Barrel Velocity Question

[jstalford]

OK, so I have looked at the following attachment from the below article a few times and the dip at 12.5" always confused me.

I have seen numerous barrel length threads were people have said something like:

"Well, I would go 10.5 for the shortest configure...if you're going SBR, might as well SBR, but 12.5 will have more velocity. 11.5 is a compromise, but gets added dwell time from 10.5"

Or

"I love the 12.5, it's my go to gun. Still short, but less velocity loss. Great general purpose carbine."

Of course I'm paraphrasing here, but most discussions contradict the results from the below test. On the other hand, most discussions to actually provide any additional concrete numerical data.

Does anyone have any additional velocity data on 10.5-12.5 barrel lengths? Does it support or contradict the below?

37037

From:

http://www.sadefensejournal.com/wp/?p=1093

CLIFFS: WTF is with the dip 12.5" at in the attached graph. Is 12.5" actually pointless vs. an 11.5" other than rail lenght (which 11-11.75" rails are not as prevalent anyway, e.g. KMR, Geissele don't offer anything that would be advantageous on a 12.5")

[MegademiC]

When using limited samples, anomalies happen and need to be taken into account. You won't increase barrel length and lose velocity, all things equal. This is how it should look: http://www.saysuncle.com/2006/08/30/9mm_performance_with_barrel_length/

There was a dip at 21 and 22" also, which does not make sense. Maybe their contraption leaked and they fixed it after shots at 12" and 22"?

[californiasushi]

I just chronied my two SBRs today back to back with a Magnetospeed V3. I used the rail attachment since there wasn't enough barrel sticking out to use the normal Magnetospeed strap mount. It didn't work very well as I had to mess with sensitivities and it was picking up two velocities, one not even close to being realistic so I just kept the shot velocity that made sense. But I did get some velocity data:

(from previous session for reference)

16" DD with lightweight barrel (5.56 chamber)
24.4gr Varget + 75 Hornady BTHP = 2603 fps

20" RRA stainless barrel (Wylde chamber)
24.4gr Varget + 75 Hornady BTHP = 2773 fps

(from today, 51F, 29% humidity)

12.5 Centurion CHF barrel (5.56 chamber)
24.4gr Varget + 75 Hornady BTHP = 2465 fps
Atlanta Arms & Ammo AMU 77gr = 2493 fps
PMC X-Tac 62gr NATO = 2724 fps

11.5 DD CHF barrel (5.56 chamber)
24.4gr Varget + 75 Hornady BTHP = 2396 fps
Atlanta Arms & Ammo AMU 77gr = 2444 fps
PMC X-Tac 62gr NATO = 2622 fps

[lysander]

OK, so I have looked at the following attachment from the below article a few times and the dip at 12.5" always confused me.

I have seen numerous barrel length threads were people have said something like:

"Well, I would go 10.5 for the shortest configure...if you're going SBR, might as well SBR, but 12.5 will have more velocity. 11.5 is a compromise, but gets added dwell time from 10.5"

Or

"I love the 12.5, it's my go to gun. Still short, but less velocity loss. Great general purpose carbine."

Of course I'm paraphrasing here, but most discussions contradict the results from the below test. On the other hand, most discussions to actually provide any additional concrete numerical data.

Does anyone have any additional velocity data on 10.5-12.5 barrel lengths? Does it support or contradict the below?

37037

From:

http://www.sadefensejournal.com/wp/?p=1093

CLIFFS: WTF is with the dip 12.5" at in the attached graph. Is 12.5" actually pointless vs. an 11.5" other than rail lenght (which 11-11.75" rails are not as prevalent anyway, e.g. KMR, Geissele don't offer anything that would be advantageous on a 12.5")
First , all the ammunition is not exactly the same, you will have variations in measured velocity. Due to the small sample size, you will get scatter in the data. Two, he drilled all the holes first, maybe the 20" and 11" gas port were leaky, this would have cause the velocities ahead of them to be lower than expected.

Raw data is not what you base assumptions on, you need smooth out the curves.

A 'best fit' curve through the data would be something like this:

http://i2.photobucket.com/albums/y9/lysanderxiii/barrel6_zpsvlpmarzc.jpg

[rep0055]

First , all the ammunition is not exactly the same, you will have variations in measured velocity. Due to the small sample size, you will get scatter in the data. Two, he drilled all the holes first, maybe the 20" and 11" gas port were leaky, this would have cause the velocities ahead of them to be lower than expected.

Raw data is not what you base assumptions on, you need smooth out the curves.

A 'best fit' curve through the data would be something like this:

http://i2.photobucket.com/albums/y9/lysanderxiii/barrel6_zpsvlpmarzc.jpg

Full disclosure: I am in no way an expert in firearms. My assertions are based solely on observations of the graph presented.

Lysander is on the right track here. By looking at the best fit curve data you can start to see why the 12.5 barrel is chosen. The best fit curve approximated as straight lines would have 2 different slopes. The two lines would intersect at about the 12.5 barrel mark. So in short: extra distance past 12.5 increases the velocity you get from the barrel but at a slower rate than the velocity you'd lose for every inch cut down past 12.5. Its the shortest barrel before additional loss of length really starts to degrade velocity. Think rate of diminishing returns and 12.5 appears to be the optimum.

[lysander]

Full disclosure: I am in no way an expert in firearms. My assertions are based solely on observations of the graph presented.

Lysander is on the right track here. By looking at the best fit curve data you can start to see why the 12.5 barrel is chosen. The best fit curve approximated as straight lines would have 2 different slopes. The two lines would intersect at about the 12.5 barrel mark. So in short: extra distance past 12.5 increases the velocity you get from the barrel but at a slower rate than the velocity you'd lose for every inch cut down past 12.5. Its the shortest barrel before additional loss of length really starts to degrade velocity. Think rate of diminishing returns and 12.5 appears to be the optimum.
It also has to do with the gas systems available and that work well with the overall barrel length. You can go as short as 10.5 with a CLGS, but you have to open the port up to compensate for the reduced dwell, and that makes for rougher operation.

The PLGS gives port pressures nearly 140% than those of the CLGS and 250% higher than the RLGS, so it is always going to be pretty rough of the action.