Tuning Larger calibers seem much more efficient?

[EPG]

Had today off from work - beautiful day in mid 70s (!) in Connecticut - so I played …

I had created a “40 Yard Challenge” target card. Tried it with my RTI Prophet Performance V1 .22 LR - set a new personal best.

My Prophet .22 was tuned for 100 yards - shooting 25.4g MRDs at 912fps.

Since its shooting so well, I didn’t want to touch the tune, but I had a new (to me) .30 cal barrel and Huma Mod 40 moderator in .30 that I was dying to try out on my Prophet…

… so I swapped barrels and pellet probes and shot my 1st .30 cal pellets with the Prophet. The 1st 2 groups were at 40 yards and the next 3 groups were at 50 - all were 1/2” or less ctc - some were really tight! Using FX (JSB) 44.8g pellets.

To my surprise, when I chronoed the 44.8g pellets they were going 850.2 fps…

I thought they would be going much slower.

In .22 cal it was shooting 25.4g @ 912fps = 47 ft/lbs.

In .30 cal (changing only barrel and pellet probe) it shoots 44.8g @ 850fps = 72 ft/lbs.

So it seems .30 cal is much more efficient than .22 - in that I gained 25ft/lbs of energy simply by swapping from .22 to .30 - leaving hammer spring and regulator exactly the same.

Does this sound correct ?

-Ed

 

[madeintheuk]

Yes. When I swapped from a 22 380mm barrel on my crown to a 500mm 30 barrel, I went from shooting 25.39 at 940, to 44g 30 cal at 850. Given the barrel was a bit longer, but I touched nothing else.
I agree, they must be more efficient to work with.

 

[MadCowCrazy2]

My guess would be that if the only thing changed is caliber then the air released should be the same. A bigger caliber means a bigger area to push on, there has to be some diminishing returns in the physics involved.
Smaller caliber means smaller hole but the air can only move at a certain speed, larger hole means larger area to push on and heavier pellet means more efficient use of the air.
I'm sure it has something to do with lighter pellets wasting air energy whilst a heavier pellet is harder to push so it uses the air more efficiently?

 

[Stubbers]

Larger caliber do have higher efficiency potential, for a .177, .22, .25, the .25 will will be at most 22% more efficient than .22, and at most 50% more efficient than the .177 while the .22 will be 35% more efficient than the .177, based on available surface area your average shot pressure can apply to.

Going from .22 to .30 cal, at most 47% gain in efficiency.

47/72 = 35% gain in energy (and thermal efficiency)

Now once you push each caliber to their respective limits, that relationship changes, fast....and larger calibers are no longer 'more efficient', rather require more air to push their respective boundary (this is because the pressure drop is much greater down the bore the larger diameter you go). That 30 cal has far more energy potential left in the system then the .22 at 47 fpe for example.


-Matt

 

[MadCowCrazy2]

Larger caliber do have higher efficiency potential, for a .177, .22, .25, the .25 will will be at most 22% more efficient than .22, and at most 50% more efficient than the .177 while the .22 will be 35% more efficient than the .177, based on available surface area your average shot pressure can apply to.

Going from .22 to .30 cal, at most 47% gain in efficiency.

47/72 = 35% gain in energy (and thermal efficiency)

Now once you push each caliber to their respective limits, that relationship changes, fast....and larger calibers are no longer 'more efficient', rather require more air to push their respective boundary. That 30 cal has far more energy potential left in the system then the .22 at 47 fpe for example.


-Matt

How does weight factor into this? I would assume the heavier the pellet the more energy is needed to push it which maximizes the energy efficiency but there has to be a point where you lose efficiency. There has to be some sort of bell curve where if you used 1000cc of air at 1750 psi, lower weight means wasting potential energy but too heavy means you lose more energy than gained in the form of lower foot pounds of energy per shot.

Is there a math equation that allows you to calculate this? or is it impossible as each rifle is different? You should at least be able to calculate estimates of expected values though?

 

[lbc_PSI]

Had today off from work - beautiful day in mid 70s (!) in Connecticut - so I played …

I had created a “40 Yard Challenge” target card. Tried it with my RTI Prophet Performance V1 .22 LR - set a new personal best.

My Prophet .22 was tuned for 100 yards - shooting 25.4g MRDs at 912fps.

Since its shooting so well, I didn’t want to touch the tune, but I had a new (to me) .30 cal barrel and Huma Mod 40 moderator in .30 that I was dying to try out on my Prophet…

… so I swapped barrels and pellet probes and shot my 1st .30 cal pellets with the Prophet. The 1st 2 groups were at 40 yards and the next 3 groups were at 50 - all were 1/2” or less ctc - some were really tight! Using FX (JSB) 44.8g pellets.

To my surprise, when I chronoed the 44.8g pellets they were going 850.2 fps…

I thought they would be going much slower.

In .22 cal it was shooting 25.4g @ 912fps = 47 ft/lbs.

In .30 cal (changing only barrel and pellet probe) it shoots 44.8g @ 850fps = 72 ft/lbs.

So it seems .30 cal is much more efficient than .22 - in that I gained 25ft/lbs of energy simply by swapping from .22 to .30 - leaving hammer spring and regulator exactly the same.

Does this sound correct ?

-Ed

Yep. Sounds about right. Did approximately what you did but -25 to .30. 24” bbls in both calibers. No other changes. .25 FX Hybrids @ 940 to .30 FX Hybrids @ 840 if I remember correctly

 

[Stubbers]

How does weight factor into this? I would assume the heavier the pellet the more energy is needed to push it which maximizes the energy efficiency but there has to be a point where you lose efficiency. There has to be some sort of bell curve where if you used 1000cc of air at 1750 psi, lower weight means wasting potential energy but too heavy means you lose more energy than gained in the form of lower foot pounds of energy per shot.

Is there a math equation that allows you to calculate this? or is it impossible as each rifle is different? You should at least be able to calculate estimates of expected values though?

Lighter pellets are less efficient as their stroke is greater during your valves 1.5~ (avg pcp) ms dwell, they move further down the bore, allowing more air to be released within that given barrel volume the pellet has exposed in such time.

Keeping pellet weight the same, say 25 grain for .177, .22, 25 cal, the 25 cal will absolutely destroy the other 2 in efficiency by the number I quoted in my previous post, depending on barrel length and pressure. FPE/CI is basically how much energy you're extracting per air released, expressed in cubic inches. In a well tuned system that can be derived by FPE / barrel volume used while valve is open. A 40 fpe .25 cal should have a higher fpe/ci than a 40 fpe .22 cal for this reason. More bore volume in a given distance means the valve can shut sooner, and allow the projectile to build energy and or coast on the remaining pressure.

For a pcp system (energy storage/release device) there are too many variables for a one size algorithm that fits all. An algorithm setup for a 12" barrel .22 cal, with only .1" ports at 2000 psi will be very much different than a 24" barrel 22 cal, full ported at 3200 psi, nominal pellet weight changes drastically in each system.

There is probably a general goldilocks formula that will work under certain circumstances, but would break if taken to extremes, for that I have yet to see any proof or data collected that has provided me a pattern other than, heavier pellets shot slower result in great efficiency, until it tapers off around 2.4 fpe/ci, which is a constraint in extracting energy from air in the temperature and pressure ranges we currently operate. That limit is generally obtained well below the ideal velocity for pcps, as in I have only achieved around 1.8- fpe/ci @ 25 fpe in 25 cal personally, which is basically lobbing 25 gr at 670 fps out of a ported marauder/20" barrel @ 2000 psi for 25 fpe.

A pcp's projectile is generally absorbing only 15-25% of the energy released from the system (25% in well tuned), doesn't matter if its a .177 or .25 cal shooting 40 fpe, both system would need to eject *nearly* the same mass of air to obtain that 40 fpe, and simply put, the .177 will need a lot more barrel length to make up for the lack of barrel diameter, rendering it less efficient in a system with equal barrel length, given the system with equal barrel length, the .177 will need to release a bunch of more air. Now give the .177 more barrel, and what happens, oh more pressure drop over the distance the valve remains open, therefore you have to keep it open even longer, hence *nearly*, as in the .177 should always require more air to produce 40 fpe than the .25 cal unless you short hand the .25 cal.

-Matt

 

[Scotchmo]

Assuming the same amount of compressed air used for a .22 vs a .30 shot.

The volume of expansion determines the energy potential. For equal barrel lengths, the .30 has about 90% more barrel volume.

A 30” barrel on a .22 caliber airgun would have about the same energy potential as a 16” barrel on a .30 caliber airgun.

 

[Stubbers]

Assuming the same amount of compressed air used for a .22 vs a .30 shot.

The volume of expansion determines the energy potential. For equal barrel lengths, the .30 has about 90% more barrel volume.

A 30” barrel on a .22 caliber airgun would have about the same energy potential as a 16” barrel on a .30 caliber airgun.

Energy potential yes, but how would the efficiency be effected in either system given different projectile weights, and energy output, is there a formula or equation that simplifies the effect across the board? Idtso?

-Matt

 

[Stubbers]

I suppose bore volume could be used grossly, but it will not scale imo nor account for pressure drop down the bore.

IE: a 30" 22 cal barrel should be less efficient than a 16" barrel 30 cal, because the 30" .22 bore sees greater pressure drop down the bore than the 30 cal (not per inch, but in the needed distance to remain open), and that initial burst of pressure (first 8" of a pcp) determines the majority of your terminal velocity. This would be if both were AIMED for peak efficiency in terms of fpe/ci.

however the above is conjecture, but at the same pressure/plenum and % equal porting, I don't doubt the 30 cal would be more efficient in the above tests for peak efficiency (higher energy output on equal air mass ejection).

-Matt

 

[Scotchmo]

Energy potential yes, but how would the efficiency be effected in either system given different projectile weights, and energy output, is there a formula or equation that simplifies the effect across the board? Idtso?

-Matt

Yes there are formulas .

For a given amount of air used:

1) Projectile mass won’t change total system energy available. But the heavier projectile will be a little more efficient in terms of muzzle energy.

2) Increased barrel volume increases air efficiency. Say you had an airgun tuned for maximum fpe, if you increase the barrel length by 2.414x, the system efficiency (and energy) is doubled.

 

[Stubbers]

Lets take my 25 cal shooting 34 gr @ 60 fpe for 1.32 FPE/CI for example against a .22 cal doing 40 fpe @ 1.33 fpe/ci...

For the 25 cal that is 5.26 CC ejection, bore volume used while valve open is roughly 6.5" (or 9" for adiabatic expansion)

A buddy recently shot his .22 cal @ 1.33~ fpe/ci @ 39.8 fpe with 25.4's... That is 2.95 CC ejected @ roughly 4.7" barrel distance (or 6.6" adiabatically).

I see no correlation to the above at nearly the same fpe/ci, because the two guns are wildly different, from barrel length, pressure used, caliber, heck the only thing identical is porting.

If you can arrange a formula that expresses how that 1.3 FPE figure comes about across the board, I will very, very slowly...clap.

-Matt

 

[Stubbers]

Yes there are formulas .

For a given amount of air used:

1) Projectile mass won’t change total system energy available. But the heavier projectile will be a little more efficient in terms of muzzle energy.

2) Increased barrel volume increases air efficiency. Say you had an airgun tuned for maximum fpe, if you increase the barrel length by 2.414x, the efficiency (and energy) is doubled.

What is the formula to express how fpe/ci works across calibers given a barrel volume? I don't see it here? I'd love to test it...

FWIW the 25 cal above used a 19.5" barrel and the .22" used 23.2"...

15.7cc barrel volume vs 14.5 cc, 8% bore volume difference.

Yet the .25 cal makes 34% more energy at the same FPE/CI...

At the same energy level for both (40 fpe) there was closer to a 9% difference in efficiency, but I do not consider pitting 2 calibers against each other the same energy fair. So while that may be close for equal energy output, it's not how you would compare a .22 to a .30 cal.

Not to mention pellet weigh skews the above incredibly, and its not ideal to maintain projectile mass from caliber to caliber, so a .25 and a. 22 cal both shooting 25.4 at 40 fpe may be an outlier.

Not to mention the .22's avg pressure was 2500 psi (3000-2000 psi test) and mine was 2000 (regulated in both 40/60 fpe tests)...so on avg the .22 also has 20% more pressure.

 

[Stubbers]

An algorithm that expresses all the changes to efficiency across calibers would have to include at the bare minimum the following dominant variables that effect efficiency...without these the algo would fail to express the changes clearly

Caliber (assuming full bore to omit porting from algo)
Barrel length
Pellet weight
Pressure

Bore volume and pressure are what predominantly make FPE, be there a projectile to absorb it or not...pellet weight effects FPE, so you cannot omit pressure, nor pellet weight from such formula to determine how fpe/ci changes across calibers. Likewise pressure, you won't get the same efficiency in a system designed at 400 psi and large volume vs 4000 psi and small volume

 

[Stubbers]

Being that the first few inches dominates the outcome (terminal velocity) of a pcp, I think surface diameter players a far greater role than bore volume in fpe/ci personally...although purely conjecture until I play with calibers more in my life span.

-Matt

 

[Stubbers]

Suffice to say if fpe/ci was based on and could be formulated on barrel volume alone an infinitely longer barrel would have infinitely more efficiency. There are too many constraints.

Even being more modest and ideal, a 50" .22 cal will not be as efficiently capable as a 39" .25 cal barrel, the further you take these lengths out, the less capable the smaller caliber becomes...this suggests surface area matters more than volume.

FPE/CI is simply the energy obtained within a given barrel length used while valve is open...surface diameter matters more here to reduce / minimize the barrel volume used to obtain X fpe..

-Matt

 

[Stubbers]

I think the simplest formulation would be along the lines of...

Bore diameter surface area increase = potential efficiency and power increase.

So if you go from .22 cal to .30 cal, and the energy potential of the .22 is 50 fpe and .30 cal is 90 fpe (both 20" barrels, 2150 psi shooting 25.4 / 44 gr, 90% bore port and 1 fpe/cc of plenum) the 30 cal can be tuned to be nearly 46% more efficient be that in power output, or air saved at identical power, while needing identical increase in projectile mass for that gain, based on their surface area alone, using the barrel volume follows the same ratio until it breaks by going too far in length.

So doubling your caliber gives roughly 4x the potential energy/efficiency (not precisely), as surface area pi * radius squared .... and would require 4x the projectile weight to maintain the increase expressed in power (likely needing more dwell to express max increase in power), or the same weight to express it in efficiency alone (Likely requiring longer barrel to see peak efficiency equal to difference in caliber surface area).

I don't think that its perfectly linear as bigger surface area has more friction that must be accounted for, among other factors, such as sharper pressure drop behind the projectile, needing more space to accelerate (caliber length/pellet mass co-efficient)...so its really not a perfect rule of thumb or a linear style expression.

(To be clear, caliber, pellet weight, barrel length, port size, state of tune and other variables greatly effect efficiency but are not the subject matter of OP)

-Matt

 

[weevil]

While the hammer strike opens the valve to the same degree for each caliber, the pressure differential between the air in the plenum and air behind the projectile dictate how long the valve stays open. With the lighter projectile, the differential needed to close the valve is arrived at in a shorter time, hence less air is used in spite of having the same hammer strike.

 

[explorer]

Lets take my 25 cal shooting 34 gr @ 60 fpe for 1.32 FPE/CI for example against a .22 cal doing 40 fpe @ 1.33 fpe/ci...

For the 25 cal that is 5.26 CC ejection, bore volume used while valve open is roughly 6.5" (or 9" for adiabatic expansion)

A buddy recently shot his .22 cal @ 1.33~ fpe/ci @ 39.8 fpe with 25.4's... That is 2.95 CC ejected @ roughly 4.7" barrel distance (or 6.6" adiabatically).

I see no correlation to the above at nearly the same fpe/ci, because the two guns are wildly different, from barrel length, pressure used, caliber, heck the only thing identical is porting.

If you can arrange a formula that expresses how that 1.3 FPE figure comes about across the board, I will very, very slowly...clap.

-Matt

Not sure how much common it has with it but the projectile surface/projectile mass ratio is nearly the same at both.

 

[explorer]

While the hammer strike opens the valve to the same degree for each caliber, the pressure differential between the air in the plenum and air behind the projectile dictate how long the valve stays open. With the lighter projectile, the differential needed to close the valve is arrived at in a shorter time, hence less air is used in spite of having the same hammer strike.

Im not sure, if this approximation is right. Okay if we are talking about pressure assisted valve like RTIs it can be a mess like this but in a case of normal valve i think the plenum pressure is much more dominating the valve speed and/or opening depth than the barrel/port pressure.