OK - This is something real new to me. But here's my problem. 1) Subsonic 2) Small calibre 3) Pure lead rounds No hiding - it's an airgun - but a severly overpowered one. It's an itch I just have to scratch. Her's my dilema - To keep it subsonic I have to use Heavy rounds. And to keep tehm stable I need to use a hollow base it seems - the result is LONG rounds (3 calibres and rising) and I'm getting concerned I may need to alter twist rates. Standard airgun barrels from Walther are 450mm twist - the option is to use a .17HMR barrel put the jump is HUGE. from 1:16approx to 1:9 Found this "Twist = 150 X D2/L" But seems that speed is a factor and I'm aiming for around 300m/s. The Greenhill is apparently for twice that. HELP! - If you can.