OK, I'm going to contradict myself here.
I just bought a 177 HW45 and a genuine HW shoulder stock from a forum member.
It was inevitably missing it's extended front sight, so I threw an old double clamp scope mount in the milling machine and cut the top flat with a post sticking up as the front sight blade - works a treat.
Initially the results were as previous, pretty disappointing, but I spent a couple of hours experimenting with different holds and I've got it sussed.
Basically the gun just needs to be supported under the muzzle in an open left hand. The right hand needs to be "thumb up", not gripping the pistol grip at all, the three lower fingers just touching the grip frame at the front and then the index just touching the trigger.
With this hold and shooting Superdomes the HW45 is extremely accurate, cloverleaf groups on my 10yd indoor range.