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'Nother Semi-Techie Question

Discussion in 'Uncategorized Threads' started by hmb, Nov 27, 2007.

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  1. hmb

    hmb Well-Known Member

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    I don't understand the question. HMB
     
  2. Neil Winston

    Neil Winston Well-Known Member

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    (Steve asked why, in the pressure and velocity tests I report on my website, there seems to be such a variable connection between the two. In one shell type, though high pressure in a single shell is often matched with an elevated speed, more often it's just average. Why this rubber-band effect? Why can't you draw a straight line through the pressure readings and lay them right down on a similar straight line for speed? Doesn't pressure cause speed? Why only sometimes?)

    I answered:

    Steve, while it's surely pressure that causes speed, there's the second element of time. We're not getting into any of that push vs shove stuff here; we're going to look at time in a different way.

    Disclosure: I had the same question for ages myself. What I say here I got from a great reloading book by - I think - Nolte.

    We are looking at 19 grains of Green Dot (or any other powder, it's just at all the shells have the same powder.) In some shells the powder burns in much the same way as many others and these will have about the same pressure and about similar speed. But in a few it will burn a little fast, the peak pressure will be a little higher - so why isn't the speed higher? The answer is that the other shells still have some powder to burn and get more speed from, but this shell doesn't; it's powder is gone . A similar argument can be made for a shell that starts a little slow. The peak pressure is down, true, but there's more powder than usual to burn later and keep accelerating the shot.

    Nolte, writing a good while ago, blamed these differences on the inconsistency of non-fouling primers. If we just went back to mercury, we'd get a lot more consistent results.

    Anyway, Steve, his account made sense to me. You can, of course, extend the line of argument to slower and faster powders as well.

    Neil
     
  3. crusha

    crusha TS Member

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    So, aren't we talking about the area under the curve, then, rather than the peak height?

    (Or, put another way...does the Winchester's flat-topped "sine wave" have more area under it than the Pat Trap's :))
     
  4. Harold

    Harold TS Member

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    Buzz-gun, I think you posted your answer on the wrong thread, but still it is a correct answer for this thread.

    The velocity should be proportional to the area under the pressure vs time curve because the acceleration is proportional to pressure and the speed is the area under the acceleration vs time curve.
     
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