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RCBS Precision Mic
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<blockquote data-quote="Bart B" data-source="post: 369241" data-attributes="member: 5302"><p>After checking with some metrology lab (place where precision mechanical measurements are made and tool gages calibrated) machinists, they all said that two V blocks were best for measuring a cartridge's concentricity. Two of them reloaded their ammo so they had some credibility.</p><p></p><p>The rear V block's best located only at the pressure ring. That's the fattest part of the sized rimless case about 2/10ths to 1/4th inch in front of the head. It's about 1/16th inch in front of the belt on a magnum case and 1/8th inch in front of the extractor groove on rimless cases or in front of the rim on a rimmed case. This is the point at the back end of the case that's against the chambr wall when the round's chambered and fired. It's pushed there by the extractor in virtually all rifles. Even though it might be a tiny bit out of round, that's not important if it's not out more than 1 thousandth inch which most sized cases are; any out of round there has minimum effect on bullet concentricity to the case.</p><p></p><p>The front V block should center about mid point on the shoulder. That part of a rimless bottleneck case is perfectly centered in the chamber shoulder when the firing pin slams it there when the round fires. And on belted cases, it's away from the body diameter at the body-shoulder junction as well as the neck, both of which may not be perfectly round. If they're out of round, their dimensions will add or subtract from the out of round condition of the pressure ring and cause bigger errors in runout readings on the bullet or case neck than the same amount at the pressure ring. A round gaging ring whose diameter is what mid point on the case shoulder is would be better than a V block as it better represents where the case shoulder fits when the round's chamberd.</p><p></p><p>The plunger on the dial indicator should be vertical and put on the bullet about 1/10th inch back from its tip, the case neck about 1/16th inch back from the mouth. This setup will also measure runout at the pressure ring or shoulder-bldy junction. Just keep the two case contact points horizontal and the dial indicator plunger vertical.</p><p></p><p>Cartridge concentrictiy tools that rest only the case body in a V block will have errors introduced by the body out of round condition. Those that have the bullet tip held in a coned collet with the back end in a V block don't take into consideration how the bullet's aligned with the center of the shoulder.</p><p></p><p>I built my own gage in the 1960's 'cause nobody had one that did all the right stuff on the market. Then clamped a standard dial indicator in on it to measure any point on the round. It was nothing more that two 1/4th inch threaded rods about 5 inches long bolted in two small plates at each end. Two nylon washers, 1/16th inch thick were spaced 1/4th inch from one end and the other two spaced at mid point on a case shoulder with its head against one end plate. Those four washers formed a "V" block at the pressure ring and mid point on the shoulder. Testing a precision 1/2 inch gage pin on them showed zero runout on a 1/1000ths dial indicator in repeated tests; so the nylon washers did well. Any wear that may have changed the exact location where the case contacted them didin't matter. I later replaced those nylon washers with ball bearings. Go spend ten bucks on parts and build yourself one, then you'll only have to spend money on a dial indicator (good cheap ones are often found in pawn shops). I used a large nylon washer whose hole was .39 inch that fit case shoulders at a good point. But that washer was hard to mount good with its hole at the same place as the outside edge of the washers used as a V block, so I went back to just a V block setup for the front.</p><p></p><p>First thing I learned was bullets seat well aligned to the neck. If the neck's crooked, a $12,345.67 custom made bullet seating die won't seat bullets straight; crooked case necks bend back from being held straight less than a second of time by such tools. Redding's web site has quite an article about this. If the neck's straight, one can seat bullets very straight with a ball peen hammer, just smake the bullet tip very square and gently. It really helps if the case mouth is square and only slightly smaller than bullet diameter. Deburring the mouth with an Eazy-Out screw extraction tool twisted clockwise leaves a better angled edge after trimming the case to length. I tried Vickerman and Wilson chamber type in line seaters, Bonanza benchrest seaters and a few others I now forget. I settled on an RCBS competition seater 'cause it's easy and fast to use.</p><p></p><p>In talking with highpower rifle folks who win the matches and set the records, any runout under 2.5 thousandths for 30 caliber cartridges is good enough. The ammo a bunch of us developed and loaded in 1991 for Sierra's prototype .308 Palma bullet had up to 4 thousandths runout. It tested 2.7 inches at 600 yards for 20 shots. Top long range competitors from around the world said it shot about 1/2 MOA at 600 yards in their rifles and I know it did that in mine. Not too shabby for new cases with metered (not weighed) powder charges in standard SAAMI chambers for headspace, body and neck diameters.</p></blockquote><p></p>
[QUOTE="Bart B, post: 369241, member: 5302"] After checking with some metrology lab (place where precision mechanical measurements are made and tool gages calibrated) machinists, they all said that two V blocks were best for measuring a cartridge's concentricity. Two of them reloaded their ammo so they had some credibility. The rear V block's best located only at the pressure ring. That's the fattest part of the sized rimless case about 2/10ths to 1/4th inch in front of the head. It's about 1/16th inch in front of the belt on a magnum case and 1/8th inch in front of the extractor groove on rimless cases or in front of the rim on a rimmed case. This is the point at the back end of the case that's against the chambr wall when the round's chambered and fired. It's pushed there by the extractor in virtually all rifles. Even though it might be a tiny bit out of round, that's not important if it's not out more than 1 thousandth inch which most sized cases are; any out of round there has minimum effect on bullet concentricity to the case. The front V block should center about mid point on the shoulder. That part of a rimless bottleneck case is perfectly centered in the chamber shoulder when the firing pin slams it there when the round fires. And on belted cases, it's away from the body diameter at the body-shoulder junction as well as the neck, both of which may not be perfectly round. If they're out of round, their dimensions will add or subtract from the out of round condition of the pressure ring and cause bigger errors in runout readings on the bullet or case neck than the same amount at the pressure ring. A round gaging ring whose diameter is what mid point on the case shoulder is would be better than a V block as it better represents where the case shoulder fits when the round's chamberd. The plunger on the dial indicator should be vertical and put on the bullet about 1/10th inch back from its tip, the case neck about 1/16th inch back from the mouth. This setup will also measure runout at the pressure ring or shoulder-bldy junction. Just keep the two case contact points horizontal and the dial indicator plunger vertical. Cartridge concentrictiy tools that rest only the case body in a V block will have errors introduced by the body out of round condition. Those that have the bullet tip held in a coned collet with the back end in a V block don't take into consideration how the bullet's aligned with the center of the shoulder. I built my own gage in the 1960's 'cause nobody had one that did all the right stuff on the market. Then clamped a standard dial indicator in on it to measure any point on the round. It was nothing more that two 1/4th inch threaded rods about 5 inches long bolted in two small plates at each end. Two nylon washers, 1/16th inch thick were spaced 1/4th inch from one end and the other two spaced at mid point on a case shoulder with its head against one end plate. Those four washers formed a "V" block at the pressure ring and mid point on the shoulder. Testing a precision 1/2 inch gage pin on them showed zero runout on a 1/1000ths dial indicator in repeated tests; so the nylon washers did well. Any wear that may have changed the exact location where the case contacted them didin't matter. I later replaced those nylon washers with ball bearings. Go spend ten bucks on parts and build yourself one, then you'll only have to spend money on a dial indicator (good cheap ones are often found in pawn shops). I used a large nylon washer whose hole was .39 inch that fit case shoulders at a good point. But that washer was hard to mount good with its hole at the same place as the outside edge of the washers used as a V block, so I went back to just a V block setup for the front. First thing I learned was bullets seat well aligned to the neck. If the neck's crooked, a $12,345.67 custom made bullet seating die won't seat bullets straight; crooked case necks bend back from being held straight less than a second of time by such tools. Redding's web site has quite an article about this. If the neck's straight, one can seat bullets very straight with a ball peen hammer, just smake the bullet tip very square and gently. It really helps if the case mouth is square and only slightly smaller than bullet diameter. Deburring the mouth with an Eazy-Out screw extraction tool twisted clockwise leaves a better angled edge after trimming the case to length. I tried Vickerman and Wilson chamber type in line seaters, Bonanza benchrest seaters and a few others I now forget. I settled on an RCBS competition seater 'cause it's easy and fast to use. In talking with highpower rifle folks who win the matches and set the records, any runout under 2.5 thousandths for 30 caliber cartridges is good enough. The ammo a bunch of us developed and loaded in 1991 for Sierra's prototype .308 Palma bullet had up to 4 thousandths runout. It tested 2.7 inches at 600 yards for 20 shots. Top long range competitors from around the world said it shot about 1/2 MOA at 600 yards in their rifles and I know it did that in mine. Not too shabby for new cases with metered (not weighed) powder charges in standard SAAMI chambers for headspace, body and neck diameters. [/QUOTE]
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