The stock that comes on the Howa Hogue rifle is NOT the full length bedding block model. The ones without the bedding block tend to flex alot. So much that I'd be willing to bet the stock is touching the bottom of the barrel whenever you sit it on a rest. If you're shooting off bags, try moving the front bag back from the forend of the stock toward the action and see if your groups tighten up. I have a Howa .25-06 that I ordered as a barreled action and bought the Hogue full-length aluminum bedding block stock for it. Even with it I felt the need to trim a little of the overmolded rubber material out of the barrel channel. It consistently shoots sub-MOA with factory ammo. It might help to trim material out of the barrel channel of your Hogue stock until you're unable to make the barrel and stock touch by squeezing the two together at the forend. Also loosen your action screws a little and check how much side to side play you have. If the action easily moves side to side that could be a problem. A few layers of electrical tape can temporarily shim it up tight to see if it makes a difference. And those action screws need to be torqued to 50-55 inch pounds, says Howa/Weatherby. Snug the back one first, then the front, then torque the back before the front. Your Vanguard is the same action as a Howa, so any stock you find for a Howa will fit your rifle. If your'e not satisfied with your factory stock or the Hogue, I suggest either getting a Hogue w/the bedding block, or a Bell & Carlson Medalist.
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