Riddle me this, Chrono-man?

[Alfie]

I've run two different recipes through two of my 4-1/4" M&P40's and my 5" M&P40 and I get consistently faster chronograph speeds out of the short barrel pistols.

• One 4-1/4" has a Storm Lake barrel and one has the factory barrel, the 5" pistol has the factory barrel.
• One recipe used 3.8 grains of Titegroup behind a 180 grain jacketed bullet, the other used 4.2 grains of VV N320 behind a 180 grain jacketed bullet.
• Both 4-1/4" guns shot similar velocities and both were noticeable faster then the 5". Specifically, the shorter barrel pistols yielded a power factor of about 140 - 145, whereas the longer barrel pistol only nets me a power factor of 132-137.

Is there any reason for me to see this result? I expected the longer barrel to produce faster velocities. Is it the burn rate of the powders that is effecting this?

[CHA-LEE]

Maybe you have a slow barrel in your 5 inch gun? You could try swapping the 5 inch barrel with an aftermarket one that is of higher quality to see if the fps improves. Find another local shooter that has a 5 inch M&P 40 and shoot your rounds through it to see if you get the same fps results.

The 5 inch barrel should produce more fps with the same rounds.

In the big picture, does it really matter as long as you are still making Minor? If it shoots the way you want it to and is still accurate what does it matter? Mass produced parts on a $500 guns are going to have variances from one to another.

Edited April 25, 2011 by CHA-LEE

[Sarge]

My M&P Pro 9mm was very slow compared to most other guns in the price range. I know Glocks are faster but it took quite a bit more charge to get the pro up to speed.

[Corey]

Sounds like a slower barrel. It happens.

+1 to the idea of shooting your loads through another 5" M&P to see how the velocities compare.

[98006]

most likely with your load, all the powder is being consumed by the 5" barrel before the bullet leaves the muzzle. The best scenario is when the bullet leaves the barrel right when all the powder is burned. this is why very short barreled gun have a big flash, as the powder is being burned outside the barrel.

Anything after the peak pressure is just drag.

Try a slower powder like WSF or Universal Clays and see what happens.

[want2race]

most likely with your load, all the powder is being consumed by the 5" barrel before the bullet leaves the muzzle. The best scenario is when the bullet leaves the barrel right when all the powder is burned. this is why very short barreled gun have a big flash, as the powder is being burned outside the barrel.

Anything after the peak pressure is just drag.

Try a slower powder like WSF or Universal Clays and see what happens.

This is my thought too. With a small charge of a fast powder it's possible. I wouldn't think it likely from .75" difference in length but perhaps the powder is spent after only 3 inches. There is drag in both barrels, just more of it in the longer barrel.

I first encountered something similar when switching from a 5" 2011 to a 6" 2011. Same ammo produced only 1pf increase in the 6 inch barrel. Everyone says longer barrels can run reduced powder to make the same power factor. Not so in my two barrels (same barrel manufacturer for both 5" and 6").

[G-ManBart]

most likely with your load, all the powder is being consumed by the 5" barrel before the bullet leaves the muzzle. The best scenario is when the bullet leaves the barrel right when all the powder is burned. this is why very short barreled gun have a big flash, as the powder is being burned outside the barrel.

Anything after the peak pressure is just drag.

Try a slower powder like WSF or Universal Clays and see what happens.

That's almost all completely untrue. Anything after peak pressure is still pressure, and it's accelerating the bullet, just not as rapidly as during peak pressure. For the OP's listed load with N320, the barrel would have to be approximately 1.75" long for the bullet to exit at the end of combustion, and peak pressure happens far earlier than that (.18" according to Quickload). Barrels longer than 1.75" offer more velocity regardless of the burn rate of the powder. Granted, QL is theoretical, but it's going to be in that ballpark.

In fact, go look at what cowboy action folks are loading, then compare the data published for rifles and pistols (they use the same ammo in both, in many cases). Look at something like .38Spl with fast powders, and you'll see that the rifle loads are often something like 200FPS higher, with the same load, and the powder finishes burning waaaaay before the bullet exits the muzzle.

Short barreled guns produce a flash due to a number or reasons (byproduct of combustion reacting to oxygen being one of them). In most normal cases, the powder that produces the highest velocity with a longer barrel, will still produce the highest velocity with a shorter barrel, even if the powder doesn't get completely burned in the shorter barrel.

[RH45]

I have 2, 5", 9mm pros that chrono different, and 2, .40, STI Trojans that chrono different.

[Alfie]

Thanks for all the info guys. I guess it just is what it is and I shouldn't be too concerned. In the end I'm only addiing one more 1/10th of a grain of powder. Once I get used to shooting the longer slide it really won't matter.