You have gotten some good advice so far. Some of it may indeed solve your problem. But I would start from step one and move on.
Here is what would do:
1) Disassemble and thoroughly clean rifle. Action & barrel. (FAQ's give pointers)
1a) Pay close attention to the barrel. Remember, this was purchased used. You have no way of knowing the PO treatment, esp being a 223, I'll bet it had a lift of surplus or at the very least cheep ammo. I'll bet its badly fouled.
1b) Remove scope AND base. Clean screw holes of all debris and oil. Reattach base with lock-tite. If you have another scope, even if you have to remove from known good shooter for this test, do so. If no scope available, try open sites. OR maybe check the SCOPE on another rifle that is a known good shooter.
2) Check the muzzle, look close! Even utilizing a magnifying glass and good light. It doesn't take much to "ding" the crown and ruin accuracy. Should be clean and sharp.
3) Ammo, we have already established ANY mil type is a NO-NO. So either load some quality loads or buy them. Even if this will not be the rifles reg. diet. All we are trying to do is attain accuracy. The odds are if one load will work well another will as well.
4) Check the for-end for binding, pinching to tight spots. If suspect, shoot WITH OUT it. This will temp. simulate a free floated barrel.
4a) You (someone) mentioned shooting positions. Put the front rest under the hinge at the front of the receiver. NEVER on the barrel itself.
Again, we are trying to eliminate probable problem spots. Once found you can work of eliminating that problem. Either by just not doing what ever it is (like ammo), or adjusting other things (like adding a "O" ring or cleaning up the fore-end barrel channel)
I hope this helps,
CW
BTW, your on your way to being a handi haulic. I can tell!!!
