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Why do heavier bullets require LESS powder?

Discussion in 'Ammunition & Reloading' started by zippygaloo, Jun 30, 2012.

  1. zippygaloo

    zippygaloo Oregon Member

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    Can someone explain to me why larger (heavier) bullet weights require less powder vs more powder? Shouldn't you have to put more powder behind a larger (heavier) bullet to get it to move?
     
  2. deadeye

    deadeye Albany,OR. Moderator Staff Member

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    The physics of loading cartridges indicates that a heavier bullet will build pressures faster than a lighter bullet owing to its mass. The greater mass of the heavier bullet resists change (acceleration) more than a lighter mass so the powder charges for the heavier bullet will nearly always be lower than those for the lighter bullet of the same construction.
     
  3. Longshot34

    Longshot34 Moses Lake Member

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    Think of it like throwing a whiffle ball. Takes a lot more umph to throw a whiffle ball 30 yards than it does to throw a baseball 30 yards.
     
  4. jake2far

    jake2far Portland Bronze Supporter Bronze Supporter

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    Physics.
    A body at rest will stay at rest unless an outside force is placed upon it.
    Case volumn is fixed, bullet sitting controls volumn, larger bullets lower volumn.
    Peak pressure is fixed at a top value, larger bullets resist moving so using the same powder needs less powder to stay inside safe fixed pressure limits.
    If you use a larger bullet general rule, you use a slower burn rate powder so the larger bullet can be pushed to its maximum velocity within safe pressures, moving to a slower powder also generally allows more powder to be used, assuming it will fit inside the case.
    Expansion volumn ratio based on a fixed top end pressure is what controls the powder speed and weight.

    Jim
     
  5. Spitpatch

    Spitpatch Forest Grove, Oregon Well-Known Member

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    Yeah. What he said. Or, in layman's terms:

    Put a jelly bean in your mouth, inhale, then project it out of your mouth as fast as you can with air pressure.

    Now put a hardboiled egg in your mouth. If you use the same amount of force to try to project it out of your mouth, you might blow your eyeballs out of their sockets.

    If ya don't get it through that blue-bonneted head of yours, boy, you'll wish you'd numbered your feathers for just such an occasion.
     
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  6. Longshot34

    Longshot34 Moses Lake Member

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    You make it sound so complicated.
     
  7. FarmerTed1971

    FarmerTed1971 Portland, Oregon, United States Well-Known Member

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    This thread is full of win! LOL
     
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  8. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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    And if you're not careful, your eye socket won't be the only orifice something might "blow out of":cool:
     
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  9. Varmit

    Varmit Beaverton, OR Member

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    If you wanted the heavier bullet to travel at the same speed as the lighter one, you would need more powder. Unfortunately the limiting factor is how much pressure your gun can take, as described above, the added resistance of the heavier bullet would increase chamber pressures beyond dangerous levels. Reading several reloading manuals completely is the first step in reloading.
     
  10. jquirit

    jquirit Forest Grove, OR Member

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    Reading this thread, the thought is right but the explanation is flawed.

    It's not that the bullet is heavier, it's that heavier bullets tend to be larger. Larger bullets means it needs to be seated further into the case (due to max OALs for the cartridge design), thus resulting in lower case volume. This lower case volume means two things:

    * less powder that can be put in the case (physically, unless you want to go into compressed loads)
    * smaller initial volume in which ignition can occur in

    Sure, you could try to use the same amount of powder from a lighter load in a heavier load but when considering those two above factors it causes one thing: significantly higher chamber pressures. Igniting the same amount of powder in a much smaller volume results in much higher pressures that eventually lead to (potentially) catastrophic results. That's why most SAAMI specs also list a maximum chamber pressure.

    In layman's terms, and the analogy I've seen used and I've always liked is light a firecracker in your hand. One time, with your hand open, and another time with your hand squeezing it tightly. Which will hurt more? The open hand instance is with a smaller/lighter bullet with a larger chamber volume, and the closed hand is the larger/heavier bullet in a smaller chamber volume. Both have the same amount of powder, but depending on the space provided, the explosive reaction needs to go somewhere. That somewhere is your hand, or in the case of the firearm, the receiver. The receiver can only take so much pressure until it fails.

    So when reloading specs are done, it's a balancing act (for the sake of this discussion) between the factors of case volume, powder burn rate (which we haven't discussed, but also why you see powder recommendations change depending on the weight/design of the bullet as there are tricks to using a slower burning powder so you don't see that huge pressure spike), and maximum case pressures to insure that there is some safety margin in which the firearm doesn't turn into an IED.
     
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  11. Spitpatch

    Spitpatch Forest Grove, Oregon Well-Known Member

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    I'd easily challenge this premise, jquirt, by noting that if I load heavier bullets (and am allowed by my action/throat) to the IDENTICAL depth in the case as a lighter bullet, I better darned well decrease my powder charge REGARDLESS. The factor is mass of the projectile.

    Seat a 110 grain Spitzer into a .30-06, with a max charge of powder type that might be also applicable to a heavier bullet. Before you seat a 220g Roundnose to the SAME DEPTH in the case, with the same powder, you better reduce the powder charge...

    Or, maybe you keep your feathers numbered for just such an occasion.
     
  12. jquirit

    jquirit Forest Grove, OR Member

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    Spitpatch, I am not recommending reloading above the design specifications as you're trying to use to prove your point. I too am saying that the powder used should be lower as bullet weight increases, but what I'm trying to illustrate is that simply by saying mass alone determines powder charge is over-simplification (flawed is probably too harsh of a word).

    One could simplify it down to "more mass = less powder" but the specifics behind it I clearly explained as a product of the dynamics within the cartridge. If you want to say it that way, that's fine. You have many years of reloading experience to back up your understanding of it, but someone who's new and does not have that basis of experience, what I explained provides them with a good understanding of what's happening inside of the case when you fire it and what happens when you step outside of those boundaries.
     
  13. zippygaloo

    zippygaloo Oregon Member

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    All of this information is great and collectively helps me better understand. Can someone add some information about slow vs fast powders?
     
  14. Spitpatch

    Spitpatch Forest Grove, Oregon Well-Known Member

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    GENERALLY: A faster powder may be more suitable to a lighter bullet in the same cartridge. GENERALLY: A slower burning powder may be better for a heavier bullet.

    GENERALLY: A faster powder may be better suited to a smaller case. GENERALLY: A slower powder may be better suited for a larger case.

    There are overlaps and exceptions (and complete contradictions) to be found to these GENERAL statements, but an attentive overview to any loading manual, accompanied by the knowledge that MOST loading manuals list powders in order, fast to slow (or better yet if the reader holds a "powder burn rate chart" in his hand), will soon deliver to the student a pattern emerging that GENERALLY conforms to these statements.
     
  15. Page.k

    Page.k Seattle Active Member

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    I think if you did that you'll not like the out come. If the round is too long the PSI in side the case will be too great. Have you ever read any reloading books? With out seating the 220gr deeper in the case the full round will not fully enter the chamber unless the head space is off.
    What it comes down to is the case size and tip in most cases.
     
  16. Page.k

    Page.k Seattle Active Member

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  17. Spitpatch

    Spitpatch Forest Grove, Oregon Well-Known Member

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    Oh, please.

    I am more willing to learn than most people I meet, profess my ignorance regularly in order to gain what I lack, but I get off the boat when discussion turns to condescension.

    Of one thing I am certain: rare (or nonexistant) is the cartridge that headspaces on the bullet.

    And that lilttle gem I picked up when I began reloading in 1968.

    Somebody please step in here. I've had my fill.
     
  18. Spitpatch

    Spitpatch Forest Grove, Oregon Well-Known Member

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    Since a picture is worth a thousand words (for some, at least), I offer this:

    Today's project was wringing out a new .270 for a 13year-old boy in Montana who has chosen to upgrade from a .243. This makes it very convenient (with all the bench dedicated already) to illustrate my previous point. (My deepest apologies that I did not dig out the .30-06 stuff, but the concept is precisely the same).

    On the left you have a heavy roundnose bullet (150g). On the right you have a light hollowpoint (90g: sorry it's not a spitzer). Both are seated to the precisely identical depth in the neck.

    Unless I am prepared to exercise Foghorn Leghorn's preparatory system for reassembly of my body, I CANNOT charge the case on the right with a near-maximum load for the 90g bullet, and put the same amount of the same powder into the case behind the 150g bullet on the left.

    Oh. Another marvelous discovery: Since the .270 is not one of those very special cartridges that headspace on the bullet, both these cartridges chamber in each of three .270's currently at the house.

    No charge for this service. Drive Safely.

    P7010119.jpg
     
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  19. jake2far

    jake2far Portland Bronze Supporter Bronze Supporter

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    This is wrong, longer and larger bullets offer greater mass, and a larger bearing surface, they also can include a different angle to the ogive, the increased bearing surface cause much higher friction, this friction and mass results in much higher resistance to movement. You are also forgeting the coatings on powders react to higher pressures, the flame front gets driven into the powder faster as the pressure increases.
    All you have to do is study reloading manuals and you will find that as a bullet weight increases the powder charge decreases.
    Your open hand closed hand example is a laugh, all bullets are fired with a closed hand. How on earth can a bullet be fired in an open hand? All cartridges are fired contained, a larger or smaller bullet is still in the same closed hand, or chamber, in fact if fired in the same gun it is an identical closed hand.
    Here, try this, seat your bullet out till it touches the lands, tell me it doesn't increase the pressure, in fact it can cause dangerous pressures, and this is with the same bullet and a larger case volume, all that was done was an increase in resistance to movement of the same weight bullet.
     
  20. saxon

    saxon springfield Active Member

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    think of it this way when it comes to powder and bullet weight
    a 100 lb man pushing a cart that weighs 100 lb will need X amount of ummph to get cart to move
    if the 100 lb man needs to push a cart that is 500 lb if he used x amount of ummph to try and move the cart
    he would hurt himself by using to much presure to move heavy weight to fast
    so if he used y amount of umph one m less and he took just that one m longer to get the same pressure
    the 500 lbs would then start to move auntill it reached the same as the 100lb cart