Madbull Airsoft continue to produce unique innovative accessories. Their barrel extension includes an inner barrel stabilizer to reduce inner barrel wobble:
Madbull Product Update: 2" Barrel Extension Now Includes Stabilizer O-Ring for Improved Accuracy
Product Update! The forthcoming 2" Outer Barrel Extension will now also include a BONUS inner barrel stabilizer o-ring.
The rubber o-ring IMPROVES ACCURACY by wedging between the inner barrel and outer barrel extension, reducing inner barrel vibration during shooting. Note: designed & tested for Madbull brand inner barrels.
Pretty clever, right?