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Ammo "set back" question

Discussion in 'Caliber Corner' started by ghr1142, Nov 9, 2011.

  1. ghr1142


    May 26, 2011
    Hello to all,
    I;v been reading lot's threads on this subject on GT, And have a question . I shoot 3 calibers . 9mm. 40 S&W, 357 sig.
    I read about "SET BACK" on ammo right out of the box ! How can I find out the tolerance on this issue ? I can measure to .005 OAL.
    My concern is that major set back can cause major pressure problems !
    I don't re load so any help would be helpfull,

    G-26 G-27 G-19 G-33
  2. Dexters


    May 3, 2004
    If you don't reload compare the round you are concerned about with the others you bought of the same mfg.

    If the round is off turn it in to the range officer to be disposed of properly.

    Don't be concerned with the OAL in this situation - it isn't worth the consideration.
    Last edited: Nov 9, 2011

  3. ghr1142


    May 26, 2011
    Thank's for the info. but, If the OAL is not important ! how do you check for set back ?
    I read that this is frequent in ammo right out of the box. I guess i'm looking for a +/- tolerance , Maybe a reload manual would spec. that ??
  4. Zombie Steve

    Zombie Steve Decap Pin Killa

    May 31, 2007
    Old Colorado City
    In reloading, you'll quickly find that variations in bullets will give each cartridge a different overall length. Less with flat nose bullets seated with a flat stem, more with soft point rifle bullets where the seater stem is pushing on the ogive.

    On the rounds you shoot, I'd say plus or minus 2 or 3 thousandths would be the norm. If you don't have a set of calipers, line them up on a flat surface. If one looks weird / noticeably short, stay on the side of caution and don't shoot it.
  5. ghr1142


    May 26, 2011
    Zombie Steve, Thank's for the info. I'll try that with my next ammo buy..
    I was not aware that a couple of thousedths could make that much of a problem ? Boy I guess when you roll your own ! one has to be very tolerance minded .
    How much "set back" would cause these problems must be enough to 100% inspect each load ..
  6. Zombie Steve

    Zombie Steve Decap Pin Killa

    May 31, 2007
    Old Colorado City
    A couple of thousandths probably won't matter much at all. Like I said, some variation is normal.

    It's a function of how fast the powder they use burns and how much volume there is in the case. On a 9mm, there's isn't a lot of room in the case to begin with, so the setback issue is more important than on, let's say, a .45 Colt that was designed for black powder and has a bunch of room. Usually factory rounds will use slower burning powders to achieve max velocities, so there's a bit of wiggle room there...

    FWIW, I've had a load for an M1A that ran really well then switched brass to Lake City (thicker / less room in the case) and had signs of overpressure.

    An example of the overall length / bullet seating depth, I've loaded my cast 230 grain lead tc in my .45 auto and tested it at 1.210" OAL. It averaged 832 feet per second. The same load seated out further / 1.230" averaged 784 fps. Just an example - there's no rule for this. .45 auto is a really low pressure cartridge to start with, and I'm using Unique... a medium-slow burning powder. It would be more dramatic with a 9mm using a fast burning powder.

    So all in all, there are a lot of factors involved. If it sets back a little, I'd probably still shoot it. If it's easy to see compared to the others in the batch, I'd probably pull it.
  7. Tiro Fijo

    Tiro Fijo

    May 31, 2011
    Lube, load & forget. Don't do what many knuckleads do & monkey with the gun. Treat every gun as if it were loaded. If you need to unchamber then mark the primer after unchambering and don't use that round again as your chamber round.

    If you want to know how much a round has set back then measure OAL BEFORE you chamber it and later when you unchamber.

    I unchamber whatever semi-auto I am using once a month to clean out dust bunnies & relube. I then rechamber ANOTHER round.

    Don't over complicate what is merely common sense. ROTATE the chambered round when you unchamber or buy a revolver.
  8. dkf


    Aug 6, 2010
    So if the OAL isn't a consideration how exactly do you measure the amount of bullet setback then? The OAL is in fact important.

    Ammo with bullet setback is not worthless and still be shot or used for components. You can either tap the bullet out further (too the proper OAL) with an intertia bullet puller and shoot it or pull the bullet and use the components.

    If you don't have one pick up a precision vernier, dial or digital caliper. The cheaper ones will work I just don't recommend plastic calipers though. Its a tool that can last a lifetime if taken care of so I only buy the good ones.

    Using the calipers measure a new full box of ammo and note the OAL of the rounds. A .357sig Speer Gold Dot measures 1.125" if memory serves me and they usually are within + or - .001". So if I measure one of those GDs that is supposed to measure 1.125" and it only measures 1.119" you know you have some setback. Bullet setback is especially critical on rounds with high case pressures like the .357sig. The .357sig is especially bad because of the small neck and the fact that you can't put a crimp at the base of the bullet to prevent setback. Once I get more than .002"-.003" setback with my .357sig carry ammo the round gets set aside.

    Below is a chart that shows what can happen to internal pressures with different amounts of setback. I posted it a few times before but I think its informative and gets the point across.
    Last edited: Nov 9, 2011