Help understanding Chronograph data

[Ghuqu2]

Well, I tried out my new Prochrono digital and it worked great (until the thunderstorm hit). The chrono was set 10ft from the muzzle. I was testing my normal loads of BL-c2, lighter loads and a new powder wc846. The WC846 is a canister powder and is supposed to use BL-c2 data.
Rifle: AR15 20" target barrel
Lee's data 25.5 gr blc2 3099fps max load 27.5gr 3313fps min oal 2.2
I load mine to 2.26 OAL, all using trimmed brass, PMC SR primer, 55gr Sierra blitzkings, Lee dies and Lee FC die.
Average from string
25.8gr BLC2 2830fps
26.4gr blc2 2870
27.2gr blc2 2930
26.4gr wc842 3088

I understand that a gas operated semi-auto would have a difference but that much? The wc842 is closer to the printed data, has the blc2 formulation changed? Any suggestions would be appreciated.

[DefensiveCarry.com]

[glockman10mm]

I'm not sure what you are asking. There is no consistancy in the load data supplied, so the velocities are going to have a spread. However, even using the same load for 5 different shots, there will be a spread due to several uncontrolable factors.

Welcome to our world. I have never assembled a load and gotten the same results as any printed data. Even using same components. If you are on an average 50-100 fps, you are doing good.

Many would also be disapointed inthe actual velocities of their favorite factory carry load.

They also use special proof testing barrels, sometimes longer than the one commonly found on most sporting rifles. And yes, the gas operated semi would show a reduction in velocity. Combine that with other factors and, Shazam!

[glockman10mm]

Bmcgilvray should weigh in here. I highly respect his opinion, and is the guy I go to when I encounter something I don't understand. He can probably expand on what I've stated.

[mlkx4]

I'm not sure what you are asking. There is no consistancy in the load data supplied, so the velocities are going to have a spread. However, even using the same load for 5 different shots, there will be a spread due to several uncontrolable factors.

Welcome to our world. I have never assembled a load and gotten the same results as any printed data. Even using same components. If you are on an average 50-100 fps, you are doing good.

Many would also be disapointed inthe actual velocities of their favorite factory carry load.

They also use special proof testing barrels, sometimes longer than the one commonly found on most sporting rifles. And yes, the gas operated semi would show a reduction in velocity. Combine that with other factors and, Shazam!

No consistancy? I think you read through his post to fast. He posted the average of 4 different loads.

[Archie]

Ghuqu2, if I'm understanding your post, you're concerned why your chrono readings are different from those in the book, is that correct?

The simplest answer is, your rifle and brass and stuff isn't exactly the same as those used in the manual. Among other things, strength of the main spring has an effect on velocity.

Probably the two things to watch for using a chronograph is A. Consistency or uniformity and B. is the reading a lot higher than the book shows? Higher (substantial, more than 10 percent or so) velocity means more pressure. Typically pressure difference is twice as much as velocity difference; if one gets 10 percent more velocity, one is getting 20 percent more pressure. However, if one cannot directly test for pressure, one cannot assume much of anything.