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> Pressure Question
Pressure Question
@ 05:47 pm (GMT) |
VinceHi AllI'm interested to know why there are different pressure maximums for different cartridges in SAAMI specs. In the case of something like a 7x57 or a 6.5x55 I understand they are setting lower limits based on the risk of them being fired in circa 1900 rifles but take for example 7mm08 60,000 psi 308 62,000 psi 6.5 cm 63,000 psi 6.5 prc 65,000 psi Just curious Cheers Vince |
Replies
@ 10:31 am (GMT) |
Andrew MurrayRe: Pressure QuestionI've been pondering this.I wonder if it has to do with with neck size (with 7mm08 and 308). Squeezing the same volume of powder through a smaller hole surely would stress the case, increasing pressure and/or velocity. The PRC and CM are differently sized, so the suggestion regarding the neck size may not be as valid, however I'm sure the overall volume requires a higher pressure to generate a similar performance. It's an interesting question and one that I assume can be explored further from a purely "physics" based point of view, though I am also sure that wildcatters of the early 20th century did not have degrees in physics. |
@ 07:04 am (GMT) |
Nathan FosterRe: Pressure QuestionHi Vince, sorry for the late reply. I was going to answer this but I thought instead that you might prefer to have an answer from somebody closer to the coal face. To this end, I waited for Dave Manson to return from a road trip and then wrote to him, asking if he would mind writing an answer for you. His reply is as follows:You and I have had discussions about SAAMI dimensional specs and tolerances in the past. Aside from some older cartridge/chamber combinations (e.g, .30-06, 303 British, 45-70), most modern designs have fairly consistent body/neck/COAL clearances and typically use the same shoulder angle for both cartridge and chamber. Throat configurations differ quite a bit, however. This, of course, affects pressure levels. In general, dimensional and pressure standards for old calibers, such as the aforementioned, were inherited by SAAMI when it was formed as a standards organization (1930s?). More recently, SAAMI relies on the developer(s) of a new cartridge to provide testing results from the development process in formats that meet SAAMI criteria. In other words, cartridge developers have to do the testing work in an approved fashion and then present it to SAAMI for review. The "package" presented to SAAMI includes (but isn't limited to): dimensional drawings, with tolerances, of both cartridge and chamber, and pressure testing protocols and results. SAAMI can approve the new caliber, or it can ask the developer to provide additional test results in one or more areas. (The 458 SOCOM has been in approval limbo for quite a few years. The last I knew of it, when still working with SAAMI, was that an Italian armory was working with CIP and SAAMI in order to achieve standardization.) The "bottom line" on this explanation is that pressure (and other data) submitted for approval does not have to be identical to testing data for other calibers; only in the same previously-established ballpark. Given that calibers are developed by different entities, at different times, it's unlikely for any to agree. Because of SAAMI's testing protocols, the consumer can be confident that the SAAMI/CIP-spec 308 Win, or 12 Ga x 3", round he buys in NZ, will fit and fire safely in a SAAMI/CIP-spec rifle or shotgun, made in the US or Italy. Unfortunately, I gave up my consultancy with SAAMI when the company was sold. Therefore, please note that the information I provided here for your forum member question is 1 1/2 years old. End. As Dave suggests, the bottom line is that data submitted for approval does not have to be identical to test data for other cartridges. Hope that helps to answer your question Vince. |
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@ 05:37 pm (GMT) |
VinceRe: Pressure QuestionThanks for the responses Andrew and NathanI had assumed it would be some scientific calculation involving throat geometry or leade angles (which I don't really understand at this point) and it sounds like that does play a roll but hadn't considered it would come down to whatever was submitted. The reason the question originally occurred was that anecdotally, you hear of quite a few people going over max recommended charge weights on specific cartridges, the two that I was looking at were the 7mm08 and the 280AI as I have both. Maybe for a company like Hornady who are designing their own cartridges it might be worth pushing a bit further for maximum performance out of SAAMI specs to promote their own product, but for Remington and Nosler who were essentially just legitimizing an existing and already popular wildcat, there are too many years and variables involved to push the performance too much? Thanks again - more food for thought Cheers Vince |
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@ 05:38 pm (GMT) |
VinceRe: Pressure QuestionAlso thankyou very much to Dave for the reply! |
@ 12:45 pm (GMT) |
Scott StruifRe: Pressure QuestionAs an aside, Hornady had a third-party ballistics expert on a recent podcast about cartridge case design. He said you can easily double the rearward force on the bolt by failing to remove all the case lube from a cartridge. The brass is supposed to grab the chamber walls. If lube prevents that from happening, the bolt face takes all the punishment. This could lead to false-positive pressure signs. The same goes for not cleaning all the oil out of the chamber after lubricating the bore between range sessions. |
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