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Thread: Case filling ratio

  1. #1

    Case filling ratio

    Just re-reading a chapter in RA Rinkers 'Understanding Firearms Ballistics' and he talks about how the fuller a case is, the better and more consistently it will perform. this is because there is less trapped air/oxygen, and the propellant obviously has its own oxygen built into the granules, so excess will reduce efficiency.

    I was then starting to think that maybe I shouldn't be using Varget for my .243, even though accuracy is great, as the case is hardly 2/3rds full. maybe I should be using a faster burning propellant that fills the case more??

    anybody else an equally big loser to start thinking about these things?

  2. #2
    dont think you want a faster burning powder for 243 certainly not for the heavier bullet weights.

  3. #3
    I believe this is why short fat cases are popular ie 6mmbr and short magnum cases..

  4. #4
    Quote Originally Posted by PKL View Post
    Just re-reading a chapter in RA Rinkers 'Understanding Firearms Ballistics' and he talks about how the fuller a case is, the better and more consistently it will perform. this is because there is less trapped air/oxygen, and the propellant obviously has its own oxygen built into the granules, so excess will reduce efficiency.

    I was then starting to think that maybe I shouldn't be using Varget for my .243, even though accuracy is great, as the case is hardly 2/3rds full. maybe I should be using a faster burning propellant that fills the case more??

    anybody else an equally big loser to start thinking about these things?

    Ahhhh actually it's a slower burning powder that fills a case more.................................

    Howve the old adage comes to mind:-

    If it's not broke ................................ don't fix it.

  5. #5
    BH, of course, slower burning,,DOH! must've been that pack of Carlsbergs - LOL..

    Anyway, one of my concerns with a 'less' full case, is that what if on a shot the propellant has not had a chance to full 'back' fully against the primer, but is distributed more towards the neck of the case, 'could' that in theory create a different ignition and propulsion rate of the bullet than otherwise normally experienced?

    I'm asking this because I took some shots off sticks where the rifle was pointing downwards, ie. the propellant would naturally fall towards the neck of the case,,and the shots were falling lower than normal...so my concern was whether it 'could' be for these reasons, or simply due to other variables...

  6. #6
    Quote Originally Posted by PKL View Post
    BH, of course, slower burning,,DOH! must've been that pack of Carlsbergs - LOL..

    Anyway, one of my concerns with a 'less' full case, is that what if on a shot the propellant has not had a chance to full 'back' fully against the primer, but is distributed more towards the neck of the case, 'could' that in theory create a different ignition and propulsion rate of the bullet than otherwise normally experienced?

    I'm asking this because I took some shots off sticks where the rifle was pointing downwards, ie. the propellant would naturally fall towards the neck of the case,,and the shots were falling lower than normal...so my concern was whether it 'could' be for these reasons, or simply due to other variables...
    Your shots either uphill or downhill WILL HAVE slightly different point of impact due the effects of gravity. It's no longer puling the bullet directly downwards but acting at a tangent to the axis. If you fire directly upwards the force of gravity is working solely to slow the bullet down and not trying to make it deviate off it course of flight. Long range "snipers" have tables and even little electronic gizmos which give the difference in MOA and there are even some scopes with such gizmos built in for uphill and downhill shots. I believe this covers it:-

    http://www.chuckhawks.com/shooting_uphill.htm

    Hope that helps.

  7. #7
    Cheers BH,

    This was only at 60yds, and in theory, with a rifle zeroed at 100yds, the POI should be 'high', esp. shooting downhill..that's what surprised me to be honest. it could of course, just be the rifle acting differently from the sticks than off the bipod/bag/sandbags/etc. but I'd be surprised if that was the case. During this test I was shooting into a 15-20 mph wind directly, you don't think that would knock the bullet 1" down at such short range?

  8. #8
    Are you sure this is correct?. Suggest you check ther POI at closer ranges before anything else.

    I think this is a very complicated area and might be advised not to worry.

    D

  9. #9
    I guess I can try some uphill shots, and if POI at 60 yds off the sticks is on target or high, then there's a chance that without driver error, the non-full case is the culprit - ie. at uphil, the propellant would be forced back to the head of the case, so ignition and combustion would perform equivalently to when zeroing was done....I guess this gives me something to do this weekend -

  10. #10
    I'll bet a pound to a penny that any major POI change @ 60m uphill or downhill off of sticks will be down to - I won't call it 'operator error' - let's say variation in shooting position and differing stresses being imposed on the body. IMO changes in internal ballistics of the .243 due to powder shifting will be negligible and external ballistics marginal @ 60m unless very extreme angles are involved. I think the Hornady manual, (might be one of the others I have), gives charts for uphill/downhill shooting but I'm not sure if it even goes down to below 100yds.
    Last edited by Orion; 14-07-2011 at 12:30.

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