I know what you mean about having the sliding ring getting stuck, it got so bad that eventually the sliding ring shaped the adapter so it wouldn't get stuck anymore.
I did two minor mod's from this website on my gun. The first one was enlarge the drain ports on the muzzle because my older model sten had tiny holes (way smaller than the newer stens and cyrano). When sent the gun in for maintenance, I also had them eliminate the pvc type material that's used in the High/Low power regulating system.
The pneumatic guns I hunt with are over 120+cm and I hold them by the middle of the barrel most of the time. Even while descending, I'm still holding the gun by the middle and trying to hide it with my body so the bigger silhouette (having the gun extended) doesn't scare away weiry fish away. Only time I hold it by the handle is when I'm pulling out the gun and getting ready to shoot.