Pellet MV; is there an optimum speed ?

For a .22, my guess is 850-875 fps.

This is from a post I started a couple of days ago: Power, reinventing the wheel, airgun myths, etc

My experiences: First up, power. A lot of times I refer to guns’ FPE but I don’t go for a gun strictly based on their power output. I base my desired power on heavy pellets at 940 fps (25 cal). This leads me to the next subject of reinventing the wheel. When I first got into airguns I heard all about the “perfect” speed for accuracy. Most agreed that 880 fps was best for accuracy and you should not go over 900 fps. After shooting many many pellets I discovered my 25 cal Sumatra seemed to shoot best at around 940 fps for long distance (100-150 yards). To me it seems that when shot at slower speeds the pellets seemed to drop more the longer distances. When I shot them at 940 fps I don’t have to holdover as much. With my 22 cals I always shot them at 900 fps. I recently changed my perspective on this also. I have one of my 22 cal guns regulated and tuned to shoot 18.13gr JSBs at 900 fps and it’s very accurate at long distances. PA was out of 18 gr pellets so I ordered some Air Arms 16gr pellets just to try them. My plan was to shoot them in my 22 cal Sumatra at 900 fps and they are dead on accurate in this gun. On a whim I tried them in my regulated gun and they averaged around 930 fps. And they are just as accurate as the 18gr JSBs at 150 yards.Let me put the obligatory disclaimer: I realise my results maybe different from other airguns because they all seem to perform differently with different pellets and speeds.

The magic number for my .25 smooth twist is 850. I have tried upward to 900 but my group size doubles

For me, optimum speed is what ever fps that pellet shots the most accurate. That varies based on many things. I have a gun that shoots a given pellet the best at 960fps and another that shoots a different pellet the best at 840fps.

How about for .177?

I think with the barrels available today and the increasing quality of pellets and the procedures some are using to extract even more accuracy from a pellet, the sky is the limit. I read a post the other day on another forum of a fellow shooting a .177 with 2 different pellets right around 1100fps that produced amazing results!! Knowing this person is a very talented shooter and does not post BS, I believe this to be a fact. I had a .177 that would shoot CPH's up to 1000fps and remained very accurate at that speed. Also seen other .177's that didn't like speeds past 850fps. Lots of variables and no 2 barrels are the same.

AJ- thanks!

My distance is limited to 30 yards and I mostly shoot 8.12 gr wadcutters. These work well at this range but any more distance than that seems to work better with domed pellets.

When tuning my PCP, the dreaded spiral started to occur with the wadcutters at velocities as low as 850 to 875. Choking the port screw just a hair made this go away and now the rifle shoots accurately at my distance with muzzle velocity at 830 to 840.

I feel sure domed pellets would allow a somewhat higher stable velocity.

Matt

Yes, WC's are not intended for longer distances and surely become unstable at higher velocities. Domes are the only way to go for longer ranges. Some hp's shoot decent at longer range as do pellets like polymags, but domes still reign supreme in my experiences both in accuracy and ability to remain stable at higher velocities.

Sounds like you found a good tune for wadcutters.

"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

I've found with my .22 cal. HW100/STB that 865fps with the 18 gr. FX STB pellets to be extremely accurate out over a 100 yards while hunting Californian ground squirrels. The only factor that makes it more challenging is doping the wind at longer ranges.