300 grain, .338 Matchking - True BC ??

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Len Backus, Jan 2, 2003.

  1. Len Backus

    Len Backus Administrator Staff Member

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    Let's try to agree on what the real BC is. Tell me how many MOA you come up from 100 yards to be on at 1,000 yards.

    We'll need your MV, temp, elevation and humidity if you have it. Then we'll run the numbers iteratively through Oehler's Ballistic Explorer to get the BC.
     
  2. Guest

    Guest Guest

    Len

    We have tested several bullets to a very fine resolution on b.c.

    My point is that the same gun at different pressures and velocity will shoot the same bullets from the same lot at significantly different b.c's

    The 30 cal. 175 is an example where the same gun fired the same bullets at b.c.'s as low as .472 and as high as .518

    Bullets often vary .018 when fired at near the same velocity. Just look at Brent's picture of his 43 software. The reasons for the variation are several. It is easier to understand if you break the components of drag down into several parts. Frontal drag, rear drag, and frictional drag. Frontal drag exists because of work or energy needed to push an object with frontal area through a fluid, and the higher pressures that result on the Ogive of a bullet.

    Just the variation in the size of the Meplat from lot to lot of the Sierra 300 gr. Mk and even from bullet to bullet much less gun to gun will make nailing this number down with confidence very difficult, and the true answer will change with every load in every gun in every dissimilar wind condition.

    Having said this, I do think that when you get a load you really like, it is very usefull to note the actual b.c. with that load in that gun. Keep in mind that this number will change as the bullet flies, some of this is due to small changes in angle of attack, but I believe most of the change is a result of the boat tails design and how it interacts with he atmosphere at different velocities. Rear drag is mostly made up of vacuum, and I have noticed trends over the years that boat tails of a certain design cause a bullets b.c. to go down as the bullet loses velocity, and other designs to influence a bullets b.c. to go up as the bullet slows.
     

  3. Brent

    Brent Well-Known Member

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    Hi Len,
    I can't remember where at I posted the pic S1 refers to but if you havn't seen it I can post it here in a bit. I'm uploading some pics to ImageStation of our bowlingball cannon shoot and it's taking its time doing it, I'm stuck on the 56k dialup still and this part bites. [​IMG]

    I think the BC is usually over .760 at the range most shoot, many times it has been right up to the high .790s. Might have been warmer then? I do know I've never tested it that it fell below .760 though, at least the average anyway. Most were quite a bit higher than that though.

    I remember some time back Darryl saying he figured them to be running about .797 or therabouts, and Warren telling him they found something different at Yuma during some tests he did, but he wouldn't say what it was. [​IMG] [​IMG] The first batch I tested were almost exactly identical to those of Darryls findings. I never thought too much about it after that. I have never tested his rfle at 1000 yards for the moa needed so this doesn't answer the question I know your really looking for. I've wondered the same thing before starting out with my Ultra.

    Len, keep one thing in mind I've often taken for granted when backing in for the BC, that is multiply the total moa needed by the actual fraction/decimal of MOA the persons scope is really calibrated at.

    Did that make sense?

    Some are really .95 and some are 1.12, or whatever it is, it will skew the BC bigtime either way.

    Either way, I think the ones that shoot 300s at 1000 yards should be able to tell both, at least I hope. [​IMG]
     
  4. Len Backus

    Len Backus Administrator Staff Member

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    Thanks, all. My reason for caring to know is that a new rifle I ordered from Dave Tooley in 338 Yogi will be throated for that bullet. I hate to make a major equipment commitment if the bullet BC is not really as high as it's supposed to be.
     
  5. Brent

    Brent Well-Known Member

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    You will NOT be disapointed about that part Len. I will tell you that when throating for it my brother ran into a problem! The bullets measured .3089", yes almost .309". The problem was the throater was .3080" and they had to open the throat a little at a time then use sandpaper on a deal they use for polishing the chamber, I can't remember what he called it, but it opened it up just enough to keep from pulling the bullets. It was a SLOW, VERY SLOW process they termed... A PAIN IN THE ASS!

    I think this was the first one Dave Caboth had throated for the 300gr SMK, but I think Dave Tooley probably has quite the handle on this with all his experience with them.

    What OAL will it be throated for? The 300s eat up alot of case capacity at mag fed OALs unless you have a long one.
     
  6. Len Backus

    Len Backus Administrator Staff Member

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    We'll be firing single shot so mag length won't matter.
     
  7. ewallace

    ewallace Well-Known Member

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    [​IMG]

    Len
    You will be happy with the Yogi and the 300g SMK I have been shooting it and the BC is .780 to .796s in my rifle.
    Crow Mag
     
  8. Nicholas

    Nicholas Well-Known Member

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    Crow Mag

    How deep in the case is the 300 matchking in that picture?

    I am going to be shooting 300 matchkings at an AOL of 3.625" and I am wondering how much case capacity they are going to eat up in my 338 lapua.
     
  9. Brent

    Brent Well-Known Member

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    Very cool!
     
  10. Steve Shelp

    Steve Shelp Well-Known Member

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    sr90,
    I've looked at that case/bullet combo enough over the last 3+ years to know the junction of the boattail and body are probably right at the junction of the shoulder/neck of the case. If I had to guess I would say in this picture it might be just a little bit below. That's how Dave Tooley throats them usually.
    Remember though this is a blown out case so the neck is approx. .050" shorter (base of the old neck is now in the shoulder) than a std Lapua case like your going to use. So that will work to your advantage some but your still going to have some of that 300gr down inside the case with the 3.625" OAL limitation.

    Rough calculation would be 3.875-3.625=.250" into the case assuming all else is the same. Those 300gr bullet tips vary a lot so the 3.875 measurement will vary from round to round even if your seating depth is constant.

    Hope this helps,
    Steve
     
  11. ewallace

    ewallace Well-Known Member

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    sr90
    Steve is correct the junction of the boattail and body of the bullet is seated a little deeper than the neck-shoulder junction of the case.
    Crow Mag