Pellet weight irrelevant?

[antithesis]

Ok this is the first time I've had this happen personally.

On my 1322, no matter whether I use cphp (14.3) or skenko new boy(25 or thereabouts), or anything in between, I'm getting just on the hot side of 13 ft lbs no matter what (at 13 pumps).

I'm fully dumping the valve. But usually all things being the same, any pneumatic should be putting out energy proportionate to projectile weight. Can anybody with the proper physics and airgunning background tell me what this says about the characteristics of this setup?

I'm happy overall, but find this kind of peculiar.🤨

[rsterne]

any pneumatic should be putting out energy proportionate to projectile weight

Not sure where you got that idea, but it's not even close.... On the other hand a constant FPE is not the norm, either, but it can happen if the valve is open until all projectiles, light and heavy, leave the barrel (typical for a pump and dump pumper).... and if the velocity with the lightest is not too high.... In your case, with a 14.3 gr. only doing about 640 and a 25 gr. under 500, it is not that surprising....

In theory, the maximum FPE of a pneumatic is proportional to the pressure x bore area (which gives the force in lbs.) x the barrel length (in feet)…. If we had 100% efficiency, that formula would give you the maximum possible FPE.... but in practice we don't even hit half that number with our best pneumatics, and many guns only get to 25% of that.... If you are developing 1000 psi at 13 pumps (just a guess), the maximum would be about 32 FPE with a 10" barrel, and you are getting about 40% of that....

The biggest loss is the energy taken to accelerate the mass of air in the barrel (significant at 1000 psi)…. that is subtracted from the ~32 FPE.... Then we subtract frictional losses, and all the others.... What is left over is the FPE of the pellet.... A heavier pellet is a larger percentage of the total mass (pellet plus air), which is the main reason heavier pellets produce a bit more FPE.... but nowhere near in proportional to their weight.... For your pistol, constant FPE is not unusual, although I would expect to see a slight (maybe 1 FPE) edge with the heavier pellets.... providing they aren't a tighter fit in the bore, and therefore have increased frictional losses....

Bob

 

[Tom Tucker]

The formula I use (after getting some guidance from Bob) to estimate the effects of pellet weight differences in a pumper is:

Mass of Pellet1 / (Mass of Pellet1 + 1/2(Mass of air charge in grains))
-----------------------------------------------------------------------------
Mass of Pellet2  / (Mass of Pellet2 + 1/2(Mass of air charge in grains))

Simplifying this expression will give the proportion of Pellet1's FPE to Pellet2's FPE

1L of air weighs 18.9 grains.  so 1.44cc of air at 1000 PSI weighs (0.00144*1000/14.7)*18.9, or 1.85 grains

[your air mass is probably a bit larger than this if your gun is generating 13FPE, but it's close enough for demonstrative purposes]

So to compare a 14.3 grain to a 25 grain:

14.3 / (14.3 + 1/2(1.85))
----------------------------
25 / (25 + 1/2(1.85))

Simplifies to:

0.939 / 0.964

Further simplifies to:

0.974, meaning that the 14.3 grain pellet will receive 97.4% of the energy that would be received by the 25 grain pellet.

HOWEVER - it's possible that this particular 25 grain pellet is generating more friction in the barrel, which would negate the gain.

[antithesis]

any pneumatic should be putting out energy proportionate to projectile weight

Not sure where you got that idea, but it's not even close.... On the other hand a constant FPE is not the norm, either, but it can happen if the valve is open until all projectiles, light and heavy, leave the barrel (typical for a pump and dump pumper).... and if the velocity with the lightest is not too high.... In your case, with a 14.3 gr. only doing about 640 and a 25 gr. under 500, it is not that surprising....

In theory, the maximum FPE of a pneumatic is proportional to the pressure x bore area (which gives the force in lbs.) x the barrel length (in feet)…. If we had 100% efficiency, that formula would give you the maximum possible FPE.... but in practice we don't even hit half that number with our best pneumatics, and many guns only get to 25% of that.... If you are developing 1000 psi at 13 pumps (just a guess), the maximum would be about 32 FPE with a 10" barrel, and you are getting about 40% of that....

The biggest loss is the energy taken to accelerate the mass of air in the barrel (significant at 1000 psi)…. that is subtracted from the ~32 FPE.... Then we subtract frictional losses, and all the others.... What is left over is the FPE of the pellet.... A heavier pellet is a larger percentage of the total mass (pellet plus air), which is the main reason heavier pellets produce a bit more FPE.... but nowhere near in proportional to their weight.... For your pistol, constant FPE is not unusual, although I would expect to see a slight (maybe 1 FPE) edge with the heavier pellets.... providing they aren't a tighter fit in the bore, and therefore have increased frictional losses....

Bob

Well dang it then..I was always under the impression of a general trend of heavier projectiles making better use of the air behind them due to longer effect of pressure acting on the pellet..

But the more I think about it, it's probably( excluding Springers) more a matter of (way) too light a projectile leaving too quickly to take advantage of what's behind it...that is probably where I picked up the impression, and extrapolating the logic to heavier projectiles. Is this likely where I went wrong, Bob? Is this at least partially true? Please tell me I don't have
To reassess everything I learned till now...

But in all the time ive been doing this, I always took for granted a slight edge in favor of heavier ammo, it usually seemed to happen that way, not extremely so but still..

[antithesis]

And this particular pistol has a 16 inch barrel. I'm just trying to figure out with the given information if I'm trying to optimize performance using middle heavyweight pellets (maybe 18 to 21 gr.)
If hogging out a tiny bit of the valve could possibly help. Do you know the approximate capacity of a standard 1322 valve? Iirc, Bob, isn't an optimal capacity it the neighborhood of like 35 to 50 percent barrel volume? I really have no interest in pumping more than 20 times for obvious reasons, and I've already cracked one pump link on there. Given the very satisfactory functioning of the ftp assembly I currently run, It really shouldn't be necessary IMHO

[Tom Tucker]

I think of it like this - The pellet is like a car, and the air is like its engine.  The engine supplies the power driving the car, but it has weight of its own that detracts from the overall acceleration it supplies to the car.  The less the air charge weighs compared to the pellet, the more energy can go into the pellet.  This is why helium can produce more power than HPA in a PCP, all things being equal...

[antithesis]

I think of it like this - The pellet is like a car, and the air is like its engine.  The engine supplies the power driving the car, but it has weight of its own that detracts from the overall acceleration it supplies to the car.  The less the air charge weighs compared to the pellet, the more energy can go into the pellet.  This is why helium can produce more power than HPA in a PCP, all things being equal...

So what I gather then is that if my porting is reasonably good, I'd better leave well enough alone, as a larger volume of lower pressure air is doing no good in a dump valve situation... pellet likely long gone before the supply of air is spent for all intents and purposes. In fact I've seen guys post increased performance with decreased valve volume in these pumpers for a given number of pumps. I've no means of taking pressure readings in this case.

[Mole2017]

I tested 8 different pellets from 8.4 to 15.5 grains in my BSA R10 and if I round the FPE to the nearest whole number, all but two were 19 FPE. Those other two were 18 FPE. But the lowest and highest energy pellets differed by only 1.4 FPE (not rounded).

Looking at it overall, a span of +/-0.7 FPE with an average of 18.8 FPE in this case is only +/-3.6% variation. Close enough for engineering, as they say. Actually pretty good--10% is often considered "close enough".

The energy stored in the compressed air in the plenum is the same within the tolerances of the regulator, or in your case the pumping input to the gun. Ditto for any losses through the valve--there can be some mechanical variation there from shot to shot, but air flow dynamics are also compounded by interaction with the pellet. The pellets vary slightly in frictional characteristics, due for example to things like skirt expansion and head size. All considered, +/-3.6 % variation was pretty remarkable.

Note: The interactions will more complex for a springer, in which case "tuning" will really play a part in efficient energy transfer to the pellet.

But here's another way to think about it in pneumatic systems, pumper or PCP: consider the force acting on each pellet as it goes down the barrel. If all the losses and interactions were equal, that force vs distance curve is going to look the same for any pellet, and thus every pellet leaves with the same energy level. (In loose terms, energy = force*distance. But with force changing over distance it is calculated via some calculus over the length of the barrel--think area under a force vs distance plot. If they are all the same curve, they all have the same area, which is the energy.)

Where pellet weight pays off is down range. Usually (not always) more weight comes with a better ballistic coefficient and correspondingly less energy loss by the time it reaches its target.

[rsterne]

If your valve dwell is relatively short, so that the light pellet is only getting to 30-50% of the barrel length when it closes, then a heavier pellet, moving slower, driven by the same valve dwell, will not have moved as far when the valve shuts.... This does two things, it means the valve releases less air in the same amount of time.... and that air has more barrel length into which to expand after the valve closes.... This will extract more FPE from less air, increasing the efficiency as well as a small increase in the FPE.... Obviously if the pellet is MUCH heavier, it will have moved such a short distance, so little air will have been released that it won't have enough pressure left at the muzzle to be adding much to the pellet.... In such a situation, adding pellet weight actually loses FPE, and that is why you will eventually find an "optimum" pellet weight in any setup....

In a pump and dump setup, however, the valve is still open when the pellet leaves the muzzle, so any air still in the valve adds NOTHING to the FPE.... The largest valve that makes sense is one of 50% of the barrel volume, after that you gain so little FPE for the extra air wasted it isn't worth it.... Most pumpers run a valve about 1/3 the barrel volume.... A 16" barrel in .22 cal has a volume of 10 cc, so yes, a stock 1322 valve is a bit small.... Don't forget, if you increased the valve volume by 50%, you would have to increase the number of pumps by 50% (ie 20 pumps), just to get back to the same pressure.... However, during the shot the pressure would not drop as fast, increasing the FPE (but lowering the efficiency)…. You might gain 3 or 4 FPE for your 50% increase in air used (pumps)....

Interestingly, Tom's estimate of 97.4% of the FPE for the 14.3 gr. pellet, compared to the 25 gr. pellet means a difference in FPE of about 1/3 FPE.... hardly enough to notice.... and easily used up by even a slightly tighter bore fit of the heavier pellet....

Bob

[antithesis]

"Don't forget, if you increased the valve volume by 50%, you would have to increase the number of pumps by 50% (ie 20 pumps), just to get back to the same pressure"

Yes, and that is why I hesitate to make any unnecessary changes in volume. I'm not one of those guys who will pump a 13xx 40 times to show em what can be done..in one dump...that's not a pumpers place IMHO. After a certain point it loses its appeal, and illustrates why pumpers enjoy limited popularity among some crowds. Given what I'm accomplishing with a 16" barrel on this platform, I'm quite happy all considered.

But who knows, I may get a wild hair and decide to go .25 just to see how close to 20 ftlbs. I can get...in 20 pumps. Before it gets said, I know that ain't gonna happen within my self imposed pump limit and without an even longer barrel.

This gun is kinda doin the t/c contender thing in pistol format, and 16 is as long as this barrel is getting. So 20 ... nah.

17 maybe😁

[Tom Tucker]

One of the main advantages of increasing the valve volume is that pumping effort decreases, so you can pump higher without causing undo strain on the pump arm.  The other advantage is that head space exists as a percentage of total valve volume, such that as you make the valve larger, you cause a smaller percentage of air to be lost with each pump.  The effect is cumulative, so while the larger valve is inherently less air efficient, it eventually overtakes the smaller valve and leaves it in the dust by holding on to more of the air that enters it.

I have 2 modded 1322's - one pistol with a 10-inch barrel and 1.7cc valve and a carbine with an 18-inch barrel and 2.4cc valve.  They are otherwise nearly identical [edit: the carbine also has a 2100 hammer spring].

Average FPS with 14.3 grain pellet:

                       1.7cc / 10"    2.4cc / 18"
3 Pumps              330              330
6 Pumps              440              460
10 Pumps            530              575
12 Pumps            560              615
15 Pumps            ----              660
20 Pumps            ----              710

I won't pump the smaller gun more than 12 times - I have worn out several pump arms doing so, and I won't do it again.  The carbine has been pumped to 20 many times with no degradation.  It also has a piston that can be adjusted while still in the gun, so I won't have to tear it down if that were to happen...

[rsterne]

Yep, that was my point.... the strain on the pump linkage depends on the pressure.... so 20 pumps in a 50% larger valve is the same load as 13 pumps with the stock one....

Bob

[antithesis]

Yep, that was my point.... the strain on the pump linkage depends on the pressure.... so 20 pumps in a 50% larger valve is the same load as 13 pumps with the stock one....

Bob

When kept properly moist, the pump effort definitely climbs, but on this platform is a non issue to me..

But for one shot tunes, I don't like to stand pumping for three minutes.

[Tom Tucker]

That's true, oil in the valve can cause pumping to get way too hard.  In fact, I think that's why I used to wear out pump arms all the time.  All my pumpers have oil wipers now and I hardly ever have to add any oil.  The o-rings still look wet after hundreds and hundreds of pellets, and all I need to do is add two drops to the felt every 1000 shots or so.

[antithesis]

That's true, oil in the valve can cause pumping to get way too hard.  In fact, I think that's why I used to wear out pump arms all the time.  All my pumpers have oil wipers now and I hardly ever have to add any oil.  The o-rings still look wet after hundreds and hundreds of pellets, and all I need to do is add two drops to the felt every 1000 shots or so.

Well I meant when I keep it lubed it pumps very efficiently...but you are correct, and I'm glad this particular maker uses felt in his design.

In fact while I'm overhauling it I'm putting one on the Benji now that I'm thinking about it