OCW vs Ladder Test; the significance of the "scatter group"

[green 788]

OCW vs Ladder Test; the significance of the \"scatter group\"

I was asked in another thread to provide some better explanation of OCW load development, and also to explain why the method is superior to the conventional Ladder Test. I would imagine that some of you will understand this, while others may not--at least not immediately. For those who don't understand it, I will assume the blame for not having handled the explanation well. If there is anything I can clarify, please let me know and I'll give it a shot. /ubbthreads/images/graemlins/smile.gif

If you're not sure about what OCW load development actually is, go here for the treatise: www.clik.to/optimalchargeweight

One more thing--before I get into this. Certaily there will be folks who will want to take issue with my line of reasoning--and that will be great. Likely we'll all learn from a spirited but cordial discussion. However, toward the end of keeping things as civil as possible, it might be necessary to ignore intentionally abrasive comments--so please don't interpret my failure to respond to such as any indication that 'so and so' "got me." /ubbthreads/images/graemlins/wink.gif /ubbthreads/images/graemlins/laugh.gif

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/dannewberrysoptimalchargeweightloaddevelopment/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:

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

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.

<font color="blue"> 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.</font>

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 Newberry

 

[Buffalobob]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

Just my free advice.

I would hope that you and Bounty Hunter would not let things get out of hand.

There is no "winning" internet arguments. It is like peeing into the wind. Much better to pee downwind and let it go where it may and be done with it and walk away.

P.S.

I forgot to add not to pee uphill either when you are barefooted /ubbthreads/images/graemlins/grin.gif

 

[green 788]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

True. /ubbthreads/images/graemlins/smile.gif

This thread is not an attempt to invite an argument--it is only a response to a question I was asked by another member here, and I wanted to get the info out there for those curious. And of course I didn't want to continue the hi-jack of another man's thread. /ubbthreads/images/graemlins/blush.gif

As I've already mentioned, I won't feed into a rhetorical argument, so it shouldn't wind up in the gutter. /ubbthreads/images/graemlins/smile.gif

Dan

 

[BountyHunter]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

Dan

OK I give, OCW will give you as you describe "practical rifle accuracy of .5-1 MOA and 1 to 1.5 MOA at 1000 as stated on your forum.

However, if you want extreme accuracy use the ladder and a chrono. It is amazing how you keep ignoring the fact that most ladders are done with a chrono and give twice as accurate data as the OCW without a chrono.

It is disengenous to only compare half of the ladder process to OCW. You conveniently forget to mention that most knowledgable shooters who shoot the ladder use a chrono which confirms a node with close MV along with the similar POI.

I have have shot ladders with clusters of 4-5 shots into 1/3 MOA at 300 and extreme spreads on the MV of 15 fps. My ladder clusters beat normal OCW groups.

How do I do that well I use a modicum of commen sense which OCW does not think exists. As I see it if you have a factory gun and no chrono and want .5-1 MOA or MOB (Minute of 5 gallon bucket at 800) use OCW because it gives you something. However if you are after extreme accuracy OCW is significant handicap. OCW is based on people being stupid (ie when and how to test) and not knowing if their gun has a bad or good barrel.

So everyone can pick the accuracy standard they want and methodology.

Lets look at why I think the ladder with chrono beats OCW.

1. I test in early mornings/late evening NO-Wind conditions. There goes the need to worry about wind blowing one off. I use wind flags all the way out to confirm.

2. As for barrel warming and fouling, that is only a problem in extremely poor barrels. Custom match grade barrels with "one shot a minute" will not heat up and "walk shots" nor foul as we shoot matches of 80-120 shots with extreme accuracy not MOB as defined by OCW.

3. Now lets talk human factors, my HG weighs 65 lbs and has 2 oz trigger, and is set of with super light touch. No human factor there. Other custom guns are shot from front rest (yes even sporters) and rear bags. A true custom gun will shoot bug hole groups at 100 yds with just about any load that no one is going to perform a adequate analysis on as OCW says. So that is a human factor against OCW and really accurate guns.

But lets say it is correct, I will spot it more readily at 300 than anyone else will at 100. Not to mention what it will show on the chrono. I am sure that it will shot a MV anonoly that automaticlly throws it out anyway. If I cannot get my all of the MVs to cluster with my POI I keep looking.

As for the "scatter group" with practical rifles, bipods and such, I would bet that many of those throw outs ie scatter shots are inherent inaccuracies of the barrel or the human factors. Why is it that the ladder is the only technique afflicted with those problems?

OCW says you do not look at the lower nodes because you want more velocity. However in the extreme accuracy game many of the guns shoot the best at the lower nodes and throat erosion is minimized. Once again a case of not knowing the extreme accuracy game which has readily evident recently.

In the extreme accuracy and LR game we tend to use custom actions that are much stronger than the practical rifle, we use minimum SAAMI chambers, neck turn and things that do not work the brass, we seat out in the lands which reduces pressures. We can end up running much higher loads than out of the book with less pressure signs than a practical factory rifle.

Therefore in our ladder testing, we occasionally run into 3 and 4 nodes on a 20-30 shot string and the upper one is often above the "books" recommended. So I see how Dan has never seen this as OCW stays in the norms of the reloading manuals with "practical rifles and practical calibers" where as the LR game often uses wildcats and data often does not even exist.

You cannot load develop for those calibers without a chrono which OCW says is not needed.

A supposed advantage of OCW is you only need 100 yards. Now this is a LR forum, and no one should be shooting LR w/o testing LR ie past 100 yards, so if you have to test loads past 100 why limit yourself to load development at 100 so again no advantage to OCW.

Bottom line as I see it OCW is a poor attempt counter "percieved human factors, poor barrels, no chrono" because they think everyone must be afflicted with than and obviously the knowledge of the extreme accuracy game and LR hunting is not there. That means there is absolutely not one bit of first hand experience at extreme accuracy, yet somehow we are to believe that OCW is the cure of all ills, because he could not get the ladder to work. It is like coming on a NASCAR forum talking about basic engine tuning. Look around the internet, I have yet to see one LR competitor, Palma, Fclass, Highpower, BR etc say OCW is the ticket. Why? OCW is for the practical rifle and inexperienced rifleman and reloader at best. If your shooting methodology fits that pattern then OCW is for you.

BH

 

[royinidaho]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

This must be important to me, as it is taking away from my stock sanding/bedding/magazine filling project. /ubbthreads/images/graemlins/ooo.gif

I really appreciate the level of effort that has gone into this OCW/Ladder Test thing.

I'm having a bit of a learning problem, though. Maybe 63 years of living is catching up to me. Had similar experience with Solid Geometry, so I don't think age is the problem. I'm just a knucle head /ubbthreads/images/graemlins/crazy.gif

First comment: So that's how segue is spelled? Never saw it in print before. /ubbthreads/images/graemlins/blush.gif /ubbthreads/images/graemlins/blush.gif

Dan, bear with me, OK? It's tough to get inside someone else's head through an LCD monitor.

RE: The 52gr SMK targets.

What was the reasoning behind selecting 41.8gr as the OCW?

All of the targets except #2 look pretty good. If I reshot target 1 and 2 loads in a round robin fashion and got the same results, I'd go with the 40.8 grain load.

If 41.2gr is the scatter group, I would want to avoid that load/velocity by all means.

Targets 3, 4 &amp; 5 are all pretty good. Targets 3 &amp; 4 show a bit of vertical stringing. Target 5 has a stronger horizonal stinging tendency.

If I reshot loads 3, 4 &amp; 5 and came up with the same results (a shift from vertical to horizonal stringing) I'd try halfway between #s 4&amp;5 If the result is a small bug hole group with no stringing up or across then that's what I go with.

What I seem to be gathering from the OCW process is that IF I pick 41.8gr as you did, I should never expect any groups larger than targets 3 &amp; 4, all things being equal. Is that the jist of it??

Now it gets tougher...... As this is where I am.

The 75gr Sierra loads w/4350. I didn't notice that the OCW was identified. I would pick halfway between 45.7 and 46.0 which would be 45.85gr (Wonder if the Chargemaster could be programed to that number of decimals /ubbthreads/images/graemlins/confused.gif Remember when I asked if OCW would work for an OCD? Yep, that's me /ubbthreads/images/graemlins/blush.gif /ubbthreads/images/graemlins/blush.gif)

With 45.8 gr of 4350, looking at the velocities, I would expect a group no larger that what's shown for the 45.7 and 46.0 gr loads with a velocity spread of say 3313-3218 or 105 FPS.

This is where my world is at the moment. I shoot on the order of less than 0.5 MOA at 200 yds (btw a lone bullet hole out of my rifle is larger than your small groups, ya gotta get in the big bore league /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif) I missed my life size steel coyote more often than not at 700 yds until I tightened up the SD of the load. It was kind of like a tuning process within a tuning process.

I applogize for rambling but I think there is something I'm missing and being a bit obsessive I can't seem to let it go. (frownie face)

I am moving into a cartridge(s) that don't have the barrel life luxury of the 338 Win and don't want to waste barrel life doing load work up. (Besides being old, I'm po /ubbthreads/images/graemlins/grin.gif)

 

[remingtonman_25_06]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

I to am scratching my head on why you picked 41.8g when the 1st target looks the best. I dont know. I have only tried the ladder method 1 time at 300 yards with a 7 STW. It was not a very accurate rifle, I think I got a bad barrel by Winchester. I think the bottom line is that with a factory rifle, your best bet is to just load up 3 shells each in .5g increments. What seems so difficult about that?? I usually only do 1g increments and when I get technical, sure I'll do .5g increment. Doesn't seem to much different then the OCW method. I personally dont like the ladder method because I had shots everywhere and where I "thought" there would be a good load, I loaded them up and they weren't any better or any worse then what I did when I just loaded up 3 shells each with x,y,z grains of powder. I also dont really think the ladder method is very accurate fora factory rifle, unless you have an accurate factory rifle. I have some very small groups in front of me with my winchester in 25-06. All I ever do to that is pick a powder, load up 3 shells in 1g increments and shoot them. Actually, thats all I do with any of my guns. Have shot some very good groups that way. Usually only takes 12 shots as well. 4 loads with 3 shots for each load equals 12 shots. I guess if you want to get technical about it, you could then go in .5g incremnt to fine tune it. OCW seems confusing to me when all it really seems like is loading up 3 shells with x,y,z and shooting them.

 

[green 788]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

Here is the reason for not choosing that 40.8 grain charge...

All it would take is a little extra pressure to make that 40.8 grain load act like the 41.2 grain load. And that little extra pressure could come from a warmer than average day, or a tight brass case that you forgot to cull, or perhaps just a hotter primer. Once the pressure was increased just a bit, that 40.8 grain load would act exactly like the 41.2 grain load does--and it would throw flyers.

Many folks get flyers every now and then and really never realize where they come from. Sometimes it's the shooter, sometimes just a bad bullet or fouled barrel or problem scope, etc. But sometimes--it's from a pressure changing event like the tight (or loose) brass case. This causes that load to act like the powder charge just above or below it on the test target.

And that's why we look for a consecutive POI of three groups if possible, and we choose the center group as the OCW.

With a properly identified OCW load, the slight pressure difference won't move the point of impact outside of MOA. (This, by the way, is where some folks get the wrong idea that I'm claiming that OCW only yields MOA results, which isn't the case at all).

I chose 41.8 grains because that charge weight would allow 41.4 and 42.2 grain charges to fall inside an MOA group with it. Pressure change events might cause the 41.8 grain charge to behave like a 41.4, or a 42.2 grain load. But judging from the target, the shots would not fall outside an MOA circle.

Roy, you're wise to question the 75 grain Sierra/IMR 4350 target (.243 win, by the way). That OCW test had to be continued on another page, and the actual OCW was determined to be 46.4 grains. I shot a bracket group at 100 yards to prove the 46.4 grain charge was the correct OCW. A bracket group is a three shot group with the normal (suspected OCW) charge for one shot, and a 1% low charged shot, and a 1% high charged shot--all three fired at the same bullseye. If you've identified the OCW correctly, all three shots will fall close together on the target. Here is the bracket group for that OCW test. The rifle was a glass bedded and free floated factory Remington 700 VLS.

Again, the 46.4 grain charge shown above will be more resistant to pressure change events than a typically developed load.

Regarding the 22-250 target, I checked that load recipe (52 grain Matchking, 41.8 grains W760) at 410 yards and literally fired a 1.5 inch group of five shots with it--so the load worked beautifully. I haven't tested that load at any other ranges, and have actually come to favor another OCW recipe, 39.0 grains of H4350, WLR primer, and 55 grain Nosler BT in my 22-250 these days.

Basically, OCW is about finding a powder charge that won't start to act funny on you when you forget to weigh your brass cases, or when you get a hotter or cooler than average lot of primers, or when the outside temperature changes considerably from what it was when you developed the load.

Have you ever worked up a load at 60 degrees F and found out it wouldn't shoot worth a darn when the temperature went to 90 degrees? That's exactly what would happen to the 40.8 grain charge on the first target shown above--it would become the "scatter group" when the outside temperature went up.

Thanks for the interest, and especially thanks for the questions.

Dan

 

[royinidaho]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

OK, I think I'm catching on.

Gotta do some more shooting. I'm gun shy at the moment. New one not here yet. The 338 Win is being rebarreled to 338 RUM and son in laws 243 is being recrowned. And I'm doing stock work.

We'll see. Thanks

 

[remingtonman_25_06]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

OK. I think its starting to make sense now. So basically you need to work up a load in about .3g increments and when you find a load that shoots good, then you should go up and down .3g to see if it stays about the same. Seems easy enough now.

 

[green 788]

Re: OCW vs Ladder Test; the significance of the \"scatter group\"

That's pretty much it. /ubbthreads/images/graemlins/smile.gif

Now, to be fair--that's exactly what a ladder test does--it finds a wide "sweet spot." When everything works out perfectly the ladder will give you the same result--only with about 2/3's less supporting data and considerably less certainty. Creighton Audette (who reportedly didn't design the ladder test, he just advocated it. FWIW, it has been said that the ladder test was a derivative of an accuracy range test done with deck guns on Navy ships in eons past)... but Mr. Audette as I was saying also chose the center powder charge from a string of shots which impacted the target in the same spot. The thing is that sometimes barrels really will heat and foul. Sometimes a shot just pulls away from the group for no obvious reason--we'll all readily admit that, right? So if such a shot occurs while doing a ladder test, we might be steered toward a bad area, or away from the good area. Remember the scatter groups shown earlier--one shot of each of those scatter groups would have really fooled us on a traditional ladder test.

Bounty Hunter makes a good point about some custom rifles being extremely accurate and not offering much POI shift to analyze at 100 yards. If you look closely, though, the data is usually there. A friend of mine, Donovan Moran, has a 1000 yard BR rig chambered in 6mm Dasher. It printed like this at 100 yards, in initial testing of the 115 DTAC bullet.

We corresponded about these target results, and I'll reprise my comments about the groups:

I would say that, based only on the groups you're showing, that 33.5 may be the center of the high OCW.

At 32.0 grains there is lateral dispersion, and this lateral dispersion marks the trend toward the POI of the 32.5 grain charge. In other words, one shot (perhaps a couple shots, it's hard to tell from the target) of the 32.0 grain group "acted" like 32.5 grain, and moved right. By the time we're at 32.5 grains, the entire group prints to the right, which would definitely rule out 32.0 or 32.5 as the OCW.

At 33.0 grains, the group center wants to move back to the left. It does, however, show a straggler shot which acts like the 32.5 grain charge, and prints where the 32.5 grain charge would have printed. This would mean that 33.0 can't be the OCW because if a shot were to lose some chamber pressure due to odd brass case (I know, not likely to happen under your supervision Smile ) or a weak primer or anything such that would drop the pressure, we'd see the 33.0 grain charge act like a 32.5 grain charge and the shot would go to the right. Again, it appears that at least one shot of the 33.0 grain group did trend toward the 32.5 grain POI.

This would mean that 33.5 grain could be the only possible OCW, but that could only be proven with a higher charged group, to see where it printed.

I would normally work in .3 grain increments with a charge in this weight range, but the fact that your rifle is so stunningly accurate with .5 grain graduations is a testament to the rifle's and cartridge's robust potential for accuracy.

But we can still use OCW to see what powder charge it's going to favor. A round robin test might yield even more interesting results, but I would say that 33.5 is the probable high OCW, only that we would have to see how 34.0 grains looked to be sure.

I would be curious to see how 33.2, 33.5, and 33.8 grains land together at 300 yards on the target.

With no more data than the targets shown above, I'd tentatively go with 33.5 grain, simply because the other charges can be ruled out by virtue of the POI shifting right at 32.5 grains... Dan

One thing to note: In many cases, the actual OCW will be the tightest group on the paper. But in addition to being the tightest group, it would have to also share its point of impact with the groups on either side of it.

So generally, if you really look the targets over, you can spot the trend. You can see what I call "transition groups" where two shots will print in one place and the third shot will print away from those two. And if you look carefully, you will usually see that two of the shots are favoring or "pointing to" either the previous or the next POI, while the other shot trends the other way.

The 33.0 grain group in the target shown above is a "transition group." (And the 32.0 grain group may be as well, I'd have to see 31.5 to really know for sure). These are five shots, by the way--and this test was not done in round-robin OCW fashion. No matter, the data is there (and as BH would correctly point out, with the Hart barrel there was no heating/fouling to be concerned with). We can see that at 33.0 grains, some shots favored the POI of 32.5, and some favored the POI of 33.5 grains. Now. That doesn't mean that with careful case prep and load assembly, you couldn't make that 33.0 grain transition charge work. The 33.0 grain charge, when tested at 1000 yards, averaged about .5 MOA for four groups of five shots each. A careful handloader using top of the line equipment can make almost any powder charge perform well. I say "almost," because of the scatter group issue. No barrel will be totally immune to the scatter group.

But check this group out--when he moved toward 33.5 grains (actually, 33.3 in this case) he nearly duplicated the 1000 yard group size at 1320 yards, or 3/4 mile:

In my opinion, the stellar results at 3/4 mile were obtained by moving the charge weight out of the transition area... (And also, such good results are only obtainable by a well seasoned 1k BR shooter and great equipment)!

The above long range BR targets are courtesy of Donovan Moran.

Dan