Extremely consistent velocity may not equal consistent accuracy.

[Bearcat]

Found an tin of RWS Super Hollowpoint 6.9gr in the back of the pellet cabinet when cleaning this morning. Took them downstairs and shot several 12M groups with the HW50s to see how they would do. Unfortunately I wasn’t impressed. 5 shot groups averaged a little over 1/2 in.outside to outside at 12M which is twice the usual size of the gun’s favourite JSB 7.87gr. pellets. Since the RWS are very light 6.9gr. I shot 5 shots over the chrono just for fun. The results were shocking to me. It was the most consistent velocity string I have ever witnessed. FPS was  819.4,  820.1,  821.2,  819.9,  819.5  A hi/low spread of only 1.8 fps. After seeing that I thought accuracy should be better than it was and maybe my shooting was at fault. Shot a JSB 7.87 group to check my shooting, and the group was what I expected a single hole 1/4 inch outside to outside. As a comparison the JSB’s usually have an extreme spread of around 5 fps when I’m doing chrono tests. It seems extremely consistent pellet velocities do not necessarily translate into extremely consistent accuracy and vice versa. Airguns never get boring!

[Motorhead]

Absolutely CORRECT .... A lousy barrel or pellet will always be lousy in accuracy and velocity has nothing to do with it ... put another way 

[Yogi]

Absolutely CORRECT .... A lousy barrel or pellet will always be lousy in accuracy and velocity has nothing to do with it ... put another way 

Absolutely, however an esp of 10% or more make longer shots problematic. If the high and the low are less than 2%, I am happy, otherwise...

-Yogi

[HectorMedina]

Found an tin of RWS Super Hollowpoint 6.9gr in the back of the pellet cabinet when cleaning this morning. Took them downstairs and shot several 12M groups with the HW50s to see how they would do. Unfortunately I wasn’t impressed. 5 shot groups averaged a little over 1/2 in.outside to outside at 12M which is twice the usual size of the gun’s favourite JSB 7.87gr. pellets. Since the RWS are very light 6.9gr. I shot 5 shots over the chrono just for fun. The results were shocking to me. It was the most consistent velocity string I have ever witnessed. FPS was  819.4,  820.1,  821.2,  819.9,  819.5  A hi/low spread of only 1.8 fps. After seeing that I thought accuracy should be better than it was and maybe my shooting was at fault. Shot a JSB 7.87 group to check my shooting, and the group was what I expected a single hole 1/4 inch outside to outside. As a comparison the JSB’s usually have an extreme spread of around 5 fps when I’m doing chrono tests. It seems extremely consistent pellet velocities do not necessarily translate into extremely consistent accuracy and vice versa. Airguns never get boring!

Barry;

Consistency of MV is what I like to call a "Hygiene Factor" meaning that if it is NOT there, then accuracy will not NECESSARILY be there either.
Now, we need to define "consistency".
In 0.177" cal a sSD (Sample Standard Deviation) of about 4 or 5 fps over an 800 fps mean is good enough for serious target work.
in 0.22" cal a sSD of between 7 and 9 is also considered good for serious target work on means of 700 to 900

Generally, I look for sSD's of around 3 or 4 fps over means of 800 as a STARTING point. More than that, means that between the MAX and the min, I COULD find up to 24 fps difference, and that is noticeable. Now, that may only happen in 0.1% of the shots (meaning 1 in each thousand), BUT, it also means that 1 in every 100 will go out of the interval 794 to 806 fps
If you calculate the impact in the trajectory of a deviation of 6 fps from the mean (and therein lies the importance of shooting groups and correcting sights ONLY by the CENTER of groups), is barely noticeable.
My personal FT rifles shoot within ±2 fps, meaning that the extreme spread in a 20 shot string will be around 6.

Now, you need to remember that the barrels of our guns are, for fast phenomena, as flimsy as a flame in the wind.
They will dance around a given point (which is the center of a hypothetical 1000 shot group). The oscillations of the barrel make the muzzle "aim" at different spots in the target every second. And the velocity at which the barrel traverses from one edge of the target to the other is surprisingly high. These oscillations are normally called harmonic oscillations.
To TAME the harmonics, you need to ensure that the pellet leaves the barrel ata point of Maximum excursion. Meaning the point where one edge has been reached and the barrel is beginning to start its return trip. At, or near , that point is where the transverse velocity is lowest, approaching zero and then accelerating in the opposite direction.

You can change the way a barrel vibrates in the same way that you can change how a tuning fork changes "pitch" (note/frequency) and that is by adding mass to different places.
Because there is no way to measure exactly WHERE the mass should be added, what is done is to add a mass whose position you can adjust in small increments. And that allows you to "tune" the pellet exit to one of those maximum excursion points.

When you get BOTH (MV and exit point) to be what you want is where you will find the best accuracy.

If you have not done so, read these:

https://www.ctcustomairguns.com/hectors-airgun-blog/controlling-harmonics-in-a-barrel

https://www.ctcustomairguns.com/hectors-airgun-blog/truly-taming-the-harmonics-in-spring-piston-airguns

https://www.ctcustomairguns.com/hectors-airgun-blog/the-hpm-or-dianas-high-performance-muzzleweight

Keep well and shoot straight!




HM

[MDriskill]

Fascinating notes and links as always, Hector! Thank you for that.

I would add that I've found standard deviation (SD) a useful metric over the years, though how I do it is the redneck shorthand version compared to you, lol.

I record individual velocities for a 15-shot string, and then the average velocity, extreme deviation, and SD as my old Chrony calculates them. You really need more data points than that for a precise SD, but 15 is useful enough as a window into my older collectible stuff, and allows me to quickly chart five or more strings when sorting a gun out.

I also like to note the SD as a percentage of the average velocity - 1% or less likely indicates a useful pellet. It's also an interesting way to compare guns at different power levels.

[HectorMedina]

Fascinating notes and links as always, Hector! Thank you for that.

I would add that I've found standard deviation (SD) a useful metric over the years, though how I do it is the redneck shorthand version compared to you, lol.

I record individual velocities for a 15-shot string, and then the average velocity, extreme deviation, and SD as my old Chrony calculates them. You really need more data points than that for a precise SD, but 15 is useful enough as a window into my older collectible stuff, and allows me to quickly chart five or more strings when sorting a gun out.

I also like to note the SD as a percentage of the average velocity - 1% or less likely indicates a useful pellet. It's also an interesting way to compare guns at different power levels.

Your method is excellent, Mike!

In fact you are using 75 pellets/shots to characterize a gun/pellet combination and that is VERY close to what we found when running "Monte-Carlo" simulations in the millions to find out where you are at the "Optimum" where a small % of the effort gets you almost full % of the results.

Our conclusion was that the best way to spend 100 shots in fully characterizing a gun/pellet combination was 10 strings/groups of 10 shots each.
More shots do not add significantly to the overal dispersion measurements, and less groups may skew the averages.
So, you are VERY close.

If you want to read the full reasoning process of that, it is here:

https://www.ctcustomairguns.com/hectors-airgun-blog/shot-cycle-dynamics-in-3-spring-piston-airguns-chap-3

I once thought not to present the full process, but in time, and peculiarly, this entry has generated more enquiries from serious players than any other.
Yes, it is an entry for "ballistic nerds", LOL!

Congrats, keep well and shoot straight!





HM