Bolt thrust and case taper?

Discussion in 'Long Range Hunting & Shooting' started by jcoop, Nov 21, 2009.

  1. jcoop

    jcoop Well-Known Member

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    Oct 22, 2008
    Does cartridge taper and shoulder angle have any effect on bolt thrust. I was reading some of the posts on another board on not using the 338 lapua in the savage action do to its high bolt thrust. It was stated that the case head of the lapua is too large in dia. and thus created too much bolt thrust, and the smaller dia of the RUM case has less thrust. It went on to say it is all about case dia and pressure, and powder capacity and bullet weight have little to do with bolt thrust.
    This got me thinking. It seems to me that the more tapered the case is the more thrust it could create IE improved cartridges having less. My reasoning on this is the more tapered case would act more like a wedge as it expands . Or does the thrust come from the shoulder? I am not trying to build a lapua on a savage action. And I am not saying it should or should not be done (I have my own opinion on that). It just got me thinking, would a lapua improved have less or more thrust do to more powder and less taper? How much testing has actually been done on bolt thrust? Anyone have any input on this ?
     
  2. ZebDeming

    ZebDeming Active Member

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    I'm sure that case taper might affect bolt thrust, as I know that the finish in a chamber can affect it, but it would be foolish to use it as design factor for a build, because so much could affect it, a dirty chamber versus a clean one, for one. And if you had a case head seperation, the brass sticking to the chamber wouldn't help at all as the case head would generate all the bolt thrust, and if you're on the line at all, it might just be enough to do bad things. This is just my opinion take it for what you will, but when I decided to build a big 338 on a savage action I decided on an Edge, and not a Lapua. Here are some articles by Dan Lilja on the subject ( Lilja Precision Rifle Barrels - Articles: A Look at Bolt Lug Strength ) (Lilja Precision Rifle Barrels - Articles: 378 Weatherby Cases in the Remington 700 Action? ) the last one is on using a .378 weatherby case or a rigby on a 700 but It's still good reading. If you were to go the lapua case in the savage action, I wouldn't use a barrel nut and fit the barrel with a shoulder. People use the rem 700 with the lapua cases and the savage should be just as strong, and it's got finer threads on the barrel shank (same dia) so the threads cut less deep into the chamber end on the barrel. Just some info to chew on. Personally I'd use the Edge.

    Zeb
     

  3. jcoop

    jcoop Well-Known Member

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    Nice Zeb. I found it interesting how they found the mkV action to be marginable with the 378 case do to thread size. And how the safety factor on bolt thrust on the 700 is over 3x and still found to be marginal. Also that the case can absorb up to half of the thrust with friction but do not use that as a factor. The dirty chamber thing makes sense. I wonder if anyone has ever tested any of these factors or if these are just engineering numbers. Good read. Thanks.
    Again I am not trying to decide on what to build or even asking about the savage lapua thing just about the factors of bolt thrust. I am already building an edge for many different reasons.
     
  4. J E Custom

    J E Custom Well-Known Member

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    The main thing to consider is the "Pounds per square inch" smaller case head less lbs/sq.in
    the bigger the case head the more Lbs/sq.in with the same chamber pressure.

    The main reason that some actions are not recommended for the Lapua and other large case
    heads is because after you open the bolt face up there is very little bolt shroud left and the
    extractors become very fragile (Thin).

    J E CUSTOM