Pulling this one back up... I'm a silhouette shooter as well so this is what is used most by the people I compete with...
My Open gun is an Air Arms HFT500 with a few minor mods. Replaced the forearm rail, end cap, cheek riser post, and added a 1/2" buttpad spacer all made out of steel to add about 2 pounds of weight. With a scope (Weaver T-24) and D3 Precision high rings it comes in right at 11 pounds. Ergonomically, out of all my rifles it's the one that fits me the best and I really enjoy shooting it.
Other PCP's that folks use are Steyr LG100's and 110's, Anschutz 2025, or other target rifle models (9015, 8002, etc.) modified to shoot at higher velocities, a few FX's, Marauders, Daystates, and some just shoot their sporter rifle for Open class (usually a TX200).
The biggest thing for off-hand shooting is fit and ergonomics. A rifle that fits you well will be able to allow you to shoot your best.Â