I must say I think Rob is right; I'd be gobsmacked if a Tempest could do 6 ft.lbs without severe (and short lived) dieselling. If the swept volume supported that power level people would have produced tuned examples doing that power level and we'd have heard about them. They typically produce around 3 ft.lbs give or take, .22 a little more - my .22 does about 3.3. Webley are claiming a near doubling of the muzzle energy. If they'd said 4 ft.lbs I'd have raised an eyebrow. At 5 I'd be very dubious. 6? Come on! As Scotty on the Enterprise might say, they cannae change the laws of physics!