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The Basics, Starting Out
Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions
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<blockquote data-quote="Batsin" data-source="post: 3044970" data-attributes="member: 126867"><p>Thanks. That comment from The Real Gunsmith had me wondering, but then again he didn't cite any examples. When worked at a factory, we used precision grinding or honing when we had to hold under a thousandth tolerance.</p><p></p><p>I learned about the headspace and case issue back when the 6.8 SPC was going through its painful introduction. There were some problems with AR15's with in-spec chambers not loading in-spec ammo (there were a lot of bad reamers and undersized barrels, but that is another story). One brass maker commented that whereas the crush fit allowed in the SAAMI spec was not a big deal for most bolt-action guns, semi-autos were more sensitive. Brass had to be around min-spec to work well in AR15's.</p><p></p><p>The standard SAAMI tolerances have not changed for decades. Even on new cartridges like the 22 ARC, chamber specs are + 0.002" dia and +0.015" length. Cartridge body specs are -0.008". A lot of the accuracy of newer cartridges is attributed to long sections of tight freebore, but there is a big difference between 0.0005" and 0.0025" clearance. One is considered "match" and the other, "sloppy".</p><p>At least more cartridges are tightening up on max bullet diameter. A surprising number of cartridges allow the max spec bullet to be larger than the bore. The traditional 30 cal bullet spec, for example, has been 0.3090" - 0.003". At least most barrel throats have a fairly gentle final forcing cone. On the other hand, several cartridges now run such a tight min freebore spec that they had to tighten up on the bullet spec as well, since a 0.309 bullet will raise hell when it hits a freebore that is as small as 0.3084" diameter. The forcing cone into the freebore is often as abrupt as 45° per side. FWIW, Weatherby Magnums specified a smaller max bullet dia of 0.3083 and call it out on the SAAMI print. Now, the 300 Norma Mag and 300 PRC spec bullets under the traditional dimension.</p></blockquote><p></p>
[QUOTE="Batsin, post: 3044970, member: 126867"] Thanks. That comment from The Real Gunsmith had me wondering, but then again he didn't cite any examples. When worked at a factory, we used precision grinding or honing when we had to hold under a thousandth tolerance. I learned about the headspace and case issue back when the 6.8 SPC was going through its painful introduction. There were some problems with AR15's with in-spec chambers not loading in-spec ammo (there were a lot of bad reamers and undersized barrels, but that is another story). One brass maker commented that whereas the crush fit allowed in the SAAMI spec was not a big deal for most bolt-action guns, semi-autos were more sensitive. Brass had to be around min-spec to work well in AR15's. The standard SAAMI tolerances have not changed for decades. Even on new cartridges like the 22 ARC, chamber specs are + 0.002" dia and +0.015" length. Cartridge body specs are -0.008". A lot of the accuracy of newer cartridges is attributed to long sections of tight freebore, but there is a big difference between 0.0005" and 0.0025" clearance. One is considered "match" and the other, "sloppy". At least more cartridges are tightening up on max bullet diameter. A surprising number of cartridges allow the max spec bullet to be larger than the bore. The traditional 30 cal bullet spec, for example, has been 0.3090" - 0.003". At least most barrel throats have a fairly gentle final forcing cone. On the other hand, several cartridges now run such a tight min freebore spec that they had to tighten up on the bullet spec as well, since a 0.309 bullet will raise hell when it hits a freebore that is as small as 0.3084" diameter. The forcing cone into the freebore is often as abrupt as 45° per side. FWIW, Weatherby Magnums specified a smaller max bullet dia of 0.3083 and call it out on the SAAMI print. Now, the 300 Norma Mag and 300 PRC spec bullets under the traditional dimension. [/QUOTE]
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The Basics, Starting Out
Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions
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