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Thread: 338 & 7mm

  1. #1
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    Default 338 & 7mm

    How come you can get 3000 fps for 180 grain 338 win mag and for the 7mm rem mag only gets 2800 fps for a 175 grain bullet
    i checked out the reloading manual and the 338 shell is a 7mm necked up
    so how come you cant get the same speed for the 7mm 175 grainer in factory amunition

  2. #2

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    Quote Originally Posted by fish slayer View Post
    How come you can get 3000 fps for 180 grain 338 win mag and for the 7mm rem mag only gets 2800 fps for a 175 grain bullet
    i checked out the reloading manual and the 338 shell is a 7mm necked up
    so how come you cant get the same speed for the 7mm 175 grainer in factory amunition
    Because the larger bore has more bullet base to push on,therefore uses the pressure generated by the powder more efficiently than the small bore. If that was not the case we would all be shooting 200 gr at 3000 in are 22-300 WBYs[that would be cool]

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    Member Diesel Nut's Avatar
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    Quote Originally Posted by jim in anchorage View Post
    Because the larger bore has more bullet base to push on,therefore uses the pressure generated by the powder more efficiently than the small bore. If that was not the case we would all be shooting 200 gr at 3000 in are 22-300 WBYs[that would be cool]
    Also take into account the larger expansion ratio. A .338 bore is much larger than a .284 bore, which allows a quicker burning powder to be used without spiking pressure. An equal amount of a quicker powder (generally) produces higher velocity as max pressure is attained quicker.

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    Member hodgeman's Avatar
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    You may also consider the bearing surface. A heavy for caliber buller (175gr 7mm) has more bearing surface than a light for caliber bullet(180gr .338).

    Increased bearing surface equals more friction and generally less velocity. It's just one variable among several however.

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    Member big_dog60's Avatar
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    You said factory ammunition. There are many variables. are the ammunitions using the same type and amount of powder? What about differences in bullet construction?

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    Member Eastwoods's Avatar
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    Quote Originally Posted by Diesel Nut View Post
    Also take into account the larger expansion ratio. A .338 bore is much larger than a .284 bore, which allows a quicker burning powder to be used without spiking pressure. An equal amount of a quicker powder (generally) produces higher velocity as max pressure is attained quicker.
    Well put.

    You can also think about the "larger bore" as "more volume" than the thinner bore. Almost like having a longer barrel. ie. a longer barrel gives more velocity because more space ( or volume) to burn powder. A bigger bore gives more space or volume to burn powder like a longer barrel does.

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