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Thread: ballistics

  1. #1

    ballistics

    Maybe someone could explain in simple terms why a 308 with a 26 inch barrel can launch a 155gr bullet in excess of 2900fps but a 260 rem load data can only launch a 140 grainer at 2600fps ish,its the same case necked down

  2. #2
    Youch! Internal ballisitcs....
    I've no idea, but if it's true then I suspect it concerns maximum pressures and the base surface-area of the bullets.

  3. #3
    My guess would be that the 140 grain bullet has a proportionally greater area of bullet in contact with the bore than the shorter and fatter 308 bullet. The two cartridges use different burning rate powders with the 308 generally using faster burn rates than the 260.

  4. #4
    Expansion ratios.

    The volume under a 308 bullet as it travels up the barrel gets bigger exponentially faster than say a 6 or 6.5mm.

    Basically allows you to use a much faster powder in the 308 compared to the 260.
    Brian.

    Just because you are paranoid, doesn't mean they aren't out to get you......

  5. #5
    The area of a .308 bullet is about 0.0745 sq inches.
    The area of a .264 bullet is about 0.05474 sq inches. So the force of the expanding gas is applied to more area.

    Let's take a round number, maximum pressure, 60,000 lbs / sq inch.dd
    Applied to the base of the .308 = 4,470 lbs of peak force
    Applied to the base of the .264 = 3,289 lbs of peak force

    Now, the force is not continuous, of course. It rises and then decreases as the bullet travels down the bore, increasing the volume of gas.
    To get the exact amount of force, you have to integrate the area under the curves, etc.

    But to keep things simple, the volume in the 22-inch barrel of a .308 = the 29.9 inches of barrel for the .264.
    The .264 needs a longer barrel and more time to accelerate its bullet. It needs a slower burning powder.

    As to the bullets, the 155-gr Palma bullet in the .308 has a short linear bearing surface, and less friction in 1:10 or 1:12 rifling, than the long shank 140-gr bullet in the tighter twist of the .264.

    The same is true of a 208-gr AMAX compared to a 180-gr RN bullet in a .308, which is why, with the right slower powder, the 208 can be driven as fast as the 180-gr.

  6. #6
    As Brian says its mostly to do with expansion ratios. However, if the 260 also had a 26" barrel and was operating at same pressure as the 308 then I'm sure it would launch a 140g bullet considerably faster than 2600 ft/sec.

  7. #7
    Thank you chaps, that was a great explanaion southern, are you brian litz , hi andy, ive fellow shooter in my club using 260s and they run theirs at 2600fps, it maybe an accuracy thing, its poss they have pushed them faster maybe getting pressure signs or poor accuracy, atb swaro

  8. #8
    Great explanation Southern - thank for your post.

  9. #9
    Quote Originally Posted by swarovski View Post
    Maybe someone could explain in simple terms why a 308 with a 26 inch barrel can launch a 155gr bullet in excess of 2900fps but a 260 rem load data can only launch a 140 grainer at 2600fps ish,its the same case necked down
    A quicker answer would be that the sectional density (weight/calibre) of the 2 bullets quoted by the OP isn’t the same. If it was the speed achievable in different calibres using the same cartridge case would be the same.

    I’ve added the sectional density (SD) to each matching combo in brackets.

    .264” 140gr ( SD = .289)
    .284” 160gr ( SD = .286)
    .308” 190gr ( SD = .286)

    Fired in the appropriate calibre rifle i.e. from a .260 Rem – a 7mm-08 Rem – and a .308 Win respectively each of these will be going at the same speed of 2600 fps or so - if pressure & barrel length are equal.

    With a bullet of matching sectional density you could also use the same propellant powder in each calibre. Not ideal maybe but perfectly workable.

    The result would be the same with a .243” using a 120gr bullet (SD = .290) if only you could find such a bullet weight in 6mm.
    If I'm going to be accused of it then it's just as well I did it.

  10. #10
    The 140 Amax being a Smaller bullet but in terms of external ballistics the .260 Rem will out shoot the 308 with most bullets in terms of wind drift.

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