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The Basics, Starting Out
Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions
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<blockquote data-quote="ImBillT" data-source="post: 3052823" data-attributes="member: 117715"><p>Sloppily indicating the barrel, and using insufficient distance between the two points that are being held are big reasons for fat chambers. Another is not having your reamer exactly on the center line of the barrel. If you lathe weighs or tooling are worn this is possible. Some guys don't check. I assume a floating reamer holder cures that for the most part.</p><p></p><p>As mentioned, the reamers don't always come out exactly to spec. It's actually pretty hard to make something perfect, so there's a tolerance. That varies from reamer maker to reamer maker. PT&G has tighter tolerances on their "match" reamers than their regular SAAMI reamers.</p><p></p><p>The SAAMI specs for reamers and chambers are so that everything from every manufacturer(that is within the spec) will function and function safely. A .004" crush fit of the smallest chamber and largest case will still chamber with some effort. A .006" space between the largest chamber and smallest case will still function safely, but potentially cost brass life. If you're having a rifle built for maximum accuracy, you may not want to use an off the shelf SAMMI reamer, because it would be any dimension within the spec, but you also may not want a SAAMI min spec reamer that was held to very tight tolerances unless you have settled on what brass you're going to use and know what size it is consistently. Many shooters have customs spec's reamers to be a close fit to specific brass. There are reamers cut for "Gold Box" Lapua, "Blue Box" Lapua, Norma, "White Box Winchester", Winchester Palma brass, Remington BR brass, and likely others. These reamers give proper minimal clearance for those batches of brass and work very well without fire forming, and when paired with a custom sizer they give excellent brass life and excellent accuracy right out the gate. However, they may be unsafe and yield reduced accuracy with other brass or off the shelf dies. Lapua Gold Box was a smaller dimension than Lapua Blue Box, and if you have a chamber reamed specifically for Gold Box, you will have difficulty using Blue Box because the web is too large for the chamber and sizing that down is extremely difficult. IIRC Blue Box is close to SAAMI max case dimensions, which, when used in factory chambers of SAAMI specs results in better than average accuracy from virgin cases as well as less brass growth. Gold Box was closer SAAMI min case specs which resulted in quite a loose fit on factory chambers. Not only was accuracy likely degraded with virgin brass, but brass life was probably also degraded due to excessive case stretch on the first firing. If you had a reamer designed for Gold Box brass, then it was absolutely excellent brass.</p></blockquote><p></p>
[QUOTE="ImBillT, post: 3052823, member: 117715"] Sloppily indicating the barrel, and using insufficient distance between the two points that are being held are big reasons for fat chambers. Another is not having your reamer exactly on the center line of the barrel. If you lathe weighs or tooling are worn this is possible. Some guys don't check. I assume a floating reamer holder cures that for the most part. As mentioned, the reamers don't always come out exactly to spec. It's actually pretty hard to make something perfect, so there's a tolerance. That varies from reamer maker to reamer maker. PT&G has tighter tolerances on their "match" reamers than their regular SAAMI reamers. The SAAMI specs for reamers and chambers are so that everything from every manufacturer(that is within the spec) will function and function safely. A .004" crush fit of the smallest chamber and largest case will still chamber with some effort. A .006" space between the largest chamber and smallest case will still function safely, but potentially cost brass life. If you're having a rifle built for maximum accuracy, you may not want to use an off the shelf SAMMI reamer, because it would be any dimension within the spec, but you also may not want a SAAMI min spec reamer that was held to very tight tolerances unless you have settled on what brass you're going to use and know what size it is consistently. Many shooters have customs spec's reamers to be a close fit to specific brass. There are reamers cut for "Gold Box" Lapua, "Blue Box" Lapua, Norma, "White Box Winchester", Winchester Palma brass, Remington BR brass, and likely others. These reamers give proper minimal clearance for those batches of brass and work very well without fire forming, and when paired with a custom sizer they give excellent brass life and excellent accuracy right out the gate. However, they may be unsafe and yield reduced accuracy with other brass or off the shelf dies. Lapua Gold Box was a smaller dimension than Lapua Blue Box, and if you have a chamber reamed specifically for Gold Box, you will have difficulty using Blue Box because the web is too large for the chamber and sizing that down is extremely difficult. IIRC Blue Box is close to SAAMI max case dimensions, which, when used in factory chambers of SAAMI specs results in better than average accuracy from virgin cases as well as less brass growth. Gold Box was closer SAAMI min case specs which resulted in quite a loose fit on factory chambers. Not only was accuracy likely degraded with virgin brass, but brass life was probably also degraded due to excessive case stretch on the first firing. If you had a reamer designed for Gold Box brass, then it was absolutely excellent brass. [/QUOTE]
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Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions
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