"Also, above 900fps my barrel cleaning frequency increases"
Yes, this is normal and is to be expected...The higher the velocity the more lead "buttering" you get in your barrel
"With a dome headed diabolo pellet, is there a speed above which it destabilizes ?"
Technically it will depend on speed and many other variables: Barrel twist ratio, barrel bore ratio (lands to grooves dimensions), length-weight and shape of projectile, etc. we can even add gas used to propel the pellet ( While minimal gains, gases that are less violent to drastic changes in temperatures are more stable and provide better accuracy as an example: CO2 vs. Air vs. Nitrogen)
Usually no two barrels will stabilize a pellet the same and the exact velocity for each projectile/barrel combination should be found by the individual... Keep in mind that projectiles have a threshold where they are inaccurate at very low velocities, then they become accurate and then loose their accuracy as you keep on increasing speed...Nevertheless, after reaching this second inaccuracy stage and you start getting flyers, keep on increasing the velocity because MANY TIMES the groups will start to tighten...Sometimes you get more accuracy than the accuracy you had before...But it is a fact that in a pellet gun this will occur before the bullet enters and breaks the sound barrier...
Projectiles don't get along with sound barriers...As you approach the speed of sound, around 50 feet plus or minus from 1126 fps. any bullet will destabilize...This 50 + or - zone is called the transonic region...
Without getting too technical, a projectile is stabilized when it passes its center of pressure behind its center of gravity (usually accomplished by spinning) but when a projectile enters this transonic region, the center of pressure moves forward as the bullet decelerates and this makes it unstable; the problem aggravates even more when the bullet was not well stabilized before entering this zone...Longer bullets are more prone to be affected (destabilized) when close to this transonic region...
So what will all of this mean?...It basically tells you that for MOST accuracy you should stay below something like 960 fps. assuming the barrel has good stabilization properties to start with...I have seem barrels that will shoot a pellet very accurately below 800 fps but then they will enlarge the groups when the velocity was increased to 850-870 fps and then again became accurate at 915-925 fps...In these cases the pellet reached it point of inflection positively, but then it became VERY inaccurate when the velocity was increased once again to 950-980 fps...and from there on there was just no way that projectile would shoot accurate (reached the transonic region)...
In other words: Pellets may have a second accuracy stage by increasing the velocity up to the point where the velocity approaches or reaches the transonic region (Around 950-960 fps)
While some over 1,100 fps guns shoot somewhat accurately, all of these guns will shoot better at lower velocities and this is why you hear about "Detuning" guns...Nobody has ever won a major tournament or established a world/Olympic record shooting at supersonic velocities..."Big and faster is better" is not always true.
I had a RWS 54 that shot 1200 fps and made 3/4 -1" groups at 50 yards out of the box, after making some mods and reducing the velocity to 850 fps. (detuing) the groups were 1/2" pretty consistently with 10.34 gr pellets directly out of the tin.
Regards,
AZ