IV vs Barrel Pressure

Discussion in 'Reloading' started by blackfly, Oct 5, 2011.

  1. blackfly

    blackfly Member

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    Oct 2, 2011
    We all know that reloading manuals are to be used as a guide only. Each gun is different, and the variables are endless. Most reloaders are primarily concerned with muzzle velocity and are only concerned about pressure when indications start to show up on the primers or the case itself.

    Because we can’t measure barrel pressure, my question is this:

    As long as the powder type, bullet, and muzzle velocity is the same as what’s published, is it reasonable to think that the barrel pressure will correspond to the published value, regardless of what our powder charge weight is?

    For example, say I’m loading for my 30-06. The IMR manual says that with 165 grain Sierra SPBT and charge of 51.2 grains of IMR 4320, they get 2869 feet/sec, which will result in 58,600 psi. Now lets say, if for me to get 2869 ft/sec in my gun, I must use 53 grains with IMR 4320 and the same bullet, will the barrel pressure still be about 58,600 psi ?

    Also, if so, now because we are talking about muzzle velocity, would a different bullet of the same weight, say a Hornady 165 grain SST at the same velocity (2869 ft/sec) and using the same type powder (IMR 4320), produce about the same pressure (58,600 psi) ?
     
  2. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Blackfly,

    You've already touched on the answer here, but in short, no, you're talking about an entirely different set of circumstances and variables. Even if you're using the "same" components, they're from a different lot than those used in the manual. This can mean some substantial differences when speaking of powders, and can vary substantially when we're talking about different lots of the same primer. Cases vary from one run to another, even within the same make of brass. Add to that the individual history of the brass, i.e. how it's prepped, its hardness, the neck tension exerted on the bullet, etc.. Add to that the fact that bullets may vary as much as .0005" or so from one run to another with many manufacturers, and there may even be differences in jacket thicknesses or core hardness (alloy). I've seen all these factors in play during various production runs over the years, and can assure you that they do exist. And finally, we have the rifles themselves. Even if you were to use the very same ammo, actually loaded by the lab that did the manual, it's going to show different results when used in another firearm. Different throating, variable in chamber dimensions, bore/groove diameter variations, and even bore finish are all going to play a role in the final results.

    You already touched on the right answer here; the guns vary considertably, as do the components, even when we attempt to perfectly match what's in the manuals. They are a guide, nothing more, and need to be regarded as such. Personally, I've always thought it best to view any reloading manual as a simple report, essentially saying, "we tired this particular combination in our rifle, and this is the result we observed." Follow that up with the ever-present caution that "your mileage WILL vary" and you're a long way down the road in understanding how to read a manual.

    Hope that helps,
     

  3. blackfly

    blackfly Member

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    Thanks Kevin. As you can see I'm new to the site here and I figured if anyone would or could give me a reasonable answer it would be here.I don't ever see much info about pressure other than what to look for and this was something I've woundered about
     
  4. Bart B

    Bart B Well-Known Member

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    One other reason muzzle velocities are all over the place for a given load and rifle is the shooter. I've seen as much as 60 fps difference between my shooting and a friend's shooting the same rifle and ammo with the rifle resting on bags atop a bench. Others have reported about 100 fps difference. It depends on how hard one holds the rifle against their shoulder. Shooting prone usually puts bullets out faster than shooting offhand/standing.