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Rifles, Reloading, Optics, Equipment
Reloading
Problem with belt cartridge
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<blockquote data-quote="Bart B" data-source="post: 550488" data-attributes="member: 5302"><p>Belted cases have had accuracy problems since their inception near 100 years ago. But It wasn't until around 1960 that a few top competitive shooters figured out what their problem was. Until then, new belted cases typically shot the most accurate in the long range matches.</p><p></p><p>Problem is, the case body immediately in front of the belt expanded enough between the front of the belt and the chamber's headspace ridge causing a step in the case body at that point. Conventional full length sizing dies don't size a fired belted case body all the way to the belt so that ridge, or step, is there after using such a die. And, of course, no neck only die didn't either.</p><p></p><p>When a conventionally sized belted case gets chambered, that ridge will interfere with the chamber at that point causing the back end of the case to be positioned a tiny bit different for each shot. It depends on how high that secondary step is and where on the case circumference its high point is (case wall thickness playes a part here; cases aren't uniform in wall thickness at this point). When the round fires, forces at that point cause the barrel to whip differently for each shot as the bullet goes down the barrel. 'Tain't much difference between shots, but enough to open a 600 yard 3" group up to 5 or 6 inches. If that step's pretty bug, accuracy can degrade further.</p><p></p><p>So, one needs to get rid of that step. 50 some years ago, folks would take a standard full length sizing die, cut it off about 1/4th inch up from the belt clearance edge and about 1/4th inch below the body-shoulder junction. After squaring off the bottom end, it would be set up in the press such that a case already full length sized die (with its neck lapped out to 3 thousandths smaller than a loaded round's neck diameter so no expander ball's used) could be sized again in this "body" die such that that step was reduced back down to the same diameter as the body immediately in front of the belt. In other words, the belted case has its body diameters sized down to virtual new case dimensions.</p><p></p><p>Now there's no step on the case to cause accuracy problems. Accuracy's as good as it gets. Larry Willis' collet die now does the same thing with that step. But it may be cheaper to make a "body" die from an existing full length sizing die.</p></blockquote><p></p>
[QUOTE="Bart B, post: 550488, member: 5302"] Belted cases have had accuracy problems since their inception near 100 years ago. But It wasn't until around 1960 that a few top competitive shooters figured out what their problem was. Until then, new belted cases typically shot the most accurate in the long range matches. Problem is, the case body immediately in front of the belt expanded enough between the front of the belt and the chamber's headspace ridge causing a step in the case body at that point. Conventional full length sizing dies don't size a fired belted case body all the way to the belt so that ridge, or step, is there after using such a die. And, of course, no neck only die didn't either. When a conventionally sized belted case gets chambered, that ridge will interfere with the chamber at that point causing the back end of the case to be positioned a tiny bit different for each shot. It depends on how high that secondary step is and where on the case circumference its high point is (case wall thickness playes a part here; cases aren't uniform in wall thickness at this point). When the round fires, forces at that point cause the barrel to whip differently for each shot as the bullet goes down the barrel. 'Tain't much difference between shots, but enough to open a 600 yard 3" group up to 5 or 6 inches. If that step's pretty bug, accuracy can degrade further. So, one needs to get rid of that step. 50 some years ago, folks would take a standard full length sizing die, cut it off about 1/4th inch up from the belt clearance edge and about 1/4th inch below the body-shoulder junction. After squaring off the bottom end, it would be set up in the press such that a case already full length sized die (with its neck lapped out to 3 thousandths smaller than a loaded round's neck diameter so no expander ball's used) could be sized again in this "body" die such that that step was reduced back down to the same diameter as the body immediately in front of the belt. In other words, the belted case has its body diameters sized down to virtual new case dimensions. Now there's no step on the case to cause accuracy problems. Accuracy's as good as it gets. Larry Willis' collet die now does the same thing with that step. But it may be cheaper to make a "body" die from an existing full length sizing die. [/QUOTE]
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Problem with belt cartridge
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