Clean your Diamond right back to metal. Use a good bore solvent and brush and scrub that bore butter out of there. After you have brushed it a few times, start running solvent patches through it. Use a solvent by the way that removes plastic, copper and lead fowling. I use Birchwood Casey or Hoppies most the time.

Once your sure that rifle is back to bare metal, then oil it with a quality gun oil. Leave the bore butter out of the rifle This is my opinion mind you, but bore butter and sabots are not a good combination according to Thompson Center Arms.

Before you shoot it next time run a damp swab down the barrel to get out any oil that might be in there. Then pop off three 209 primers through the nipple to clear it and to "kind of" fowl the bore. Now load as normal.

I would load 100 grains of FFg powder (your choice). Thompson Center will tell you Pyrodex RS. I use Goex FFg. And a 250 grain Shockwave and it should shoot excellent. I also have real good luck with the 200 grain Shockwaves, 240 grain XTP's (Thompson Center Mag Express brand), and 375 grain Buffalo Bullet SSB sabots. Also if you want a conical, shoot a 460 No Excuse Conical... they might surprise you with 80 grains of Triple Se7en or Pyrodex P. If your shooting good groups, then work your load up from there.

Some things to check besides your scope and load.. on your locking lugs by the trigger, make sure you have not lost your washers. Also try to tighten the lugs the same amount. I will many times run a paper test to make sure the barrel is floating. The paper should slide all the way up the fore arm to the first locking lug.

Also when shooting off a bench as I am sure you know, never rest the rifle on the barrel, only the for arm of the rifle. If you rest on the barrel you throw the floating barrel off..

You are swabbing between shots I hope... other then that I have no idea why your Diamond will not group. Mine will shoot almost anything (and I mean anything) I stuff down it with very good accuracy.