Welcome Guest ( Log In | Register )

> Forum Sponsored by Black Hills Ammunition

Black Hills Ammunition
One of the longest supporters of Sniper's Paradise and an excellent ammunition manufacturer for military, police, cowboy action, as well as standard uses.www.black-hills.com

 
Reply to this topicStart new topic
> OCW vs Ladder Test..., The significance of the "scatter group"
green 788
post Feb 10 2006, 10:20
Post #1


CPL


Group: Registered
Posts: 380
Joined: 4-March 2002
From: Southwest Virginia
Member No.: 768
Rifle:Savage 10FPLE2B, Savage 10FP 24"
Optics:Nightforce NXS 3.5-15x50, Bushnell Elite 3200 10X
Ammo:200 grain Matchking handloads, 178 AMAX handloads...



There are some great guys out there who have been using the Ladder Test for eons to get to a good load recipe. While many have had good results with the loads they've developed in this manner, others have reported difficulty interpreting the results on the target. Still others have claimed that they have properly identified a wide accuracy node with the Ladder Test--but will also tell you that the seating depth is critical, or that primer selection is critical--otherwise the load "goes all over the target." If this is the case, the liklihood that the individual is truly using the best powder charge is slim. Don't get me wrong--you can use an inopportune powder charge and get away with it. You might even win a match or two with it. But if you're having to keep to critical tolerances just to keep the load from exceeding MOA, you're "walking the tightrope to accuracy." Which, by the way, I discuss HERE...

One of the reasons I designed the OCW load development method is that I was one of those guys who found that results from a 200 or 300 yard ladder test were often inconclusive. I knew that the method had (and still has) merit. But I also knew that there had to be a better way.

There are some obvious reasons that a 20 shot ladder test can give inconclusive or even out-right misleading results. I go into more of those here: http://home.earthlink.net/~dannewberry/dan...opment/id1.html

With the Ladder Test, it is generally recommended that you fire 20 shots in graduated powder charges of around .2 grains (.3 grains on larger cases). You are to begin at the published starting load, and proceed upward. You fire at the same bullseye, and watch for a succession of shots to cluster at some point (or points) along the continuum.

An aside: In my opinion (and others now share it) you will be wasting bullets, powder, and barrel life on the first 8 to 12 of those shots. While it is of course important to work up from lower powder charges (beginning published loads), you should not be too curious as to whether you find an accuracy node in the area just above the starting charge level. Do you want your 30-06 to act like a 30-30? Or your 220 Swift to act like a .223? We choose a rifle chambering based on an expected performance level from that cartridge. Since it is doubtful that you will be willing to settle for a reduced velocity load, you should move quickly through the first half of the charge weight range by stepping up in larger increments to begin with.

The main thrust of this piece is to (hopefully) bring the reader to an understanding of what can go wrong in a ladder test--even a perfectly executed ladder test--which may lead them to incorrect conclusions as to the best powder charge to use.

Engineer Chris Long has introduced the concept of the Shock Wave's effect on barrel behavior. Chris and I have corresponded for over three years on matters of load development, and with his educational background he was quickly able to discern how OCW loads work--and has even developed a method to predict OBT (Optimum Barrel Times). Note that "OBT" is Chris' terminology. RSI (a company I would assume many of you are familiar with) has a working relationship with Chris Long, and Chris has written some of the software for RSI's ballistics programs. Here are a couple of links, first to Chris Long's Shock Wave pages, and then to RSI's notes wherein they give credit to Chris for his work.

http://www.the-long-family.com/OBT_paper.htm

http://www.shootingsoftware.com/barrel.htm

I offer the above information as a segue into the "scatter group," and its significance to the powder charge continuum in a particular load work-up.

Check out these OCW targets:

(IMG:http://img.photobucket.com/albums/v114/green788/22-250w760.jpg)

(IMG:http://img.photobucket.com/albums/v114/green788/ocw.jpg)


The next targets show what an incredibly accurate (stock Tikka, by the way) rifle will do in an OCW test...

(IMG:http://img.photobucket.com/albums/v114/green788/872c6036c4351f6a22c606fcec1f16f6.jpg)

(IMG:http://img.photobucket.com/albums/v114/green788/b8d023908b70b73854ae5d21dd8563e3.jpg)

I show all of the targets above because I want to illustrate the importance of the "scatter group." This will be one group of the round-robin sequence that seems to inexplicably open up. The reason for this? The Shock Wave, as identified and described by engineer Chris Long in the page linked above, is at the muzzle when those bullets are being released. Generally, a 1 to 2 percent powder charge increase above the scatter group charge weight will have you right in the OCW zone.

But the real reason I wanted to discuss the scatter group is this: Please note that in each of the scatter groups shown above, ONE shot lands right on the same POI (or within 3/8 MOA or so) as the groups preceeding and proceeding it. This means that in one of three instances, during a conventional Ladder Test, the target would indicate a wide "sweet spot" where it should not. The OCW round-robin testing easily identifies the problem with this charge weight level, but in (at least) one of three instances, the Ladder Test will miss it--which may lead a shooter to an incorrect conclusion as to the center of the true accuracy node.

Sure you can go back and prove and re-prove your Ladder Test results--but if you'll take the time to understand the OCW/round-robin firing sequence, you'll not have to do any such re-proving; you can move right on to depth tuning (adjusting the seating depth of the bullet for absolute accuracy).

Thanks for the read,

Dan
Go to the top of the page
 
+Quote Post
E.Shell
post Feb 13 2006, 11:44
Post #2


SGT


Group: Registered
Posts: 640
Joined: 8-July 2005
From: Central Maryland
Member No.: 8,854
Rifle:M-700s
Optics:NF & Loopy
Ammo:Handloads



Thank you for posting this, Dan. Quite enlightening.

I had been using the the one-round ladder, then went to round-robin testing method for more meaningful results. I have often had an unexplained group that went bad, surrounded by other, more acceptable groups. Exactly as shown above. I had always written it off as unidentified shooter error, eye fatigue or wind.

Shooting a (1/4moa) 6.5-300Wby like that last week with 74 odd grains of Retumbo and 140 Berger VLD's, groups # 1 & 2 shot "OK" at .5 to .6 MOA, group #3 went to over 1 moa, then groups #4, 5, 6 & 7 tightened way up, with groups #5 being .200 and #6 being .330. Group 7 went back to 1/2 moa and showed pressure signs. Trying to find a replacement for my 1/4 moa, but horrendously fouling, load of 83.0 H-870.

This post has been edited by E.Shell: Feb 13 2006, 11:45
Go to the top of the page
 
+Quote Post
green 788
post Feb 13 2006, 19:03
Post #3


CPL


Group: Registered
Posts: 380
Joined: 4-March 2002
From: Southwest Virginia
Member No.: 768
Rifle:Savage 10FPLE2B, Savage 10FP 24"
Optics:Nightforce NXS 3.5-15x50, Bushnell Elite 3200 10X
Ammo:200 grain Matchking handloads, 178 AMAX handloads...



"...group #3 went to over 1 moa, then groups #4, 5, 6 & 7 tightened way up, with groups #5 being .200..."

That's generally what you're going to see. Once you pass the "scatter group" you will find yourself in the OCW zone about two graduations (or 1.5 to 2 percent) beyond that level.

I'm always really happy when I find the scatter group in an OCW test. That tells me right where the OCW is likely to be... (IMG:http://www.snipersparadise.com/sniperchat/style_emoticons/default/smile.gif)
Dan
Go to the top of the page
 
+Quote Post
marc357
post Feb 17 2006, 5:28
Post #4


PFC


Group: Registered
Posts: 12
Joined: 3-December 2003
Member No.: 3,666
Rifle:NVGW Custom M70 .300WM
Optics:USO SN-3 3.2x17x44
Ammo:Handloads Berger 210 @ 2950fps



I tried some Retumbo in the .300 WinMag, with Berger 210s. With no published data, I started a bit on the low side, but once I got up into the right zone, I had a "scatter" group, then 3 really tight groups, then another "scatter" group. Picked the middle of the 3 good ones, and I'm DONE.

Thirty rounds is all it took. If there had been published data to give me a better starting point, it would have taken less than that.

The gun will let you know what it likes, (and doesn't like) -if you are open to it.

I mean- how many times does the baby have to spit mashed peas in your face, before you realize HE DOESN"T LIKE THEM>? Your rifle will do the same.

OCW works for me. The quickest way to find a good load, in the least number of rounds.

Thanks, Dan.

marc
Go to the top of the page
 
+Quote Post
green 788
post Feb 17 2006, 6:51
Post #5


CPL


Group: Registered
Posts: 380
Joined: 4-March 2002
From: Southwest Virginia
Member No.: 768
Rifle:Savage 10FPLE2B, Savage 10FP 24"
Optics:Nightforce NXS 3.5-15x50, Bushnell Elite 3200 10X
Ammo:200 grain Matchking handloads, 178 AMAX handloads...



Marc,

I appeciate the feedback. (IMG:http://www.snipersparadise.com/sniperchat/style_emoticons/default/smile.gif)
Dan
Go to the top of the page
 
+Quote Post

Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 3rd September 2014 - 0:56