface wear the trigger mechanism pinches against the stock as this should be a tight fit.

 

[Trung Si]

1911Rookie! You Probably need a New Barrel, the Stock binding on the Barrel can cause Awful Groups like M14 stated but I have found that it only does it when the Barrel gets hot, not right away!

 

[colt100]

Buy a throat erosion and muzzle wear gage. I would guess that one or both of these readings are off the chart.

Did you just purschase these rifles? Did the rifle shoot fine then the accuracy tapper off with use? Were these rack grades? I would want more information before I said either way what might be the cause.

If you think that the stcok is binding somewhere, I would swap stocks with the better shooing rifle to rule this out.