i have a dpms varmit upper. 24'' heavy barrel, 1 in 9 twist. when it was new it would shoot under an inch at 200 yards. lately more like 2 inch + groups. bore does not seem to be copper fouled. upper has been shot maybe 700 or so. i need advice as to what the problem might be. i've tried cleaning barrel with everthing i have and it hasn't helped. what can i do to the upper to get it shooting good again.