Chamber pressure, recoil, and powder burn rate...

[jstein650]

I know, not again, but I was thinkin' - I wish I wasn't, but hey, that's part of the fun of this forum. A while back the ubiquitous subject again came up, and some lively discussion followed. I posited that the only significant factors with regard to recoil were: the mass of the firearm, the mass of the projectile, the mass of the propellant, and the muzzle velocity of said combined ejecta. This led to the path of reasoning the definition of the objective 'free recoil energy' and the subjective 'felt recoil' debate. At any rate, inevitably, the idea of chamber pressure emerged, with the reasoning being that a faster powder will produce a higher chamber pressure, whose action upon the bolt face will necessarily result in a higher linear force upon the shooter. I argued that while that may be true, the time that it is exerted will be for a shorter period of time and that should obviate any difference in 'felt recoil'.

After reading a few posts on subjects touching the same question, it occurred to me that chamber pressure itself has nothing to do with recoil actually. In the same way I can hold a fully charged scuba tank. Nothing happens. The pressure in a gun chamber is acting equally on all sides, including the base of the bullet on the opposite end of the bolt face. The exterior world (the guy holding the gun) doesn't know anything until that projectile begins to move, and is pushed in the opposite direction, due to inertia.
With regard to understanding powder burn rate, most folks agree that in most all cartridges, the powder is pretty much 'consumed' before the projectile moves more than a even an inch, yet there are significant disparities when it comes to velocity in even the shortest barrels, the slowest powders always giving the highest MV's. Of course, with current powders, it always takes more of a slow powder to attain or surpass the max vel. you can get with a faster powder. Both powders will be able to hit the pressure ceiling, but the slower can hold it there for a bit longer. After the initial ignition, there will be a perhaps slight, but significantly longer duration for which that powder is still exerting an increasing amount of force on the the bullet (in an EXPANDING combustion chamber VOLUME, no less) and that will get you a higher MV. The drawback, if you will, is that propellant mass is always given a 25 to 50% increase in recoil figures since it always exceeds the speed of the projectile when firing.
Bottom line: We're not getting anywhere a linear constant acceleration from modern handguns, shotguns, or rifles throughout of the length of their barrels, so the slower powder=softer recoil does not really make sense.

 

[Njal Thorgeirsson]

I agree, and (from extensive physics knowledge) can attest that the only thing that significantly effects recoil (besides weight of the firearm) is the momentum of the projectile that exits the gun.

Reported pressure is not an indicator, as only MAX pressure is reported. There is a large pressure spike at the beginning of the burn, which is what is reported. This pressure spike is not indicative of the projectiles final velocity, or pressures over the course of the projectile's time in the barrel. A slow-burning powder will exert a high pressure for a longer period of time. Slow vs fast burning powder is simply a question of which powder maintains the highest possible pressure for the duration of the bullet's travel in the barrel without an unsafe pressure spike at the beginning.

 

[jmr40]

You are seriously overthinking this. As you stated before recoil is easily calculated using 4 factors

Projectile weight
Rifle weight
Projectile speed
Powder weight

There are numerous online programs that let you plug in the numbers and calculate recoil

the only thing that significantly effects recoil (besides weight of the firearm) is the momentum of the projectile that exits the gun.

Technically correct, but the powder charge weight is also a factor that is often overlooked. I can come within 100 fps of my 30-06 speeds with my 308 shooting the same bullet weights, but with considerably less powder and recoil.

30-06/165, 58 gr powder, 2850 fps, 22 ft lbs recoil
308/165, 47 gr powder, 2760 fps, 17 ft lbs recoil

19% less powder
3% less speed
23% less recoil

 

[tactikel]

I would just add that felt recoil is more than just kinetic energy. Stock configuration, muzzle blast, and type of propellant and projectile, and action must be factored in. A 20 ga single shot with a heavy drop in the stock may have more felt recoil than a 12 ga 1100. A 69 cal flintlock muzzleloader may have less recoil than a .30-30 Win 94. A .308 scout gun with a 16" barrel may have more pop than .30-06 with a 22" barrel. IMHO recoil is not just about numbers.

 

[Steel Horse Rider]

I think you are all overlooking the area that the force is applied over, meaning the cross sectional area of the butt of the stock. I believe that is why folks believe the Mosin has much more recoil than other 30 caliber rifles. The Mosin stock is pretty thin and not padded so the counter force generated by the bullet leaving the barrel is applied to a much smaller area of the body and only cushioned by steel.

 

[denton]

Hatcher has a very nice discussion of recoil in Hatcher's Notebook. It works through the physics, starting with first principles, such as Conservation of Momentum. If you want to understand what's going on, that's a very good starting point.

Still, there are some problems with the classical approach.

First, we do not know the typical speed of the ejected propulsion gas. An assumed value for the average speed is used, the value being chosen to reconcile measured results with theoretical calculations.

Second, the model that is used assumes a freely recoiling rifle and reports a number that is not correlated with our sense-perception. A freely recoiling rifle, and a rifle against a shooter's shoulder are two entirely different systems. Our senses perceive pressure, which is force per unit area. The standard calculation is for momentum, not force or pressure. It assumes that a 1 ft/sec^2 acceleration for 100 seconds feels the same as a 100 ft/sec^2 acceleration for 1 second produce the same sensation. Of course, they do not.

A hunting rifle accelerates at about 500 g, for a millisecond or two, and travels only about 1/8" while accelerating.

 

[barnbwt]

Brief explanation (the real one )
Higher pressure, louder boom, rougher over-pressure --higher perceived recoil
Higher pressure, more rapid acceleration (greater "jerk") --sharper perceived recoil

Theoretical explanation(real, but not as pronounced as the above)
Slow powders tend to be used to deliver a longer duration of high pressure to a heavier projectile to increase it's power from both long and short barrels (since powder burn is over after even a few inches of barrel, normally). If the same weight projectile is accelerated to the same velocity using magnum and blank powder, you bet there'd be a difference in perceived recoil. If gas operated, one would probably fail to cycle, if blowback, one would probably rupture the case, and if bolt action with a metal plate, one will rap your knuckle on the trigger guard more (but not bump your shoulder noticeably harder ). Interestingly, the muzzle pressures would probably be about equal from a rifle length barrel --slow powder isn't really that much slower, and pressures would already be rapidly falling.

If a carbine vs. full length gas tube can appreciably change bolt carrier and recoil dynamics, it makes sense that a great difference in pressure curve would mess with them (and the shooter) in detectable ways as well.

TCB

 

[brickeyee]

The free recoil free energy calculation is simple.

Conserve momentum (ejecta momentum = gun momentum).

Convert gun speed from momentum into energy (watch units, you need mass for energy, NOT weight).

How that feels (impulse calculations cover speed, but are more complicated) is very individual and depends on stock (or grip) design, barrel height over stock (or grip), and to many other factors that are not subject to simple calculation.


Get the wrong but angle and the long gun kicks you in the face with each shot.

Get a pistol bore above the grip height and rotation in recoil becomes a larger factor.

Felt recoil is just that, felt.