223/5.56 Hornady 55gr VMAX vs 55gr HPBT HUGE Variance in FPS

[babyguppy]

Question for the team:
Why are my VMAX fps so out of wack vs the HPBTs

I loaded 15 rounds of each in mixed 5.56 brass with 25.8 grains of H335 powder, CCI #41 primers. Bolt action, 20" 1 in 7 twist. I wanted to see how much variance in performance in mixed brass --because I didn't feel like sorting the brass by headstamps.

Please NOTE - I love the VMAX bullet and was rather surprised by what I found, hence why I'm here asking

55 VMAX Hornady with Cannelure
COAL 2.24
FPS - 3288, 3215, 3289, 3355, 3343 (groups were over 1.5" @ 100 yards) Extreme spread of 73 FPS
If you pull out the slowest round, you still get a 45 FPS spread

55 gr Hornady HPBT with Cannelure
COAL 2.202
FPS - 3296, 3292, 3276, 3276 (groups were less that 1/2" @ 100 yards) Extreme spread of 20 FPS

All speeds measured on a Magneto speed sporter

Why did the HPBT perform so much better that the VMAX, all brass prepared the same, powder loads were weighed on a RCBS match master to the EXACT amount for ALL rounds.


thanks

 

[rustyinbend]

It seems like that will be a hard question to answer when the brass is mixed and variable. I'd retest with everything "constant" and then try to decipher the results. But that's just me ...

 

[supercorndogs]

Surely it couldn't be the small sample size, skewing the results from a lucky four in row.

 

[FLIGHT762]

First thing I'd do is to re-shoot the V Max with all the same head stamped brass. See if that does anything and use a larger sample.

 

[db2000]

Concur about the brass variable. I’ve had 70 fps differences when all else equal in .243 Win. Both FC brass, but some had small writing and the other larger writing.

 

[babyguppy]

yup, I was thinking about having to do an all same headstamp to eliminate the variable.
FYI - each of the samples had basically the same different types of mixed brass. Each sample had 2 PSDs and 2 FC's (the VMAX had a Rem in there for the fifth shot)

- can't get an accuracy test with a sporter attached to the barrel. so, usually I shoot a few to get a rough FPS and use the extra to get a true accuracy test.

 

[Doom]

There are a number of faults in this post related to the original assumption/theory and what was done with the analysis. In order to get to an answer you need to formulate a test plan. Design of Experiments is an excepted method to design an experiment or test. One of its primary concepts is to vary only one variable at a time. Here are some comments on what the OP did.

I wanted to see how much variance in performance in mixed brass --b

So he wanted to compare mixed brass to ??? He shot mixed but didn’t shoot a control group of same lot to compare it to. Also, His mixed brass between the two test were not the same mix. while the mix had the same head stamps they may have been from different lots. It would be of greater value to shoot the test using the same brass for each test if comparing different bullets as he did. He has not tested to his original question.

Why did the HPBT perform so much better that the VMAX, all brass prepared the same, powder loads were weighed on a RCBS match master to the EXACT amount for ALL rounds.

Surely it couldn't be the small sample size, skewing the results from a lucky four in row.

He has no way of knowing. He changed two variables, bullet (actually much more as bullet will include seating depth, bearing surface, boat tail vs flat base), and brass in this comparison. He also didn’t intend to compare these originally. He has drawn a conclusion which is anecdotal at best. He not performed any statistical analysis of the data.

The variable he chose to compare has no statistical significance. In a large population the two values which make up extreme spread lay beyond +/- 3 standard deviations of the population. By shooting only 4 or 5 shots he has a limited idea as to the the actual standard deviation but no idea as to what the two extreme values will be.

To answer the original question a different approach that allows for a better analysis would be to shoot a minimum of ten rounds of each brass type and determine statically the standard deviation and mean of each type, then compare the means and standard deviations statistically to see if there is difference. Standard methods such as the t-test and F-test are specifically applicable to this type of analysis.

 

[GONE BAD]

Why is the seating depth different?
.004 might be huge in a 233
Seat the A-max deeper?
What does the loading manuals call for in relation to the 2 bullets?
JMO.

 

[babyguppy]

Why is the seating depth different?
.004 might be huge in a 233
Seat the A-max deeper?
What does the loading manuals call for in relation to the 2 bullets?
JMO.

Bullet seater is on a Dillion, adjusted for the 55gr Vmax.
edit: remember seaters, seat off the Ogive, so even though both bullets have a different COAL, they each have the same distance to the lands.

Also, both bullets are Cannelure, and you normally seat to the Cannelure.
2.26 is Max COAL due to ar15 mag lengths
I'm gong the current route of eliminating some variables with the brass. I'll follow up, when I get the results.