Chap if you're accuracy is that shit. With all the ammo used. I'd be looking at getting the crown redone. And a scope down the barrel to check for wear..
well the OP hasn't stated what condition the rifle is in, New, Used or Abused.
I'm pretty impressed with Howas and their accuracy so if the barrel isnt shot out then I still go back to the obsessive cleaning every test string.

On your next day out shoooting make up 20 rounds sized 20thou of the lands. Load with the 77gr you want to stick with and fire them in batches of 4.
As your barrel fouls and settles down your groups will get better.
Once its proven your gun can group then start on getting a good reload. this starts with your powder load not COL

PS, If the shot groups dont have any improvement then your guns F***ed, get a new one.