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

 
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Old 02-11-2006, 12:29 AM
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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
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Old 02-11-2006, 02:04 AM
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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.
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Old 02-11-2006, 08:06 AM
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Re: OCW vs Ladder Test; the significance of the \"scatter group\"

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

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
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