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Thread: Scary variation in 308 case volumes!!

  1. #1

    Scary variation in 308 case volumes!!

    To put you in the picture I have a standard .308 load that is what does the business for me, 44gr of vit 140 with 150gr bullet. My 2 choice bullets are sst for sika or smaller and b'tip for reds. I've just got some gmx which is a new bullet for me but I'm testing it out to see how it performs because the FC is going lead free next year!

    The gmx being gilding metal has more volume built into it's length to hold the weight so it seats deeper than the sst to get the profile right. That's where my problem and investigation started. I started loading with lapua brass and right away it was obvious that 44g of powder was going to leave me with compressed loads. So I started checking through my brass by individual case weight to see what was lighter and so had more volume. The best brass type I found that was good to load the gmx was winchester. Here are the weights by case:

    Winchester - 160gr
    RP - 166gr
    RWS - 168gr
    PPU - 168gr
    FC - 175gr
    HPS-TR - 175gr
    NRA - 175gr
    SAKO - 175gr
    LAPUA - 175gr

    15gr of brass worked out to be a difference of about an extra 1.6gr of powder space in vit140. The Winchester had the volume to get the round right without fannying about. If that not a touch worrying!

  2. #2
    If I've got this right you are trying to load the same powder amount in a smaller volume case , Very dangerous my friend .
    You could use the Lapua brass and develop your load again with the new heads , and you will find that the smaller amount of powder in the smaller volume case will be in ratio to the larger case volume needing the larger amount of powder .
    All different brands of case's should be treated in there own right and loads should not be assumed to be transferred due to different case volume's ,

    ATB

  3. #3
    If you think that is bad try 7.62x39, there is about 2.7 grains of powder variation in capacity from Lapua, the smallest to IMI the largest.

    Neil.

  4. #4
    2.7gr is a lot of variation!

  5. #5
    I get about 6 gr variation in my Remington 260 Rem brass. It weighs from about 158.8 gr to 164.5 gr. I try and group it into 1 gr lots. I work up a load for each brand of brass that I use. Regards JCS

  6. #6
    Hmm, you've obviously been being lazy/lucky with your nine different cases but using just one "standard" load.

    Despite the title, what you haven't done is actually measure the volumes.

    Weigh empty case, then fill up with water to top of neck, then weigh again. You might be surprised.

    Weight of water is an exact measure of case volume.

    Weight of the cases themselves is indicative, but brass does vary in density, and details like the head design can affect weight but not volume and vice versa.

  7. #7
    +1 Sharpie youv'e nailed it there mate.

    Usually you will find that Winchester cases will have one of the greatest case volumes and Lapua one of the smallest. The same load in the Lapua case as the Winchester will usually run at much higher pressures and usually higher velocity.

    Incidentally what are the cases with the NRA headstamp, they aren't RWS (RUAG) from the NRA are they?
    It's the calibre of the shooter that counts not the calibre of the rifle.

  8. #8
    Density of brass can vary from: http://www.engineeringtoolbox.com/me...ties-d_50.html
    Brass - rolled and drawn 8430 - 8730
    Brass 60/40 8520

    The 160-175 grain variation is outside this.

    Assuming nominal 8520 (kg/m3) density, 15 grains i.e. 1 gram more brass should reduce volume by 1/8.52 i.e. 0.117 cc.

    N140 has a VMD of 13.63 according to lee so 0.117 cc reduction = 1.6 grains less room for powder. Which seems remarkably close to paul at barony's findings. That said, I would not be worried about 1.6/44 = 3.6% powder compression.

  9. #9
    I shoot a lot of military brass which is exceptionally heavy and transferring commercial loads to the military cases can be a chore. Try this:

    FL resize cleaned cases each LOT of cases and trim to identical length. Weigh the cases of each LOT. To account for the thicker brass/ decreased capacity, reduce the established load by twelve percent of the difference in case weight. Takes the guessing out of it.

    You must start with cases that are clean and outwardly as close to identical as possible.~Muir

  10. #10
    Quote Originally Posted by Sharpie View Post
    Hmm, you've obviously been being lazy/lucky with your nine different cases but using just one "standard" load.

    Despite the title, what you haven't done is actually measure the volumes.

    Weigh empty case, then fill up with water to top of neck, then weigh again. You might be surprised.

    Weight of water is an exact measure of case volume.

    Weight of the cases themselves is indicative, but brass does vary in density, and details like the head design can affect weight but not volume and vice versa.
    and if you want to be really anal use a spent primer in reverse in the primer pocket (correct way round gives tiny volume for water but not consistently enough as air space will hold water out in some but not others)

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