Alfred,It could be that the Sumo is upsetting your particular air rifle's barrel harmonics. In effect acting as a tuned mass damper, that needs to be set before it provides a benefit.Are you sure the Sumo is screwed onto your barrel tight enough to prevent it from drooping?Have you checked that the Sumo is properly screwed shut, and that there is no felt material that has somehow gotten into the projectile path? There may be something that is easily fixed, if you take a look. Have you looked inside the Sumo for evidence that the projectiles might be making contact at one or more of the baffle hole edges inside? You would be looking for light colored lead splashes or other visual indicators of such contact. If there is no evidence of projectile contact (clipping), then the mass and length of the Sumo may just not suit your airgun. This would not reflect on how well it works with other airguns. Each is its own case.
Note, the term tuned mass damper typically describes and additional spring and mass system, not just the effect of adding mass to change the natural frequency of the barrel.
In the sense of reducing building sway, you are absolutely correct with your terminology.