I just installed a Hammer Debounce Device (HDD) on my MRod .25. I took velocity, pressure and acoustic data before and after installing the HDD. I got moderate increases in shot count per fill (8 additional shots). The most significant improvement was that the velocity profile was much smoother (less shottoshot variance in velocity). The HDD produced a sweet spot of 11 shots from 2500 to 2100 psi with a standard deviation in velocity of only +/ 2.5 fps. From 2500 to 2300 the standard deviation was less than +/ 1fps for six shots.
My engineering analysis of the HDD performance can be found athttp://www.myopinion1.info/
I also present a spectral analysis of the improvement in report acoustics.
I'm an engineer but somewhat new to air guns. I'm just starting to learn how to tune the MRod 25. Any advice is appreciated.
Regards,
Frank, username = FS2020
ps: Does anyone have the differential equations describing the gas expansion and pellet velocity from an air gun reservoir? I assume this problem can be approached by placing a control volume around the expanding air. I'm working on solving it now, but I'm sure someone has solved this and programmed the differential equations already. I'm an old timer and still program in Intel Fortran.

 Joined: May 16th, 2012, 6:52 pm
Frank  great analysis. While I don't own a MRod (I have a .22 Discovery), I did forward your link to some friends that do own the .25 cal MRod. The debouncer looks like a good investment. I am a retired reliability engineer, so I liked your analysis.

 Joined: March 18th, 2012, 3:36 am
Outstanding visual representation of the measurable entities. Thanx!I just installed a Hammer Debounce Device (HDD) on my MRod .25. I took velocity, pressure and acoustic data before and after installing the HDD. I got moderate increases in shot count per fill (8 additional shots). The most significant improvement was that the velocity profile was much smoother (less shottoshot variance in velocity). The HDD produced a sweet spot of 11 shots from 2500 to 2100 psi with a standard deviation in velocity of only +/ 2.5 fps. From 2500 to 2300 the standard deviation was less than +/ 1fps for six shots.
My engineering analysis of the HDD performance can be found athttp://www.myopinion1.info/
I also present a spectral analysis of the improvement in report acoustics.
I'm an engineer but somewhat new to air guns. I'm just starting to learn how to tune the MRod 25. Any advice is appreciated.
Regards,
Frank, username = FS2020
ps: Does anyone have the differential equations describing the gas expansion and pellet velocity from an air gun reservoir? I assume this problem can be approached by placing a control volume around the expanding air. I'm working on solving it now, but I'm sure someone has solved this and programmed the differential equations already. I'm an old timer and still program in Intel Fortran.

 Joined: May 8th, 2001, 4:06 pm
Go online (amazon, pyramid air, etc) and find a copy of the Cardew books, the 'airgun from trigger to muzzle' was the first, I think, but better and more comprehensive is 'trigger to target'. Plenty of equations for you to choose from, plus the most science of any publication that I know of. Dated, pretty primitive equipment, but still relevant.I just installed a Hammer Debounce Device (HDD) on my MRod .25. I took velocity, pressure and acoustic data before and after installing the HDD. I got moderate increases in shot count per fill (8 additional shots). The most significant improvement was that the velocity profile was much smoother (less shottoshot variance in velocity). The HDD produced a sweet spot of 11 shots from 2500 to 2100 psi with a standard deviation in velocity of only +/ 2.5 fps. From 2500 to 2300 the standard deviation was less than +/ 1fps for six shots.
My engineering analysis of the HDD performance can be found athttp://www.myopinion1.info/
I also present a spectral analysis of the improvement in report acoustics.
I'm an engineer but somewhat new to air guns. I'm just starting to learn how to tune the MRod 25. Any advice is appreciated.
Regards,
Frank, username = FS2020
ps: Does anyone have the differential equations describing the gas expansion and pellet velocity from an air gun reservoir? I assume this problem can be approached by placing a control volume around the expanding air. I'm working on solving it now, but I'm sure someone has solved this and programmed the differential equations already. I'm an old timer and still program in Intel Fortran.

 Joined: April 7th, 2012, 10:05 pm
Youre the man! ntI just installed a Hammer Debounce Device (HDD) on my MRod .25. I took velocity, pressure and acoustic data before and after installing the HDD. I got moderate increases in shot count per fill (8 additional shots). The most significant improvement was that the velocity profile was much smoother (less shottoshot variance in velocity). The HDD produced a sweet spot of 11 shots from 2500 to 2100 psi with a standard deviation in velocity of only +/ 2.5 fps. From 2500 to 2300 the standard deviation was less than +/ 1fps for six shots.
My engineering analysis of the HDD performance can be found athttp://www.myopinion1.info/
I also present a spectral analysis of the improvement in report acoustics.
I'm an engineer but somewhat new to air guns. I'm just starting to learn how to tune the MRod 25. Any advice is appreciated.
Regards,
Frank, username = FS2020
ps: Does anyone have the differential equations describing the gas expansion and pellet velocity from an air gun reservoir? I assume this problem can be approached by placing a control volume around the expanding air. I'm working on solving it now, but I'm sure someone has solved this and programmed the differential equations already. I'm an old timer and still program in Intel Fortran.

 Joined: May 12th, 2001, 1:29 pm
...to your question about an analytical solution to the equations of airgun interior ballistics. Unfortunately, I don't.I just installed a Hammer Debounce Device (HDD) on my MRod .25. I took velocity, pressure and acoustic data before and after installing the HDD. I got moderate increases in shot count per fill (8 additional shots). The most significant improvement was that the velocity profile was much smoother (less shottoshot variance in velocity). The HDD produced a sweet spot of 11 shots from 2500 to 2100 psi with a standard deviation in velocity of only +/ 2.5 fps. From 2500 to 2300 the standard deviation was less than +/ 1fps for six shots.
My engineering analysis of the HDD performance can be found athttp://www.myopinion1.info/
I also present a spectral analysis of the improvement in report acoustics.
I'm an engineer but somewhat new to air guns. I'm just starting to learn how to tune the MRod 25. Any advice is appreciated.
Regards,
Frank, username = FS2020
ps: Does anyone have the differential equations describing the gas expansion and pellet velocity from an air gun reservoir? I assume this problem can be approached by placing a control volume around the expanding air. I'm working on solving it now, but I'm sure someone has solved this and programmed the differential equations already. I'm an old timer and still program in Intel Fortran.
However, if "(un)reasonable approximations," interest you, here're a couple.
The "average" driving pressure at the breech required to achieve a given muzzle velocity is determined by...
1. Caliber.
2. Barrel length.
3. Pellet weight.
4. Propellant density.
Here's the semiempirical formula I've had some predictive success with...
Feet_per_second = 172 x sqrt(Q / (grains + Z x Q))
Where: Q = Pavg x barrel_inches x caliber^2
Pavg = average driving pressure in psi
Z = 0.011 for air, 0.02 for CO2, 0.002 for helium.
In a PCP, the generation of the driving pressure occurs in two phases...
1. "Isobaric" phase = the interval while the firing valve is open and connecting the breech to the reservoir, maintaining (approximate) constant pressure.
2. "Adiabatic" phase = after the valve closes and the dispensed charge expands (and cools) behind the pellet, causing an exponential drop in pressure proportional (for a  mostly  diatomic gas like air) to volume^{1.4}.
If there's a closeform solution to this mishmash, I have no idea what it would look like. But simple iterative solutions converge rapidly and seem to lead to fairly realistic results.
Have fun.
Steve

 Joined: April 24th, 2012, 12:56 am
Steve,
For practical design purposes, the kind of empirical equations you provided are most useful. I found an full model with moving control volume from an army research report. Even has Fortran code. Will update when I have time.
Would be nice to model the whole thing with CFD, but that's a huge effort and far beyond a hobby.
Regards,
Frank, MSME,
For practical design purposes, the kind of empirical equations you provided are most useful. I found an full model with moving control volume from an army research report. Even has Fortran code. Will update when I have time.
Would be nice to model the whole thing with CFD, but that's a huge effort and far beyond a hobby.
Regards,
Frank, MSME,

 Joined: February 22nd, 2009, 11:38 pm
You mentioned that you saw an increase in usable number of shots and more consistant velocities but....my question is....did you see a drop in average velocity after the installation?I just installed a Hammer Debounce Device (HDD) on my MRod .25. I took velocity, pressure and acoustic data before and after installing the HDD. I got moderate increases in shot count per fill (8 additional shots). The most significant improvement was that the velocity profile was much smoother (less shottoshot variance in velocity). The HDD produced a sweet spot of 11 shots from 2500 to 2100 psi with a standard deviation in velocity of only +/ 2.5 fps. From 2500 to 2300 the standard deviation was less than +/ 1fps for six shots.
My engineering analysis of the HDD performance can be found athttp://www.myopinion1.info/
I also present a spectral analysis of the improvement in report acoustics.
I'm an engineer but somewhat new to air guns. I'm just starting to learn how to tune the MRod 25. Any advice is appreciated.
Regards,
Frank, username = FS2020
ps: Does anyone have the differential equations describing the gas expansion and pellet velocity from an air gun reservoir? I assume this problem can be approached by placing a control volume around the expanding air. I'm working on solving it now, but I'm sure someone has solved this and programmed the differential equations already. I'm an old timer and still program in Intel Fortran.

 Joined: April 24th, 2012, 12:56 am
Brian,
Thanks for the question. The average velocity (as averaged over all of the shots for a fill from 2900 to 1600 psi) decreased from 843 fps to 828 fps, or by 1.7%, with the HDD.
In my report athttp://www.myopinion1.info/
I show a plot of the velocity as a function of shot number over the 2900 to 1600 psig range. The plot gives a good description of the velocity curve. If you have any other questions, feel free to shoot them my way.
Regards,
Frank
Thanks for the question. The average velocity (as averaged over all of the shots for a fill from 2900 to 1600 psi) decreased from 843 fps to 828 fps, or by 1.7%, with the HDD.
In my report athttp://www.myopinion1.info/
I show a plot of the velocity as a function of shot number over the 2900 to 1600 psig range. The plot gives a good description of the velocity curve. If you have any other questions, feel free to shoot them my way.
Regards,
Frank