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Rifles, Reloading, Optics, Equipment
Reloading
Neck Bushing Sizes
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<blockquote data-quote="Mikecr" data-source="post: 1898527" data-attributes="member: 1521"><p>Redding is most correct on the topic.</p><p></p><p>They are a bit weak in this: <em>"When using brass which is at the thinner end of the SAAMI spec. we have found the bushing which is .001" smaller will provide the best result." </em></p><p>Necks come in a wide band of thicknesses, and whether 8thou thick or 18thou, the 1thou interference represents max needed. So Redding bushings are selected to get that -regardless of brass thickness.</p><p></p><p>The reason their math doesn't always work out, leading to a band .001-.002 recommended, is because of chamber neck clearances. Higher neck clearances cause larger fired necks, leading to greater downsizing requirement to achieve 1thou interference. But that doesn't mean using a smaller bushing,, it means using a <u>larger</u> bushing. This, because with greater downsizing the brass is rolling at <strong>larger</strong> <strong>angles</strong>.</p><p>With that, bushing selection is often trial & error of recommended band.</p><p></p><p>Example: </p><p>Loaded neck diameter of .248</p><p>Chamber neck at .250</p><p>Fired necks will hit the .250 chamber neck plus a tiny chamber expansion and spring back to ~.2495. This in a general sense, because neck diameter and hardness affect it a bit.</p><p>You want .001 interference after sizing(unloaded necks of .247). Will a .247 bushing sizing down from .2495 actually leave necks at .247? NO. The necks will release from the bushing springing back outward to .2475. You end up with 1/2thou interference.</p><p>You'll need a bushing ~.002 smaller than loaded necks, .246, from which necks will spring back to .2465, or 1.5thou interference. And 1.5thou interference is better to desired than 0.5thou interference, so the better bushing selection is .002 under loaded neck (in this case).</p><p>But this is considered by some as a 'tight neck' chamber, offering only 2thou clearance.</p><p></p><p>Example 2:</p><p>Loaded neck diameter of .248</p><p>Chamber neck at .254</p><p>Fired necks come out ~.2535</p><p>Desired sized neck OD again at .247</p><p>Sizing down .2535 with a .247 bushing is >5thou difference. That level of sizing ANGLE changes the result, actually causing necks to release at ~.2465, or 1.5thou interference from .248.</p><p>Not bad, so Redding's recommended .001 under loaded works out. </p><p>A 246 bushing (.002 under loaded) would be too much.</p><p></p><p>IMO, it's a greater trend in CLEARANCES that Redding recommendations are following.</p><p>You can always purchase both 1thou and 2thou under bushings, and try them both for your final selection choice.</p></blockquote><p></p>
[QUOTE="Mikecr, post: 1898527, member: 1521"] Redding is most correct on the topic. They are a bit weak in this: [I]"When using brass which is at the thinner end of the SAAMI spec. we have found the bushing which is .001" smaller will provide the best result." [/I] Necks come in a wide band of thicknesses, and whether 8thou thick or 18thou, the 1thou interference represents max needed. So Redding bushings are selected to get that -regardless of brass thickness. The reason their math doesn't always work out, leading to a band .001-.002 recommended, is because of chamber neck clearances. Higher neck clearances cause larger fired necks, leading to greater downsizing requirement to achieve 1thou interference. But that doesn't mean using a smaller bushing,, it means using a [U]larger[/U] bushing. This, because with greater downsizing the brass is rolling at [B]larger[/B] [B]angles[/B]. With that, bushing selection is often trial & error of recommended band. Example: Loaded neck diameter of .248 Chamber neck at .250 Fired necks will hit the .250 chamber neck plus a tiny chamber expansion and spring back to ~.2495. This in a general sense, because neck diameter and hardness affect it a bit. You want .001 interference after sizing(unloaded necks of .247). Will a .247 bushing sizing down from .2495 actually leave necks at .247? NO. The necks will release from the bushing springing back outward to .2475. You end up with 1/2thou interference. You'll need a bushing ~.002 smaller than loaded necks, .246, from which necks will spring back to .2465, or 1.5thou interference. And 1.5thou interference is better to desired than 0.5thou interference, so the better bushing selection is .002 under loaded neck (in this case). But this is considered by some as a 'tight neck' chamber, offering only 2thou clearance. Example 2: Loaded neck diameter of .248 Chamber neck at .254 Fired necks come out ~.2535 Desired sized neck OD again at .247 Sizing down .2535 with a .247 bushing is >5thou difference. That level of sizing ANGLE changes the result, actually causing necks to release at ~.2465, or 1.5thou interference from .248. Not bad, so Redding's recommended .001 under loaded works out. A 246 bushing (.002 under loaded) would be too much. IMO, it's a greater trend in CLEARANCES that Redding recommendations are following. You can always purchase both 1thou and 2thou under bushings, and try them both for your final selection choice. [/QUOTE]
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