Power Factor versus Energy

[Howard 538]

I was wondering if someone could explain the real world difference in these two measures.  I generally shoot 147 gr 9mm and people always comment on the sound my rounds make versus the 115 or 124 grain that most people seem to shoot.  When I did some calculations I find that this ammo (from the supplier I use) seems to have higher power factor but lower energy.  Seems strange that one measure goes up with grain weight while the other goes down.  (see attached image for numbers)

[Vlad G 345]

Power factor is momentum (p) which is mass times velocity, energy is kinetic energy (Ek) which is mass times velocity squared divided by 2. 

 

p = m*v

Ek= 1/2m*v*v 

 

As a result higher mass makes a bigger difference in the calculation of pf and velocity makes a much bigger difference in measuring E. 

 

However, the relevance of those two numbers when it comes to terminal effects is debatable. 

 

1 Oz slug is roughly 0.028kg, moving about 400m/s. Its energy is about 2240J. Its momentum is 11.2Kg*m/s

55gr bullet is roughtly 0.0035kg moving at 975m/s. Its energy is about  1663J. Its momentum is  3.4Kg*m/s

 

(this kind back of napkin math, I reserve the rights to screw it up)

 

So .. energy of the two is relatively close because one is moving twice as fast while weighing a tenth of the other. However momentum is far different.

 

I'm personally inclined to consider momentum (power factor) as a more relevant factor when it comes to ammunition but I wouldn't argue to hard with someone that holds an opposite view. 

 

Edited and kinetic energy is 1/2mvv which I fixed. 

[FishHunter 0]

Energy calc uses V squared, small increase in V makes greater calc factor than small mass diff. At same velocity, heavier bullet has more emergy

[Howard 538]

Thanks guys, I understand the physics, and did the calcs in the attached image.  My question relates more to real world shoot.  I find that I shoot the heavier ammo better than the lighter.  I was surprised when I did the calculation as it has a higher power factor than the lighter ammo and everyone seems to claim you are better off with lower power factor (that meets match requirements).

[Vlad G 345]

You are trying to compare qualitative and quantitative things. 

 

The thing that both PF and Ek leave out is acceleration, which basically becomes the rate at which recoil is transmitted. To oversimplify heavier bullets accelerate slower take longer to impart their push on the gun and that results in a "softer" feel because the "recoil" is spread over a longer period of time.  People talk about snap vs push, where some loads and calibers are snappier and some are softer and pushier. 

 

Because humans have reflexes and such that take time, the slower and pushier recoil impulse would be easier to control then a snappy one that happens before we can react and absorb it. 

[T Bill 649]

Vlad is on to this.  Heavier bullets require slower powder, taking longer to achieve full propelling potential, hence longer curve to full impact of recoil. Great for new shooters or those whose reaction times are longer.  Race gunners prefer snap and faster propellants to run faster. This of course comes with time and practice and training of muscle strength and muscle memory.  When you have both muscle memory and strength the gun will return to your sight alignment faster allowing you to increase speed.  Lots of variables both of the human kind and machine kind.  Best to shoot what you are comfortable with and move up as proficiency permits.

[GRIZ 3,369]

Everything has been well explained except the difference in sound you mention. Once an object exceeds the speed of sound, roughly 340 meters/1100 ft per second, you get a sonic boom. The lighter bullets in 9mm do break the sound barrier and you get the "crack" or sonic boom. Your 147 gr loads are slower so you don't get the sonic boom.

 

Many are unaware of the fact that 147 gr 9mm ammo was originally developed for surpressed weapons. The surpressor would work on the noise made by the gases but did nothing to surpress the sonic boom from lighter 9mm ammo. Going to a 147 gr bullet reduced the velocity eliminating the sonic boom but still gave reliable functioning.

 

It may not be you but your gun may like the 147gr ammo better. I find my S&Ws, Berettas, and Taurus 9mms shoot better with 147gr ammo and other 9mms like lighter bullets better. I'm not saying you will get the same results but trying different bullet weights and brands might result in your gun liking one kind of ammo more than others.

[Howard 538]

Thanks for the detailed insights.  I find I shoot better and can get on target for follow-up shots faster with the 147 grain ammo.  I find this to be the same in my light Glocks, all the way up to my heavy STI 9mm 1911.