Most people have found that overtightening the howa mini cracks the bottom plastic. I'm lead to believe the correct torque is 30 inch pounds. This is a very easy and effective way of fixing this problem. All you do is drill out the plastic to accept some steel tube that the bolts fit through nicely and jb weld it into the bottom plastic basically making a mini pillar. Its a little bit fiddly but works well.
Better mod: sell it and buy a tikka
Got a tikka in x55. Nice rifle to. Action would be over kill for the tiny grendel.
6.5 06 6.5x55 and 6.5 grendel