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  1. #1
    Senior Member Mobusten's Avatar
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    Am I using my chronograph wrong, or are my bullets going super fast?

    I went out shooting today after settling down on a load.. 308 cartridge, 168gr AMAX, 42.5gr Varget. Not necessarily the most accurate load cause I'm pretty new at reloading and long range shooting, but accurate enough to let me actually practice MY part of the equation. I took my pro-chrono out for the first time since it was overcast skies, and put 10 rounds through it, and the results were as follows:

    2737, 2715, 2710, 2710, 2704, 2704, 2693, 2715, 2693, 2710.

    The average of them all is 2709.1. Not sure why the first one was so much faster than the others, I didn't clean the bore since last session other than a loose fitting dry patch to clear out any dust or debris before shooting. But I didn't shoot it much last time anyways.

    My main reason for asking, is my hornady reloading manual says that 44 something grains of varget should get me up to 2600 fps and it stops there because of pressure. So if 42.5 is getting me 2709fps, am I just using the chrono wrong? I don't see any pressure signs on my brass or primers, so I don't think my rifle is going to kerplode.. Is it not so abnormal as I'm making it out to be?

  2. #2
    Canadian ForcesOgre Haywire1's Avatar
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    Ambient temperature, elevation above sea level, barrel length all affect speeds.
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  4. #3
    Go Canucks Go! lone-wolf's Avatar
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    http://www.hodgdonreloading.com/
    For additional data
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  5. #4
    Senior Member 3MTA3's Avatar
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    Type of case and primer is important-as well as firearm and bullet seating depth, the manual usually describes this- I would be verifying my scale probably more than the chronograph.

    Were there any pressure signs?
    "So much of left-wing thought is a kind of playing with fire by people who don't even know that fire is hot.." - George Orwell
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  6. #5
    Moderator kennymo's Avatar
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    Quote Originally Posted by lone-wolf View Post
    ^^ And by this, you're in the ballpark. Different manuals give different results with makers using different rifles, brass and primers for their data. Some also measure pressure slightly differently and may have a slightly different opinion on what is safe. Plus their lawyers may have slightly differing opinions on how far to cut back the loads once they hit the pressure threshold. The manual data is tweaked so it SHOULD be safe in most rifles even with minor tweaks to components (though they recommend replicating their exact test materials). And I say SHOULD. Always a great idea to cut the powder charge back a bit from the maximum listed and work towards it with gently increasing loads, checking for pressure signs along the way.... Any way, you're probably OK if the primers aren't trying to squish out around your firing pin, the cases aren't splitting and your brass isn't getting all suspicious looking around the case head....
    Cleverly disguised as a responsible adult.

  7. #6
    Resident Combine Pilot JustBen's Avatar
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    What make of brass are you using? I've seen higher velocities than predicted when using military brass, specifically.

  8. #7
    Moderator kennymo's Avatar
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    Quote Originally Posted by JustBen View Post
    What make of brass are you using? I've seen higher velocities than predicted when using military brass, specifically.
    Thicker case walls = less volume = more pressure from the same powder charge. The difference between NATO stamped cases and .308 Win/.223 Remington are a prime example.
    Cleverly disguised as a responsible adult.

  9. #8
    Resident Combine Pilot JustBen's Avatar
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    Quote Originally Posted by kennymo View Post
    Thicker case walls = less volume = more pressure from the same powder charge. The difference between NATO stamped cases and .308 Win/.223 Remington are a prime example.
    Exactly what I was referring to. There are even significant variations in case capacity between commercial brass.

    Since the OP said .308 - there's a real possibility that he could have mil spec brass.

  10. #9
    Moderator kennymo's Avatar
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    Quote Originally Posted by JustBen View Post
    Exactly what I was referring to. There are even significant variations in case capacity between commercial brass.

    Since the OP said .308 - there's a real possibility that he could have mil spec brass.
    There's a certain reloader on the other site that really likes his Winchester cases because they're chintzy with the brass..... If his chrony is accurate he's getting some spectacular results with 'highly customized' loads, well above what anyone would dare publish for powder content.... The loads are definitely not suitable for firing in any rifle other than the one they were worked up in though....

  11. #10
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    Quote Originally Posted by Mobusten View Post
    I went out shooting today after settling down on a load.. 308 cartridge, 168gr AMAX, 42.5gr Varget. Not necessarily the most accurate load cause I'm pretty new at reloading and long range shooting, but accurate enough to let me actually practice MY part of the equation. I took my pro-chrono out for the first time since it was overcast skies, and put 10 rounds through it, and the results were as follows:

    2737, 2715, 2710, 2710, 2704, 2704, 2693, 2715, 2693, 2710.

    The average of them all is 2709.1. Not sure why the first one was so much faster than the others, I didn't clean the bore since last session other than a loose fitting dry patch to clear out any dust or debris before shooting. But I didn't shoot it much last time anyways.

    My main reason for asking, is my hornady reloading manual says that 44 something grains of varget should get me up to 2600 fps and it stops there because of pressure. So if 42.5 is getting me 2709fps, am I just using the chrono wrong? I don't see any pressure signs on my brass or primers, so I don't think my rifle is going to kerplode.. Is it not so abnormal as I'm making it out to be?
    ALL reloading manuals as well as the info from sites like Hodgdons reloading website are simply guides, nothing more than approximate load data. there are many factors that influence pressure and velocity. 42.5 grains of Varget is a very conservative load in a bolt rifle especially with a light bullet like the 168 gr.
    Case capacity will vary slightly from casing to casing even in the same lot of brass, it can vary significantly from brand to brand which is why shooting ammo made from a mismash of brands and lots is a BAD idea.
    From barrel to barrel there are slight dimensional and finish differences. A tighter bore diameter will generate higher pressures and velocity than a bore that is .0002 bigger.
    Rougher bores as are the norm in mass produced factory barrels tend to generate lower velocities than the well lapped bores that the better aftermarket barrels have. This is often offset by the bore diameter being tighter.

    Your overall E.S. is only 44 FPS which is not too bad. Depending on how you prep your cases and what you use for scale will also have a large impact on E.S. velocities. With intense case prep ( weighing casings, uniforming them etc.) and a precision scale it is possible to attain single digit E.S.'s

    In my own 308 shooting 190 gr SMKs out of the 21.75" barrel my average velocity is 2690 FPS which IS fast given the bullet weight, yet I get get good casing life and am just on the edge of pressure signs. IF I increase the powder charge by 2/10ths of a grain primers start to flow. My 10 shot E.S. is 4 FPS which is extremely low, but I also spend a ton of time on case prep and my powder charges are weighed to the 1/100th of a grain rather than the 1./10th that is usually considered acceptable.

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