Pressure Vs. Velocity - Part 2

Discussion in 'The Ammo & Reloading Forum' started by .308 shooter, May 4, 2009.

  1. .308 shooter

    .308 shooter New Member

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    Okay, thanks to all who advised from my last post. I took my chrono out, and shot several different loads with both the 150gr and the 168gr bullets.

    Conditions during the shoot were cool (approx. 64 degrees and 70% humidity)

    Results are as follows: All are with RE15 Powder
    150gr
    actual shot - 42gr @ 2578 // published - 2500 @ 41.9gr
    actual shot - 44gr @ 2789 // published - 2600 @ 43.6gr
    actual shot - 46gr @ 2917 // published - 2700 @ 45.4gr

    168gr
    actual shot - 40gr @ 2411 // published - 2400 @40.7gr
    actual shot - 41.5gr @ 2492 // published - 2456 @ 41.6gr (interpelated)
    actual shot - 43gr @ 2599 // published - 2555 @ 43.4gr (interpelated)
    actual shot - 43.5gr @ 2619 // published - 2555 @ 43.4gr (interpelated)

    In both cases my ending velocity is greater than the velocity published for the max powder charge.... 2800fps @ 47.2gr for the 150gr bullet and 2600 @ 44.3gr for the 168gr bullet.

    With the velocities being as great as they are compared to the published velocities, am I in approaching anything in the unsafe realm? Is the velocity directly proportional to the pressure? Or are they seperate issues? I individually checked each round after shooting and had no signs of high pressure.
  2. Freebore

    Freebore New Member

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    Yes there is a direct relationship between pressure and velocity. High velocity loads usually have higher pressures behind them. This was the Roy Weatherby syndrome. Weatherby factory loads ran on the high side of the pressure line compared to others.

    Faster burning powders can produce high presse with only a very slight grain increase. I did load for the 308 years ago and with that cartridge along other large capacity cases I alway got the best results with the slower burning powders, good case life, accuracy, and barrel life.


    As a point of interest, the 220 Swift when introduced got a bad reputation as a barrel burner due to high pressure/high velocity factory loads (probably bad steel in the old Winchesters barrels as well) that were put out on the market, the reality is that this round is one of (if not) the best long range .224 varmint rounds when backed off just a few FPS (with good barrel life)

    The ideal load is one that burns the last grain of powder as the bullet leaves
    the barrel, this requires a lot trial and error to develop the perfect load for each barrel. The reloading manuals usually help to identify what powder works best and gives the best accuracy, but developing the right amount is usually up to the individual.
  3. .308 shooter

    .308 shooter New Member

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    Okay..... Since my loads are faster than the published loads but my charges are lower than those published, am I okay where I'm at or do I need to back down a little?

    By the way, most most accurate loads were at the 43.5 grain mark.
  4. Freebore

    Freebore New Member

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    You're case will usually determine how the pressure situation is, if you're getting good accuracy at 43.5 with a little less FPS (and less pressure), I would take that to the bank, that load will give two one-ups, good case life and good barrel life. what more could you ask for.

    If you want flatter shooting longer distance look at something like the 300 Win Mag (bigger case, more powder, higher pressures, shorter barrel life, etc)

    One thing I really liked about the 308 was its accuracy, it probably set more records in the 30 cal class that any other .30 During the mid sixties I was on the US Army rifle team and we mostly used the M14 (7.62), accuracy out of a battle rifle is usually not the best, but the 308 round (and a little tweeking) gave the the M14 a nice edge.
  5. 312shooter

    312shooter Active Member

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    By the way, most most accurate loads were at the 43.5 grain mark.[/QUOTE]

    Personally if you've got the most accurate load figured out, I'd start playing with your COAL with this grain load. 168's seem to like the 2600 fps range in my rifles. If you have properly sized your brass and have no headspace issues and the bullet is not touching the lands ( seated too long ) you should be fine with pressure range.
  6. 312shooter

    312shooter Active Member

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    :)
    Last edited: May 5, 2009
  7. muddober

    muddober Active Member

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    .308 shooter, I think you got Freebore to buy into your idea of using your chronograph to read pressure signs perhaps because he did not see your earlier post. I said it before and I will say it again as I did in a reply to another post of yours on the same subject, there is NOT a direct relationship to pressure and velocity. As an example lets say for some crazy reason someone decided to use what would be a maximum high pressure load of IMR 4064 in a 300 Ultra mag. While it would work, meaning it go bang and push the bullet down range it is the wrong powder to use to get maximum velocity at maximum pressure. I promise that you could obtain several hundred feet per second velocity faster using a maximum load of reloader 25 at exactly the same pressure as the IMR 4064 loading. Both loads could have the same pressure yet the velocity wouldn't even be close making your chronograph worthless for checking pressure. As I also told you before every reloading manual I have ever seen has a section of reading "signs of pressure" or "pressure signs" and I guarantee the word chronograph will not be found in that section.

    By the way I did think all the work you did chronographing those loads was very cool and informative.

    Ron
    Last edited: May 5, 2009
  8. .308 shooter

    .308 shooter New Member

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    Okay.... then the short answer is.... NO velocity and pressure is not directly related and as long as I'm showing no signs of excessive pressure, the velocity really doesn't matter as it relates to the safety of the reloads - Right?

    Thanks guys... now it's just a matter of perfecting that load and tweaking it for a little tighter group.
  9. Popgunner

    Popgunner New Member

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    Muddober has it correct. Velocity is a product of presure over time. You can get short lived, dangerous pressure spikes that don't contribute much to velocity.
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