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Rifles, Reloading, Optics, Equipment
Reloading
Accuracy=seating depth or tenths of powder
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<blockquote data-quote="Bart B" data-source="post: 1132507" data-attributes="member: 5302"><p>I think the increased rough surface of the lands and grooves contributed to the accuracy loss. The slight change in pressure curve shape may also be a factor. Here's why.</p><p></p><p>Sierra Bullets' ballistic tech's testing their bullets for accuracy in rail guns record each 10-shot test group for size. Over the life of the barrels, the average group size increases at a fairly constant rate. When the average group size is 50% more than when new, the barrel's replaced. For example, the .308 Win chambered test barrels shooting bullets up through 180 grains last about 3000 rounds, then get replaced. They start out in the 1/4 MOA or so range with match bullets and reach the 3/8 MOA average point that many rounds later. </p><p></p><p>As far as I know, Sierra keeps the same OAL on all their test rounds fired in virtual SAAMI spec chambers. They don't prep cases and full length size them in Redding dies now (I think they used RCBS or Lyman ones years ago). The tech grabs 10 bullets as they come out of the pointing die (at about 90 per minute) forming the ogive on the cored jacket, then seat them in charged and primed cases and shoot them in their rail gun. They do this several times during a production run. If a run of bullets is extremely accurate, they keep some of those as "standards" to qualify test barrels; some standards shot in the 1/10th MOA range and they used to sell them at bigger rifle matches in plain brown boxes of 1000 with the sizing lube (lanolin) still on them and not polished bright and shiny like retail ones in green boxes.</p><p></p><p>Some time ago, I think someone from Black Hills posted a thread in this forum about a test of one lot their .308 Win match ammo. They fired hundreds of 10-shot test groups at 100 yards from the same rifle. Groups started out averaging 1/4 MOA or better but slowly opened up to about 3/8 at about 3000 rounds, then 1/2 at 5000 and at 10,000 rounds up to almost 1 MOA or something like that. I've searched a couple of times to find it but nothing uncovered.</p><p></p><p>One interesting thing about cartridge OAL in 7.62 NATO M1 and M14/M1A rifles used in matches. Some lots of M118 and M852 match ammo had enough muzzle velocity spread that a 1 MOA vertical shot stringing happened at long ranges. Some people brought their Lyman 310 nutcracker to the match then seated those bullets .010" deeper breaking the asphaltum seal between bullet and neck. They shot much more accurate with that much more jump to the lands as well as much less release force needed to push them out of the case neck. That modified ammo, as well as good lots of it, shot about the same accuracy level for the 3000 to 4000 round lives of the barrels. The bullet jump distance had increased about .080" or more in the chrome moly arsenal match barrel's I wore out and gauged for throat erosion.</p></blockquote><p></p>
[QUOTE="Bart B, post: 1132507, member: 5302"] I think the increased rough surface of the lands and grooves contributed to the accuracy loss. The slight change in pressure curve shape may also be a factor. Here's why. Sierra Bullets' ballistic tech's testing their bullets for accuracy in rail guns record each 10-shot test group for size. Over the life of the barrels, the average group size increases at a fairly constant rate. When the average group size is 50% more than when new, the barrel's replaced. For example, the .308 Win chambered test barrels shooting bullets up through 180 grains last about 3000 rounds, then get replaced. They start out in the 1/4 MOA or so range with match bullets and reach the 3/8 MOA average point that many rounds later. As far as I know, Sierra keeps the same OAL on all their test rounds fired in virtual SAAMI spec chambers. They don't prep cases and full length size them in Redding dies now (I think they used RCBS or Lyman ones years ago). The tech grabs 10 bullets as they come out of the pointing die (at about 90 per minute) forming the ogive on the cored jacket, then seat them in charged and primed cases and shoot them in their rail gun. They do this several times during a production run. If a run of bullets is extremely accurate, they keep some of those as "standards" to qualify test barrels; some standards shot in the 1/10th MOA range and they used to sell them at bigger rifle matches in plain brown boxes of 1000 with the sizing lube (lanolin) still on them and not polished bright and shiny like retail ones in green boxes. Some time ago, I think someone from Black Hills posted a thread in this forum about a test of one lot their .308 Win match ammo. They fired hundreds of 10-shot test groups at 100 yards from the same rifle. Groups started out averaging 1/4 MOA or better but slowly opened up to about 3/8 at about 3000 rounds, then 1/2 at 5000 and at 10,000 rounds up to almost 1 MOA or something like that. I've searched a couple of times to find it but nothing uncovered. One interesting thing about cartridge OAL in 7.62 NATO M1 and M14/M1A rifles used in matches. Some lots of M118 and M852 match ammo had enough muzzle velocity spread that a 1 MOA vertical shot stringing happened at long ranges. Some people brought their Lyman 310 nutcracker to the match then seated those bullets .010" deeper breaking the asphaltum seal between bullet and neck. They shot much more accurate with that much more jump to the lands as well as much less release force needed to push them out of the case neck. That modified ammo, as well as good lots of it, shot about the same accuracy level for the 3000 to 4000 round lives of the barrels. The bullet jump distance had increased about .080" or more in the chrome moly arsenal match barrel's I wore out and gauged for throat erosion. [/QUOTE]
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Rifles, Reloading, Optics, Equipment
Reloading
Accuracy=seating depth or tenths of powder
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