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Rifles, Reloading, Optics, Equipment
Reloading
Concentricity & Runout Correction
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<blockquote data-quote="Bart B" data-source="post: 656173" data-attributes="member: 5302"><p>Over a few decades of high power competition, I've done several tests for accuracy with ammo with bullets somewhat crooked to the case axis. Very interesting results.</p><p></p><p>First off, the tool used and how it's set up make a big difference in what a given round shows its runout to be. As the .308 case centers its front end in the chamber by its shoulder mating perfectly with the chamber shoulder as the firing pin drives it there, that's where the front of the case is when it fires. If the case neck's well centered on the case shoulder and the case neck axis is in line with the body axis, the bullet held in that case neck's well aligned with the bore. It doesn't seem to matter if the case neck wall thickness varies 1/1000th inch; accuracy is just as good with such rounds as those with zero tolerance in neck wall thickness. </p><p></p><p>Second, it makes sense to me that the runout measuring tool should support the loaded round such that the case shoulder spins in something that keeps it concentric and the pressure ring on the case rests against something as that's the part of the case that gets pressed against the chamber by the extractor when it's loaded. In other words, the runout tool should hold the case the same way the chamber does. So I built my own with a nylon washer up front with a 3/8ths inch hole in it to hold the front of the case on its shoulder and the back end at the pressure ring rides in a V block of some sort. The dial indicator's plunger touches the bullet ogive about 1/10th inch back from its nose. No other runout gage I know of holds a case that way; they support the round different ways that don't indicate how the bullet axis will align with the chamber/bore axis. Most of them have the front support on the case body just behind the shoulder and any out of round dimension there adds to the bullet runout number; cases are not perfectly round at that point nor are chambers for that matter.</p><p></p><p>Third, .308's shoot very accurate with up to about 3/1000ths runout. When a few of us worked up a load for Sierra Bullets' first Palma 155-gr. bullet, several thousand rounds of ammo using it were made on two Dillon 1050 progressive loaders. Bullet runout measured up to 3 thousandths. A random sampling of 20 rounds were test fired from a Palma rifle (with a virtual SAAMI spec chamber) at 600 yards; they all went into 2.7 inches. Not bad at all for brand new unprepped cases (other than a Lyman expander to uniform the case mouth inside diameter) and a 3/10ths grain powder charge weight spread.</p><p></p><p>Using issued Lake City 7.62 NATO M118 and M852 match ammo, I've often seen their bullet runout as much as 8/1000ths. It's easily corrected by putting a .338 caliber collet in a bullet puller, then after doing runout tests on rounds marking the high spot on the case, put that round in the bullet puller holding the round by the case neck then pressing on the back end of the case to bend the case neck a bit. I've been able to do this getting bullet runout under 2/1000ths and the ammo shot very accurate; noticably more accurate than before.</p></blockquote><p></p>
[QUOTE="Bart B, post: 656173, member: 5302"] Over a few decades of high power competition, I've done several tests for accuracy with ammo with bullets somewhat crooked to the case axis. Very interesting results. First off, the tool used and how it's set up make a big difference in what a given round shows its runout to be. As the .308 case centers its front end in the chamber by its shoulder mating perfectly with the chamber shoulder as the firing pin drives it there, that's where the front of the case is when it fires. If the case neck's well centered on the case shoulder and the case neck axis is in line with the body axis, the bullet held in that case neck's well aligned with the bore. It doesn't seem to matter if the case neck wall thickness varies 1/1000th inch; accuracy is just as good with such rounds as those with zero tolerance in neck wall thickness. Second, it makes sense to me that the runout measuring tool should support the loaded round such that the case shoulder spins in something that keeps it concentric and the pressure ring on the case rests against something as that's the part of the case that gets pressed against the chamber by the extractor when it's loaded. In other words, the runout tool should hold the case the same way the chamber does. So I built my own with a nylon washer up front with a 3/8ths inch hole in it to hold the front of the case on its shoulder and the back end at the pressure ring rides in a V block of some sort. The dial indicator's plunger touches the bullet ogive about 1/10th inch back from its nose. No other runout gage I know of holds a case that way; they support the round different ways that don't indicate how the bullet axis will align with the chamber/bore axis. Most of them have the front support on the case body just behind the shoulder and any out of round dimension there adds to the bullet runout number; cases are not perfectly round at that point nor are chambers for that matter. Third, .308's shoot very accurate with up to about 3/1000ths runout. When a few of us worked up a load for Sierra Bullets' first Palma 155-gr. bullet, several thousand rounds of ammo using it were made on two Dillon 1050 progressive loaders. Bullet runout measured up to 3 thousandths. A random sampling of 20 rounds were test fired from a Palma rifle (with a virtual SAAMI spec chamber) at 600 yards; they all went into 2.7 inches. Not bad at all for brand new unprepped cases (other than a Lyman expander to uniform the case mouth inside diameter) and a 3/10ths grain powder charge weight spread. Using issued Lake City 7.62 NATO M118 and M852 match ammo, I've often seen their bullet runout as much as 8/1000ths. It's easily corrected by putting a .338 caliber collet in a bullet puller, then after doing runout tests on rounds marking the high spot on the case, put that round in the bullet puller holding the round by the case neck then pressing on the back end of the case to bend the case neck a bit. I've been able to do this getting bullet runout under 2/1000ths and the ammo shot very accurate; noticably more accurate than before. [/QUOTE]
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