Can we talk about changes in Efficiency / Power with pellet weight changes ?

[Motorhead]

This post a side spin off of this project : https://www.gatewaytoairguns.org/GTA/index.php?topic=148788.0

Without getting too wordy in getting my question framed ... Those of us who tune PCP's are well aware that there is a strong bias that within any caliber and power gun may be tuned at ... there generally is a pretty good jump in kinetic energy ( Foot Lbs ) as you go from lighter to heavier pellets. * given gun realistically can shoot the heaver weighs 
This generally is viewed as more weight stays in barrel longer and is subject to absorbing more energy of the expanding HP air pushing it yielding more energy upon exit from barrel ... In a sense 

In reference to my Full bore ported .177 that has a SUPER short valve dwell but HUGE AREA of transfer path to pellet, the energy of the high pressure air gets in behind the pellet so quickly due to near zero flow restriction from valve.

*My thoughts are with such a rapid rise in pressure the Per-say launch of pellet is violent and it gets the full energy transfer based on its bore area very effectively.  Tho a WAG 

Reason for this inquire stems from the picture attached, That of a target shot at 30 yards ( pellet testing a new barrel )  that has pellets light as 7.9 grains and heavy as 15 grains.
The ENERGY in FT LBS hardly moves at all across the weight ranges shot which i found very interesting. The * paragraph above I'm trying to wrap my mind around and make some sense of why it is acting like it does
   Is it the short dwell that in ONLY providing a given amount of available energy and the large ports is allowing each weight to get the most out of it .... What ya got GTA think tank ?



To better follow what picture is showing ..

Crosman 10.5 @ 911 fps for 19.35 ft lbs
JSB Monster 13.43 @ 817 fps for 19.9 ft lbs
H&H Sniper Magnum 15 @ 784 fps for 20.45 ft lbs
Crosman 7.9 @ 1016 fps for 18.1 ft lbs
JSB 10.3 @ 925 fps for 19.57 ft lbs
H&N Barracuda 10.6 @ 912 fps for 19.39 ft lbs

While there is SLIGHT trend in weight verses ft lbs energy & really looking at the similar 10.5 +/- weights against the 13.4 & 15 weights ... it is FAR far far less than i have seen before in pcps running much smaller Transfer sizes ... am i imaging something or is there logic here i'm not grasping ?

Scott

[rsterne]

I dunno, I see a pretty clear trend towards more FPE with heavier pellet weight.... A 90% increase in the pellet weight results in 13% more FPE, which is on the low end, yes, but within normal ranges I have observed.... In fact, with guns tuned to use only a "sip" of HPA (ie high efficiency), I have seen a DROP in FPE in a .25 cal regulated PCP when going from a 25 gr. pellet to a 51 gr. bullet (with no tuning change) from 52 FPE to only 40 FPE…. If I increased the dwell a bit, that evened out, and eventually, by the time I got the 51 gr. into the high 800s, it had significantly more FPE than the 25 gr....

The primary reason that a heavy pellet ends up with more FPE is that when you don't retune, the mass of air released by the valve is pretty constant (in fact it actually decreases a bit, because the heavier pellet, going slower, hasn't moved as far down the bore when the valve closes), but it becomes a smaller percentage of the total mass (pellet plus air)…. This means less of the total energy goes into accelerating the air, so more is left over for the pellet.... hence the trend for heavier pellets in a PCP to end up with more energy.... The more efficient you make the gun, the smaller is the air mass.... so the losses in accelerating it are less of the total available.... This reduces the difference in the FPE advantage for the heavier pellet....

Bob

[Motorhead]

There ya go .. well explained.

Thank you

[Dairyboy]

I was just thinking about this the other day. I have a .22 that is shooting the JSB 18gr at 945fps and will then shoot the JSB 25gr redesigned at 800fps. Idential FPE essentially which I haven't seen before. So what I'm getting is that it's just that efficient on air? It's got more power to it but that's just where it came tuned from the factory and is crazy accurate so haven't adjusted it.

[Scotchmo]

The only blip in the trend seems to be that this tune likes pellets in the 10.5gr range. Especially the 10.3gr. Though I would test more <10gr and more >11gr before reaching any conclusions.

[rsterne]

More likely, Scott, is that it doesn't "like" the lightest pellet, because the velocity is over 1000 fps.... and we know what happens to the internal efficiency when you start pushing the velocity high, particularly in a 20" barrel.... You could just as easily draw your trendline through all the pellets between 10-15 gr. (in fact more pellets would be on the trendline)…. and only the light one (at high velocity) would be the outlier....

Bob