What is the highest theoretical muzzle velocity possible from a gas piston rifle

[MrD1458]

Hello, I was wondering what the highest possible muzzle velocity from a gas piston air rifle would be. I’m curious because I just graduated with a degree in engineering and wanted to construct my own air rifle. I won’t be using pellets as they are very inaccurate at supersonic velocities, so I would be using slugs. Lastly, I want to build a fast shooting air rifle just to put my knowledge into practice and ultimately see if it’s possible, I understand it’s not as efficient as smokeless powder.

[HOSPassassin]

I'm sure that it is physically possible to produce a slug-shooting gas piston powered rifle that shoots faster than what is commercially available. The real engineering problems will involve three primary issues, IMHO:

1. Weight. A Hatsan 135 is a honking brute of a rifle and it is one of the fastest shooting gas piston rifles out there. Few would want to carry it for any great distance. Bigger mechanical components that lead to bigger velocities will also lead to more weight to carry and hold while firing. I could imagine something like a 30-pound Barrett rifle shooting a wicked fast little .22 slug but it's going to end up being a bench-only weapon. This negates what a lot of people prefer about springers and gas piston guns over PCPs, which is simplicity and portability.

2. Cocking force. Higher velocity will require more force applied to cock. Look up info on the Whiscombe spring air rifles. IIRC at least some of them were designed to be cocked with multiple strokes of the lever. To spread the effort over multiple easier motions will certainly help but again it negates the "cock once and off you go" attraction of non-PCPs.

3. Recoil. Higher velocity will mean greater recoil. I'm sure you already grasp how unpleasant a heavy-recoiling powder gun can be, but the problem is even worse in a piston-powered air rifle because of the bi-directional recoil. Harsh recoil generally hurts accuracy in any gun, but it is much worse when the recoil moves in opposite directions in quick succession. It requires the shooter to develop the correct way of holding that particular rifle which is unlike anything else that is fired from the shoulder. The recoil issue of course can be dampened by greater mass, but eventually you will run into problem 1 above. There are also ways to mitigate double recoil mechanically. Look up the current Diana recoilless spring rifles and the Diana GISS system. I don't think they use GISS anymore but it was quite revolutionary in its time. You'd probably have to scale up their innovations significantly to realize your goal. Again, that will lead to even more mass and bulk.

I could see an amazing one-off rifle that shoots .22 slugs like a .22 magnum happening but it's probably going to be limited to a proof-of-concept type effort.

[rsterne]

To be honest, I don't think maximum velocity in a Springer is anything anybody has ever seriously explored.... If you have not experienced the unpleasant shooting characteristics of a Magnum Springer, once you do you will understand.... There are a few design points you should ponder in Springer design.... One is the relationship between compression chamber volume and output energy (FPE).... The other is the preference for a given combination of compression volume, spring force, piston mass and transfer port size for a given (relatively narrow) range of pellet weight to produce maximum FPE.... Springer design is very complex, and somewhat counter-intuitive....

There is an excellent article by Dr. Domingo Tavella on ResearchGate on the Internal Ballistics of Springers you may find informative....  https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

The other main consideration is the basics of how airguns develop their FPE.... It is governed by the pressure, calibre and barrel length.... The absolute maximum FPE possible can be found by multiplying the force (pressure times area) by the distance (barrel length), but don't forget that includes the energy imparted to the mass of the air discharged, and all other losses.... Springers are notoriously difficult to determine the pressure, because they incorporate Adiabatic compression, where the increase in temperature increases the pressure as well, and once the pellet/slug moves that pressure drops rapidly.... Since projectile base area increases by the square of the calibre, larger calibres have an advantage in the FPE possible, but if the compression chamber volume is fixed, that soon becomes the limiting factor....

Bob

[HOSPassassin]

Are you hoping to just do this for fun and maybe some knowledge that you could apply to another project, or do you hope to maybe make custom shoulder-fired air cannons for special orders, or do you want to try to pitch an idea to a manufacturer? If it's just for curiosity and fun a lot of the concerns I raised wouldn't really matter. It might end up like the .950 JDJ of the airgun world.  It's only if you want to make it for other people who want to shoot it for practical purposes that mass, cocking force, and recoil management become important.

I imagine a 15-pound slug-shooting gas ram rifle might have some practical applications as long as those who use it are well aware of its limitations like portability and the danger it will pose to scopes. That's another downside of bi-directional recoil: it will destroy scopes that are not designed to withstand it. Even those that are sometimes aren't tough enough to last very long on a supermagnum springer like a 135 or a Gamo Magnum. A peep sight might be necessary because there may not be a scope out there that is built to survive what you're envisioning.

It's still probably going to be a niche product. But who knows, people who just like piston guns (a small but dedicated crew) or like cool new engineering solutions to problems might like it.

[ballisticboy]

There have been a number of computer based time stepping simulations of spring airguns over the years (I even created one myself at one time) some of which were made available to the general public. Most of them at some point in their calculations require calibration factors (fiddle factors in reality) to match measured performance. Once you start trying to use them at extremes, the accuracy gets debatable. You would really need to start with something like a simulation model to avoid a lot of wasted effort in building something which is never going to work.

[Rob M]

this could have been your senior engineering project , the complexity of something seemingly soo simple would be astounding.. Not many people have succeeded in powerful springers, I think 40 fpe was achieved by CAP airgun about 14 yrs ago. The theoben dual magnum used 2 gas rams and claimed 36 fpe. Im sure whiscombe has a model thats in the 30 fpe range also

[Scotchmo]

The theoretical limit is reached as the projectile mass approaches zero and the barrel length approaches infinity (when fired into a vacuum).

Maximum velocity= SOS x sqrt((2/(k-1))+1)

Higher temperatures raise the speed of sound (SOS) in the air.

The maximum theoretical velocity of a piston gun (starting with room temperature air) will be dependent on the compression ratio. Adiabatic compression increases the temperature. Since the speed of sound is higher in high temperature air, piston gun velocity can be considerably higher than that for a PCP that also uses room temperature air.

Though there are also practical limitations.

[MrD1458]

The theoretical limit is reached as the projectile mass approaches zero and the barrel length approaches infinity (when fired into a vacuum).

Maximum velocity= SOS x sqrt((2/(k-1))+1)

Higher temperatures raise the speed of sound (SOS) in the air.

The maximum theoretical velocity of a piston gun (starting with room temperature air) will be dependent on the compression ratio. Adiabatic compression increases the temperature. Since the speed of sound is higher in high temperature air, piston gun velocity can be considerably higher than that for a PCP that also uses room temperature air.

Though there are also practical limitations.

What would those practical implications be?

Update: So far I've been doing some hand calculations and found this software that seem to show that break barrel air rifles can achieve high velocities. I attached a picture of the software that I've been using as well as the gas piston I found

[Jim-in-UK]

What would those practical implications be?

Update: So far I've been doing some hand calculations and found this software that seem to show that break barrel air rifles can achieve high velocities. I attached a picture of the software that I've been using as well as the gas piston I found

A question if I may. What are the units of friction in the software?

[rsterne]

Sorry, I have a hard time believing the peak pressure in any Springer is over 210,000 psi.... 

I am wondering about the materials you plan to use, as steel melts at 1800*K.... 

Bob

[MrD1458]

What would those practical implications be?

Update: So far I've been doing some hand calculations and found this software that seem to show that break barrel air rifles can achieve high velocities. I attached a picture of the software that I've been using as well as the gas piston I found

A question if I may. What are the units of friction in the software?

I can’t seem to link the website, but if you search the name of the software from the first photo, you should be able to find the website with the coefficients.

[rsterne]

The "initial pressure" or 15 MPa equates to 2175 psi.... and the "maximum force" (I assume the cocking force) of 7100 Newtons is 1596 lbf.... I think both of those are unlikely, and I'm guessing that using the wrong units is causing you grief....

Bob

[Rob M]

Bob has some rough numbers on the ideal piston diameter and stroke for a 70 fpe springer, Id ask him those numbers and start designing the gun. The spreadsheet is not gonna help you because of the hidden compexities of a springer/gas ram gun.
 Bob , a few years ago we discussed some red instrustrial gas ram that had the stroke and bore to produce massive power , I have not seen it mentioed since.

[MrD1458]

The "initial pressure" or 15 MPa equates to 2175 psi.... and the "maximum force" (I assume the cocking force) of 7100 Newtons is 1596 lbf.... I think both of those are unlikely, and I'm guessing that using the wrong units is causing you grief....

Bob

I took the units into consideration. I understand that the cocking effort will be high, but I would build a hand crank similar to compound crossbows. This would allow me to compress the piston. 

[jackssmirkingrevenge]

To be honest, I don't think maximum velocity in a Springer is anything anybody has ever seriously explored

A light gas gun is essentially an extreme springer where the piston is propelled by chemical means and compresses a light gas like helium or hydrogen in the cylinder:



With these setups velocities of over 5 kilometers per second are achieved, obviously they are enormous and static laboratory devices that require a burst disc to retain the light gas so completely impractical as a shoulder fired device, but it does give some idea of what is possible when the relevant parameters are maxed out.

I understand that the cocking effort will be high, but I would build a hand crank similar to compound crossbows. This would allow me to compress the piston.

As others have mentioned, if it requires a hand crank it is going to be extremely unpleasant to shoot and will need to be heavily built in order not to destroy itself at every shot.

Do you have your heart set on a gas piston rifle or have you selected this type of airgun because commercially they are the ones that seem to be producing the highest velocities?

[MrD1458]

Thank you for your input. I’m not set on break barrels, I’m just very intrigued by them and want to see if it’s possible to produce high velocities for fun. Essentially putting my engineering knowledge to the test.

Would modified burst disc be effective in a springer to generate higher velocities?

[HatsanPhantom]

I hunt with break barrels and would be 100 percent interested in a gun like this. I shoot a Walther Parrus and a Hatsan 125 right now with great success

[Scotchmo]

The theoretical limit is reached as the projectile mass approaches zero and the barrel length approaches infinity (when fired into a vacuum).

Maximum velocity= SOS x sqrt((2/(k-1))+1)

Higher temperatures raise the speed of sound (SOS) in the air.

The maximum theoretical velocity of a piston gun (starting with room temperature air) will be dependent on the compression ratio. Adiabatic compression increases the temperature. Since the speed of sound is higher in high temperature air, piston gun velocity can be considerably higher than that for a PCP that also uses room temperature air.

Though there are also practical limitations.

What would those practical implications be?

Update: So far I've been doing some hand calculations and found this software that seem to show that break barrel air rifles can achieve high velocities. I attached a picture of the software that I've been using as well as the gas piston I found

There are many practical limitations. For instance, in your example, how do you make that spring that weighs only 1/30 of an ounce but stores 500 foot-pounds of energy?

[Jim-in-UK]

There are many practical limitations. For instance, in your example, how do you make that spring that weighs only 1/30 of an ounce but stores 500 foot-pounds of energy?

I wondered that, but assume it's the weight of the gas in the ram.

[MrD1458]

There are many practical limitations. For instance, in your example, how do you make that spring that weighs only 1/30 of an ounce but stores 500 foot-pounds of energy?

I wondered that, but assume it's the weight of the gas in the ram.

I was also making the same assumption