Hey Kirby

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by 1doug, Mar 6, 2005.

  1. 1doug

    1doug Well-Known Member

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    Daveosk had a good question about over bore, what is in your opinion a good ratio of case length to bore diameter and where is the line drawn. Like why people dont neckdown a 50bmg to a 338-50 bmg? just a few thoughts. Thanks in advance
     
  2. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    D-a,

    Well, actually, the 50 has been necked down to the 338, its called the 338 Talbot. Practical, not really but impressive looking.

    Seriously though, its a fine balance I believe. Not only do you need to be aware of case length but also diameter compared to bore diameter, more specifically bore volume. Also bullet weight is a major player in this game.

    My 257 Allen Magnum is what most would call severely over bored and with 100 gr pills I may agree but when you combine this +100 gr case capacity with a 257 bullet the weight of a 130 gr or heavier, things begin to behave a little different.

    My personal opinion is that a round is not over bored in my opinion until the velocity and accuracy results become erratic with the bullet and powder being used. Lets look at the 257 Allen Magnum again.

    With a 100 gr Wildcat Bonded Core FBHP, we can get a solid 4100 fps and groups generally run in the .3"s at 100 yards and in sub 3" groups at 500 yards in the 30" #6 Lilja barrels.

    Now velocity spread with AA8700, the only commercial powder suitable for this round, will average an extreme spread of 50 to 70 fps. This is the reason you get the group variation at 500 yards. But because it is so fast in velocity, this is also the reason you only have this amount of group variation at 500 yards.

    If you had this with a round that produced 2800 fps youwould get more vertical variation at 500 yards, with 4100 fps it is usible and actually holds pretty tight groups still.

    Personally, I feel this combo of bullet and case capacity is over bored because the velocity consistancy suffers.

    Drop a 130 gr pill in with AA8700 and your extreme spreads will drop to the 30 fps range instantly with only the bullet changed. THis is simply because the heavier bullet in the same bore size offers more resistance to the huge powder charge for a more uniform burn from shot to shot.

    As a result groups at 100 yards average the same as the 100gr pill but at 500 yards average group size will dip just under 2".

    Finally if we take the huge 156 gr ULD Wildcat bullet which is simply an awsome bullet in the 257 Allen Mag again using AA8700, we will see the velocity spread drop into the teens which again helps grouping consistancy at long range.

    So with a 100 gr pill I would agree the 257 Allen is vastly overbored but with the 130 to 156 gr pills, it is actually a very consistant round, in fact as consistant as the 7mm and 300 RUM at least, probably even more so with the steeper shoulder angle.

    As far as where I draw the line in case length and bore diameter, its hard to say. Again, it is my opinion that if you can get consistant velocities out of a give round. No matter what round or case volume it is, it is a practical round.

    Now the topic of over bore rounds effecting throat life is a totally different beast to hunt. Personally, I like performance, I could care less how many grains of powder I have to burn to get to a certain level of performance as long as I get there safely first, consistantly and accurately.

    Good Shooting!!

    Kirby Allen(50)
     

  3. 1doug

    1doug Well-Known Member

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    Just wanted to say thanks again. You seem to be the go to man here.
     
  4. ricka0

    ricka0 Well-Known Member

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    The right way to do it is to use a Sabot and shoot a 338 in a 50 BMG ( .509 ) - you only get about 3 MOA but if they could figure out how to make it fly straight the physics is on your side. For a given pressure the Acceleration is directly proportional to the square of the caliber but the drag once you leave the barrel is proportional to the cube of the caliber. That's why the Abrams tanks is such an effective Kinetic Energy Tank Killer ( Depleted Uranium is twice as dense as lead and it uses a Sabot) ALS has a vendor building Sabots to shoot 338's out of a 50 BMG - if they could get them down to 1 MOA it would be the ultimate long range gun.

    Image shooting one Richards Wildcat 350 gr. .95 BC .338 at 4,500 FPS. It might just punch thru the gong at 2,500 yards (if you could hit it).

    Notice the Sabot comming apart in the pic
    [​IMG]

    From
    consider a couple of incidents out of Desert Storm. On one occasion, an M1 fired its 120mm gun at an Iraqi T-72 (Soviet built) tank. The 120mm Armor-Piercing (SABOT/uranium-depleted steel dart) round went right through the T-72 and into another one directly behind it, destroying both tanks.
    Another more amazing M1 story happened during the 24th Mechanized Infantry Division’s run to the Euphrates River. It was raining heavily, and one M1 managed to get stuck in a mud hole and could not be extracted. With the rest of their unit moving on, the crew of the stuck tank waited for recovery vehicles to pull them out.

    Suddenly, as they were waiting, three Iraqi T-72 tanks came over a hill and charged the mud-bogged tank. One T-72 fired a High-Explosive Anti Tank (HEAT) round that hit the frontal turret armor of the M1, but did no damage. At this point, the crew of the M1, though still stuck, fired a 120mm SABOT round at the attacking tank. The round penetrated the T-72’s turret, blowing it off into the air. By this time, the second T-72 also fired a HEAT round at the M1. That also hit the front of the turret, and did no damage. The M1 immediately destroyed this T-72 with another SABOT round. After that, the third and now last T-72 fired a 125mm Armor-Piercing round at the M1 from a range of 400 meters. This only grooved the front armor plate. At that point, realizing that continued action did not have much of a future, the crew of the last T-72 decided to run for cover. Spying a nearby sand berm, the Iraqis darted behind it, thinking they would be safe there. Back in the M1, the crew saw through their Thermal Imaging Sight (TIS) the hot plume of the T-72’s engine exhaust spewing up from behind the berm. Aiming carefully through their TIS, the M1’s crew fired a third 120mm SABOT round through the berm, into the tank, destroying it.

    DESERT STORM
     
  5. Pete Lincoln

    Pete Lincoln Well-Known Member

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    Big Bore, i always said a tank on top of a Scottish Hillside would be the ultimate in long range deer hunting..
    Best group i ever fired was from a Challenger 120mm canon.
    and the butts or berm on the tank ranges are a hell of a depth to be able to catch the DU APFSDS rounds. awesom !
    Pete
     
  6. abinok

    abinok Writers Guild

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    I had the opportunity to hold in my hand a round from RBCD ammo that is a sabot 50BMG. Its a 30 Cal 200gr bullet that their factory info states does 5900fps. Liquid propellant, no powder.
    Its my understanding that there are some of these in the magazines of a few canadian snipers over in the sandbox, according to the company rep anyway...