"Best?" PCP for offhand shooting...

It is more about how you shoot. Such as walking the woods squirrel hunting. You want a super light trigger and instead of trying to hold the crosshair steady on any target, either go from left to right or bottom to top and just as soon as the crosshair is on target snap the trigger. Trying to hold any rifle steady at range is an exercise in frustration. Learn to snap the trigger on target. Overall though Bullpups are a bit easier to shoot offhand. 



Cheers

Kit
 
Daystate Huntsman, pick the caliber you like. Bipod was not on this regal when I took this squirrel. Zeke 
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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. 
 
weight of the rifle , how it shoulders and fits , is it lefty friendly ? ? ( my needs ) (I KNOW! I KNOW, ) all a real personal preference, STYER HUNTER 5 , WALTER LG 400 F/T AND 10 meter rifles, ALL NICE FOR SURE!! no matter what you choose , practice is the key to offhand shooting . SO to answer the question of "best pcp for offhand shooting ??" MY ANSWER IS The one you have practiced with the most, Also NO red bulls or coffee before you shoot helps . LOL! 
 
My Air Arms S510/S500 with the Ultimate Sporter stock is by far the best rifle I have ever used to shoot off-hand. Given we are talking about sporter style rifles and not target/FT stocks.

The subtle drop in the fore-end just before the trigger guard gives you a perfect place to rest your hand for a traditional off-hand position. Also, the adjustable cheek-piece and butt pad make it much easier to fit the rifle to you, which is the most important part of off-hand shooting. YOU are the number one factor in making the rifle shoot well when standing, so getting things into a position that makes it easy to capitalize on your natural physiological strengths (i.e. bone on bone contact) is the only way to go. This is why olympic and other target rifles have loads of adjustment. I like the Huntsman a lot, but I think the adjustment of the AA Ultimate Sporter is hard to beat given we are specifically talking about off-hand shooting.

Rifle length is another important factor. I prefer a longer rifle (the full length S510 in this case), because the extended air tube/barrel create weight out front and gives you a little "hang" to counterbalance the heavier stock towards the rear. This was the same principle guys talked about when I shot high-power/CMP, and another reason why the 20" M16's were generally preferred.