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Rifles, Reloading, Optics, Equipment
Reloading
OCW vs Ladder Test; the significance of the "scatter group"
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<blockquote data-quote="green 788" data-source="post: 98577" data-attributes="member: 3781"><p><strong>Re: OCW vs Ladder Test; the significance of the \"scatter group\"</strong></p><p></p><p>Here is the reason for not choosing that 40.8 grain charge...</p><p></p><p>All it would take is a <em>little</em> 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.</p><p></p><p>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.</p><p></p><p>And that's why we look for a consecutive POI of three groups if possible, and we choose the center group as the OCW.</p><p></p><p>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).</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p><img src="http://home.earthlink.net/~dannewberry/sitebuildercontent/sitebuilderpictures/243ocw75.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>Again, the 46.4 grain charge shown above will be more resistant to pressure change events than a typically developed load.</p><p></p><p>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.</p><p></p><p>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. </p><p></p><p>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.</p><p></p><p>Thanks for the interest, and especially thanks for the questions.</p><p></p><p>Dan</p></blockquote><p></p>
[QUOTE="green 788, post: 98577, member: 3781"] [b]Re: OCW vs Ladder Test; the significance of the \"scatter group\"[/b] Here is the reason for not choosing that 40.8 grain charge... All it would take is a [i]little[/i] 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. [img]http://home.earthlink.net/~dannewberry/sitebuildercontent/sitebuilderpictures/243ocw75.jpg[/img] 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 [/QUOTE]
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