Scope gone bad?

I done some trading a few weeks ago and got a model 12 223 with a heavy 26 in barrel. It shot around an inch, figured I could make it do better so I went to work on it.
Took it apart, laped the lugs, laped the recoil lug to the face of the receiver, and the barrel nut to the recoil lug. Set the head space to a min. Done a trigger job and the bedded it.
I put a Nikon 6x18 buckmaster on it and loaded a few different loads to try. The thing shot real nice, .15 to .25 5 shot groups every time.
Well, after a few hundred shots, the dang groups started opening up, 1/2 to 3/4 in. I've cleaned it really good, tried everything I can think of. Even rebedded it.
Can't think of anything it could be but the scope but its not like some of the other scopes I've seen go bad. All my other scopes are 3x9, so it's hard to compare.
The loads are the same as when I started lot # 's ect. Everything is tight and to spec.
Any suggestions ?

Please show us some targets so we can follow the regression.

OK, 'splain the 'cleaning' process... Sounds like severe copper fouling. Do you have access to a borescope???
Personally, I won't use ANY centerfire ammo unless it's HBN coated (as well as my factory or replacement bbls). NEVER had any problems. Just ask David Tubb.....

ron

I thought it would be copper myself but its not, I've striped it, still no difference.
I think the scope just is not holding its tolerance like it should.
I had a redfield a few years ago on my 22 250 that went bad, it shot around a half inch and all of a sudden one day it was shooting 4 or 5 inches. Maybe this one is just on a smaller scale, I don't know.

It was the scope. Found an old Simons atec 3.5 x 12 in my reloading room. Stuck it on and went and shot it before dark today, shot real good. Guess it will work for now until I can get something better.

jb, glad you found the problem. Although I own a few Nikons (including a 6x18 Buckmaster), I've never been overly impressed.
Last year, I had a bunch of shooting buddies compare a brand new Monarch compared to a bunch of various priced models in the same magnification range. When the 'group' thought a $70 (wal mart) Centerpoint was equal to the Nikon, I sent the Nikon back the next day. Nothing really 'wrong', just for the $, I expected more.
I also have a Tasco 20X-40mm I bought new back in 1976. Looking through it, is amazing.
Even if my shooting buddies made fun of it, because it was "Made in Japan".... ha ha

ron

Lol, I got this one from a buddy, it's one of those refurbished nikons from natchez shooters supply. 90 day warranty so I'm not gonna bother with sending it off. I've had it for a couple years.
I've got Leopolds on my huntin rifles but thinking about trying a vortex on this gun. It's just for shooting off the bench, dang thing weighs to much to drag around calling yotes.