wide extreme spread

[sherlok]

I have extreme spreads (ES) exceeding 70 fps on my loads, yet they group consistently less than moa out to 300 yards (my current range limit) in my 700 SPS Tacticals in .308 and .223. The loads are:

.308 168gr Nosler CC over 42.0 IMR4895 LC prepped Brass.
.223 69gr Nosler CC over 24.0 H4895 Lapua Brass

They run circles around Federal GMM in accuracy, yet the FGMM shows much lower ES on the chronograph.

I'm not one to look a "gift house in the mouth", but just how important is all this ES business do you think?

Sherlok

 

[Lindy]

Re: wide extreme spread

Extreme spread is not always a very useful metric.

Think about it. If you fired 100 rounds over a chronograph, all 100 rounds would be represented in the calculation of the standard deviation of the velocity.

How many would be represented in the extreme spread? Two.

You might find this article interesting:

http://www.shootersjournal.com./Features/Haps/EnglemanChronographStatistics.pdf

 

[19Scout77]

Re: wide extreme spread

Thank you Lindy. GREAT ARTICLE!

 

[Hummer]

Re: wide extreme spread

ES won't raise up and bite you till you get to 600 yards, maybe 500.

I just pulled out my copy of Sierra Infinity 5 and plugged in 175 grain Matchking at 2550 fps and then added 35 fps and subtracted 35 fps to give your 70 FPS ES and here is what I got:

Velocity Bullet Drop Inches
2515 489.54
2550 474.36
2585 460.18

So assuming you are zeroed with 2250 fps you slowest round is going to hitover 15 inches lower and your fastest round is going to hit over 14" higher. Thusly you have an extreme spread of 28.74" of elevation alone before you add in other things like heart rate, wind, mirage etc.

The 1000 yard Highpower Target has a 20" 10 ring thusly you have ammo that wouldn't print 10 ring shot from a machine rest in zero wind conditions.

Now lets look at 20 feet per second extreme spread:

Velocity Drop in Inches

2540 478.78+
2550 474.56"
2560 470.39"

Thusly you are now looking at 8.48" vertical dispersion and the X ring is 10 inches so you have ammo capable of shooting X ring.

I saw the record 1000 yard target shot a Perry a number of years ago. 200-17X and if memory serves me correctly he had one 12 o'clock shot at the 10 line and two 6:00 O'clock shots at the 10 line so he had about a 19" vertical dispersion. If he had been a click off up or down he would have lost a shot to elevation alone.
One of the two years I came second in the Canadian Cup I was I think down six points. I lost four of those six points due to a round that gave me a 6 ring hit at 6:00. Two more points and I would have won the Long Range Championship but that is how it goes in long range.

The problem with figuring the extreme spread is the long run average. 10 rounds won't do it.

If you want to see what your ammo and rifle is capable of chronograph two sighters, record them and then shoot thirty continious rounds at say 40 second intervals and run the numbers every 10 shots. I think you will find that the longer you shoot the ES will start to creep upwards.

This is also a good indicator of whether your barrel is properly stress relieved as when they heat up they tend to change the alignment and group will start to walk.

Even better put up a target at 600 yards and have someone plot your shots while you are shooting.

The Marine Corps Teams used to train with no spotters. You got two sighters and then they left the target in the air for 20 shots and you worked off the mirage and wind flags and you didn't see your target till you got to the pits.

Bottom line is up close ES is not that critical but at long range it will rain all over your parade.

 

[sherlok]

Re: wide extreme spread

That surely makes sense, but how do you close up ES?

My rifles group better with IMR4895 and H4895 and they have been mentioned frequently as the powder of choice by many on the Hide for these respective calibers. Cases are prepped well, the charges weighed manually and primers don't seem to make a noticeable difference.

Is it possible to have a load that is more accurate at long range because of lower ES yet not appear as accurate than other loads at shorter ranges? That would seem a little strange too.

Thanks, Sherlok

 

[milanuk]

Re: wide extreme spread

sherlok,

Typically after the usual suspects like case prep, weighed charges, and trying different primers are exhausted, then ya get into sacrificing virgin chickens under a blood moon

Maybe not *quite* that bad, but it sure seems hit or miss for me. Some guns/calibers I have to go to the ends of the earth to get the ES/SD down, others just seem to fall into place easy as pie. I've tried a lot of different things, with various degrees of success - none that I can point to and say "That is the key", though.

As far as the near vs. far thing... there are a lot of people who swear by exactly that phenomenon. Most of the 'experts', and by this I mean people w/ the degrees and letters behind their names to go with and people with acoustic targets and such - tend to disagree. Whole separate debate there, though.

 

[sherlok]

Re: wide extreme spread

Well I guess I'll get some chickens.

Thanks all,
Sherlok