Big Bore Efficiency Thread

[rsterne]

HI guys.... The purpose of this thread is to gather ACCURATE information about the efficiency of big-bore airguns.... Although I'm sure you'll enjoy the "bragging rights" involved.... please treat this as a scientific, informational thread.... In other words, please make sure you supply accurate information and don't exagerate your results.... A database is only as good as the information input.... GARBAGE IN - GARBAGE OUT !!!

The information required is as follows:

1. Number of shots in the shot string (I normally include only shots within 10% of the highest velocity, 4-5% for longer range guns)
2. Average velocity of string (fps or m/s)
3. Pellet/bullet weight (grains)
4. Caliber (mm or in.)

From the above, I will calculate the total energy of the string by calculating the average FPE per shot and multiplying by the number of shots in the string.... I also need:

5. The volume of the air reservoir in either cc or cu.in.... OR the internal dimensions (diameter and length) so that I can calculate that
6. The pressure at the beginning of the shot string (psi or Bar)
7. The pressure after the last shot in the string (psi or Bar)

From that information, I will calculate the volume of air used at normal atmospheric pressure (1 Bar) by calculating the pressure drop in Bar and then multiplying that by the volume of the reservoir in cu.in. to give the amount of air used at 1 Bar....  Once we know the total energy of the string (FPE) and the volume of air used (at 1 Bar) we divide the FPE by the cu.in. to get the efficiency in FPE/ci.... The bigger the number, the more efficient the gun.... as each cu.in. of air is producing more energy.... So far, the range of values I have measured is between 0.58 FPE/ci and 1.97 FPE/ci.... although I imagine that we will see values outside this range as the number of measurements increases....

Having the added information about the type of gun, mods, and pellet/bullet would of course add useful information....

Bob

[Nomadic Pirate]

Interesting thread idea, too bad I won't even be able to read it since i can't wrap my head around those things


I personally calculate efficiency with how dead the dead critter is

[rsterne]

Just forget the mumbo-jumbo and look at the last number.... the FPE/ci.... basically, the more FPE the gun delivers compared to the amount of air used, the more efficient it is in using that air.... 

Bob

[melloroadman]

That 10% really can not be applied to high power B.B . with high velocities as far as accuracy goes .JMO.Marvin

[rsterne]

Not sure what you mean.... do you mean less spread is acceptable?.... or more?....

In any case, whatever velocity spread YOU find is acceptable is fine with me.... those were just a guideline based on what I use for .177 to .25 cal.... The calculations are based on the AVERAGE energy and TOTAL air usage for the string.... It is possible to do an efficiency calculation for each shot.... but you need an accurate pressure reading before and after each shot to do that....

If nobody is interested, that's fine too.... I am, but I'm pretty weird about this stuff anyway.... I like to know WHY things work the way the do.... not just that they work at all.... 

Bob

[Butcher45]

     Keep in mind that consistent gauge readings on one gauge may read a little different with another gauge.....though using the same gauge with the same rifle should work great.

     Some of these gauges are pretty tough to read down to a 25-or-so psi variance, so extra diligence would be required of us all when filling (especially with a tank). 

     Also, fill your reservoir to a certain pressure with a pump, then bleed the pump.  Screw the bleed valve back in, and pump just to the point you see how much air is in the rifle without putting more in.  How much was lost from the bleeding if anything?

     I've been encouraging bigbore airgun shooters to record their end fill pressures for a long time.  Seems most of them are mesmerized by the amount of FPE their rifle has, and fail to take any notice of their rifles' efficiency.  When you refill your tuned 909 every 3-4 shots using a hand pump, you soon realize an extra shot per air fill really amounts to a lot!  I'll dig up some more numbers for you

[Butcher45]

Interesting thread idea, too bad I won't even be able to read it since i can't wrap my head around those things


I personally calculate efficiency with how dead the dead critter is

Try shooting 4 roundballs thru the chrony, then record the end fill pressure as you air back up, the shoot 4 EPP/UG's and record how much air is left after shooting them.  The EPP/UG string should have more air left in it, with a tighter fps variance than the RB due to the efficiency.

[Nomadic Pirate]

Your are absolutely right Chris, I could tell that when I stated shooting the EPP/UG, but I have no chronograph

[Butcher45]

Your are absolutely right Chris, I could tell that when I stated shooting the EPP/UG, but I have no chronograph

Oh you got to get a chrony Manny.

Must.......See.......909S Tune........Numbers!

[lloyd-ss]

Bob,
Very nice work!  I am into the theory, too, and depending on a person's expectations of a gun, the efficiency of a gun in FPE/std cuin of air may or may not be important. 
I think most big bore shooters who work with the adjustments on their gun have already experienced this when they adjust to a point where they are using more air, but not gaining any velocity.  They know the efficiency is down, and your calcs will quantify that change.

If 2 or 3 high power shots is all you want, then eff really is not a big concern, but if you want to get a 20 shot string out of a .308, then it becomes a issue, maybe not for the user, but for the developer. Can you get those 20 shots with a 250cc reservoir, or do you need 500cc.  For the smaller PCPs shot count is a big deal.

I think just knowing that some guns are more air efficient than others is important,  even if understanding the math behind it is more of a personal decision.  Call it weird or a geek thing but I enjoy learning the theory behind these applications, and how we can make them better.    My wife says it is a sickness, LOL.
Lloyd

[rsterne]

If they ever find a need for GEEK bookends.... I think we make a pair.... 

Bob

[Nomadic Pirate]

Your are absolutely right Chris, I could tell that when I stated shooting the EPP/UG, but I have no chronograph

Oh you got to get a chrony Manny.

Must.......See.......909S Tune........Numbers!

LOL, I get my numbers from Will when he shoots my guns I'm not the chrony type, i look where my rounds hit

[lloyd-ss]

LOL, I get my numbers from Will when he shoots my guns I'm not the chrony type, i look where my rounds hit

Ha, Ha.  Yup!  If you can't hit the target, not much point to it...... but I do love the science part of it, too.
Lloyd

[lloyd-ss]

If they ever find a need for GEEK bookends.... I think we make a pair.... 

Bob

Bob, I think you are right!
Now I can tell my wife that I am not the only one with the sickness LOL.
Lloyd