Seating Depth / OAL

Discussion in 'The Ammo & Reloading Forum' started by Airborne1101, Dec 10, 2009.

  1. Airborne1101

    Airborne1101 New Member

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    I have a question and I know this is the place to ask it. I am extremely careful when loading, I research multiple manuals, start loads well below max shown and work up or down in some cases to achieve the most accurate best handling, or most effective load for that particular caliber or weapon. My question is this is there a formula for predicting the effects of pressure based on the seating depth or OAL of a particular bullet type or weight? put simply how much increase in pressure or decrease in pressure am i going to get by missing a seating depth by say 0.1" up or down?

    AIRBORNE......All the Way!!
  2. Bindernut

    Bindernut Well-Known Member

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    It'll pretty much be dependent on each rifle...and also each brand/type of bullet, internal volume of each brand/lot of brass, the type/lot of powder you're currently using, etc, etc.

    How the chamber is cut (tight, loose, short, deep), how long of a jump til the bullet engages the rifling (Length of rifling leade) and how much bullet is stuffed inside the case at a given COAL (which will affect the internal volume of the case), powder burn rate isn't a linear curve, etc.

    All of these factors are a variable in one big complex equation and I don't think there is any truly accurate way to calculate all of those out on paper. And to to any figuring on those variables, you'd need the full array of tools used by the manufacturers when developing data...pressure measuring equipment, chronograph, the complete data for the powder's burn rate at a given volume/weight charge/compression factor/temperature/etc. All of that attached to your particular rifle instead of the test piece used when the maker worked up the load data.
    Thus the standard-issue disclaimer to always start at the minimum load and work up...and do the same whenever you change lots of powder. brass, etc. and the same reason that each maker can be using the same powder, bullet, etc and still get different results.

    It can be kinda frustrating, but also kinda fun.
  3. Gearheadpyro

    Gearheadpyro New Member

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    Unless you have an extremely nice custom rifle .001" will not make nearly any difference at all. My quality standards are my given C.O.L. +/- .001" and I routinely shoot sub .5". I've even seen on berger's website that the sweet spot is usually .015" long, although that will vary with the gun.

    I agree with bindernut on pretty much everything else. There are just too many other factors to accurately predict pressure for your load in your gun. Do a good load development, and go with what you find. I wrote an article on my website detailing how I do load development, you can find it here.
  4. LDBennett

    LDBennett Well-Known Member

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    Airborne1101:

    If you develop a load at the listed COL (cartridge overall length) and you don't exceed the max load recipe and the fired cases show no signs of excessive pressure, you have a base line. If you seat the bullet to a COL greater than the COL of the base line, the pressure will go down. The pressure will go up if you seat the bullet deeper (less COL). In the latter case if the baseline load is near or at maximum you may generate pressures that will show signs of excess and that, of course, should be avoided.

    So briefly, differences in COL that make the cartridge longer are no problem whereas making the COL shorter may be and will require you to develop the load all over again using the shorter cartridge COL.

    But there is a problem with longer COL. The result may not fit into the gun's magazine. Often a compromise in the COL is necessary to allow use of the cartridge in the magazine of the gun.

    In general, making the COL greater is used to move the bullet closer to the start of the rifling in the barrel. Some place the bullet so it barely touches the rifling or at least a few thousandths of an inch off that point. The bullet should never have to be forced into the lands of the rifling when the bolt closes. That's because the pressures will skyrocket while the stalled bullet overcomes the huge friction. And because if you decide to remove the unfired cartridge, the lands may pull the bullet out of the case dumping all the powder into the chamber and the action, leaving the bullet in the barrel and the now empty case in your hand. Tough to make a followup shot while hunting when that happens.

    For my rifle reloads I almost always make the bullet be a few thousandths of an inch off the lands. Finding that point is not as easy as the tool manufacturers make you believe with their tools designed to measure that point. I do this to maximize the accuracy but sometimes I have to break that rule to be able to fit the cartridges into the magazine. I have been known to modify the magazines to accept longer cartridges, if possible, to be able to put the bullet "on the lands". But this technique means ammo developed this way is for this gun and this gun only, and should not be used in any other gun.

    LDBennett
  5. steve4102

    steve4102 Former Guest

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    I assume you are refering to bottleneck rifle loads? Straight wall Pistols are a different animal when it comes to COAL.

    Do not go below Min published charge. This can be just as dangerous as going above Max. There is a Min for a reason. Some powders like H 4895 work well with reduced loads, but there is data to support this.

    This is not necessarily true. There is countless studies that show in a bottle neck rifle cartridge pressures usually go down when the bullet is seated deaper, not up. Here is one such study by Barnes. Scroll down to "From the Lab"
    http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

    Here is what Accurate Powders says about COAL.
    SPECIAL NOTE ON CARTRIDGE OVERALL LENGTH “COL”
    It is important to note that the SAAMI “COL” values are for the firearms and ammunition manufacturers industry and must
    be seen as a guideline only.
    The individual reloader is free to adjust this dimension to suit their particular firearm-component-weapon combination.
    This parameter is determined by various dimensions such as 1) magazine length (space), 2) freebore-lead dimensions of
    the barrel, 3) ogive or profile of the projectile and 4) position of cannelure or crimp groove.
  6. LDBennett

    LDBennett Well-Known Member

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    steve4102:

    That is an interesting article. But I take from it that the pressure change based on COL is rather random. If that be the case and it can go up or down independent of the COL then a prudent reloader should develop the loads from a starting load point with every change is bullet seating depth (COL).

    It goes to show that reloading is not so much scientific as it is trial and error. The variables are many and not totally predictable, it appears. Hopefully the error will not end up in a catastrophe and damage to the gun or the shooter. I have been successful with the concept I presented (but that success today is not an indication of the future ???) and will continue to use it, but now with caution.

    Thanks for the information.

    LDBennett
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