Pellet weight decreases ... FPS increases, relationship?

[Marc In Iowa]

I got these fps & pellet results from a single 177 tune:

10.65 100% H&N BM
  8.44  -21% JSB Exact +  8% fps
  6.80  -35% GTO           +16% fps

Dropping the pellet weight 21% caused speed to increase 8%.

Is there a well understood relationship here?

[Airnut]

Generally speaking the lighter the pellet the faster it travels. Also the lighter pellet has less fp of energy compared to a heavier pellet at a slower speed. As far as your% go unless I miss my guess you could run the same test with different brand pellets same weight  and get different %

[Marc In Iowa]

... my guess you could run the same test with different brand pellets same weight  and get different %

That is the part I'm wondering about. The three pellets I've listed are the three best performing pellets in my rifle. So ... at least understanding those particular pellets in my particular rifle is useful to me as I think about tuning.

[Marc In Iowa]

I see this blog post:

https://www.pyramydair.com/blog/2012/05/power-and-pellet-weights/

My rifle is a PCP (Brocock Bantam). Maybe this is better addressed in the Darkside gate. The blog post discusses that springer and PCP power plants react differently.

[Motorhead]

There is also a shift in speed due to LEAD hardness and SIZE of a pellet within the same weight range.  Issues of bore drag I have seen / documented to be @ +/- 25 fps in such cases.

[cosmic]

Also the fit or size of the pellet...

[Taso1000]

One other thing to consider is that the lighter pellets is spending less time in the bore because it's moving faster.  I can't take full advantage of the current tunes blast of air. 

The lighter pellet would make better use of a shorter and higher psi air blast to use all that energy before it exits the barrel.

Thanks,

Taso

[Marc In Iowa]

The lighter pellet would make better use of a shorter and higher psi air blast to use all that energy before it exits the barrel.

Hmmm. A shorter blast of air is at higher pressure to cause the valve to shut sooner?

[rsterne]

As a basic rule of thumb, the FPE will remain roughly the same, which means the velocity will change by the square root of the ratio of the weights....

For the same energy as the 10.65 gr. we get....

10.65 / 8.44 = 1.26.... square root of that is 1.12, so you would expect a 12% increase in velocity (and you got 8%)….

10.65 / 6.80 = 1.57.... square root of that is 1.25, so you would expect a 25% increase in velocity (and you got 16%)….

The difference between predicted and observed could be due to several things.... PCPs generally produce more FPE with heavy pellets than with light ones.... and when the velocity gets over about 800 fps, the internal efficiency tends to drop as well.... Both those factors would cause light pellets to not get to the predicted velocity.... Pellet hardness, and/or fit in the bore, can also influence the results....

Bob

[Marc In Iowa]

The difference between predicted and observed could be due to several things....

I suppose, in the end, "there's no reality like real life." What the rifle is really shooting, with these pressures and these pellets, is ... what it is.

🙂

[Taso1000]

The lighter pellet would make better use of a shorter and higher psi air blast to use all that energy before it exits the barrel.

Hmmm. A shorter blast of air is at higher pressure to cause the valve to shut sooner?

Right.  You'll also need more spring rate to knock the valve open.  That may not be preferred as it will make the gun harder to cock and will put more stress on cocking bolt and lateral forces when cocking the striker.

Taso

[Doug Wall]

"Dropping the pellet weight 21% caused speed to increase 8%.
Is there a well understood relationship here?"
As a generalization, absolutely! For any specific gun and tune, a set amount of FPE is generated, This translates to higher velocities at lower weights, to get that FPE I remember Hard Air Magazine doing a graph of pellet weight vs. velocity for a whole bunch of pellets out of a single gun. There was pretty much a linear correlation. Things like pellet fit and friction play a smaller part but with certain pellets, it can be more significant. Have you ever noticed that lots of guns are advertised as having XX fps with alloy pellets? The manufacturers are looking to get the highest velocity to advertise, and light alloy pellets are the way that they push the velocity up.