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how does bullet shape affect pressure?

Discussion in 'Handloading and Reloading' started by chache, May 20, 2010.

  1. chache

    chache Active Member

    I had a discussion with a fellow reloader who told me that the shape of a bullet directly arffects the pressure in a gun. He said that regardless of the seating depth, some bullets crewate more pressure than others. The bullet we were talking about was a barnes ttsx. Can anyone explain this to me, because he couldn't?
  2. 243winxb

    243winxb Well-Known Member

    When a bullet takes the rifling, its compressed by the lands/rifling. A soft lead core will compress very easy. A solid copper bullet or even Nosler's Partition bullet will increase pressure more than a normal pure lead core jacketed bullet.Shape> Then you have the bearing surface of the bullet that is in contact with the rifling. More bearing surface, more pressure, more friction, more heat.
    Last edited: May 21, 2010
  3. tehweej

    tehweej Well-Known Member

    I would imagine that the bearing surface of the bullet (the part that actually touches the barrel) would affect pressures (greater bearing surface, greater friction resistance, and vice versa), but I don't think that would be as big of a factor as bullet weight. Maybe somebody else knows more?
  4. 243winxb

    243winxb Well-Known Member

  5. ants

    ants Well-Known Member

    Chache didn't say Material, he said Shape.

    The gunpowder provides finite pressure. Everything else is a matter of whether that pressure is contained for useful work, or is wasted through leakage and inefficiency.

    Experts on this forum have mentioned that small amounts of gas sometimes leak by a boat tail bullet where a square base tends to seal more positively. Boat tail has its advantage during flight, where a square base SP or HP is generally more efficient. I am not sure if this will affect pressure very much. One of our experts may join the thread and help us out.

    Long bullets with fat ogives that catch the rifling sooner can truly affect pressure. If I'm not mistaken, Hornady has a section on Internal Ballistics that helps explain this.

    What other ideas do we have?
  6. Oyeboten

    Oyeboten Well-Known Member

    I do not see how shape of Bullet would effect Pressure behind or against the rear of the Bullet in interior Ballistics, other than, as you mention ants, that a Boat Tail ( or a Round Ball ) may allow a little more early gas to blow by, and, a flat base would not be imagined to do so as much...upsetting being left aside.

    I doubt very many people shoot Boat Tails in Pistols, unless an old Remington XP or some other relatively exotic ones, chambering what are Rifle Cartridges, anyway.

    The stories of the early WALKERS 'blowing up', because of people loading Conicals upside-down, did not make any sense to be.

    Round Balls have about as much Aerodynamics, as the Cone of a Conical, if they were imaging the Gass to be deflected against the Bore because of the shape? makes no sense, regardless.

    The pressure of the expanding Gasses behind the Bullet, will not be any different because of what the shape of Bullet, or the rear of the Bullet is...the shape, whatever it is, gets pushed against the same as if it were merely a cross section of the Bore...however many parts of a sqaure inch that is as a bore section, is what is being pushed against by the PSI of the Gasses.
  7. ants

    ants Well-Known Member

    The term is "Bevelled Base" when used in handguns. Look for the initials BB in the bullet description. Most common in cast lead bullets. Found not only on commercial cast bullets, but molds from Lyman, RCBS, etc.
  8. Steve C

    Steve C Well-Known Member

    To be more accurate the gunpowder provides a finite amount of gas depending upon the mass of the powder that's generated as it burns. The rate of gas generation is the major factor in determining what the peak pressure will be. The pressure in ammunition is a dynamic that involves the rate the gas is generated and the volume it is contained in over time. Remember that the powder is generating gas during combustion but the volume of its containment is expanding as the bullet moves down the barrel.

    Anything that slows the bullets movement down the barrel decreases the volume the gas expands into over time which increases pressure over time. If the bullet shape increases bearing surface and resistance in the barrel it slows the rate that the bullet moves and thus increases peak pressure.
  9. chache

    chache Active Member

    thanks everybody...its starting to make sense now.

    the discussion i was having was in regard to a fellow shooters custom loads he recieved after sending his gun to corbon. in 20 rounds, the majority had flattened primers and a few even blew the primers out. he noticed it when he almost couldn't open his bolt.

    my first reaction was that the loads were too hot, because after looking at the bullet (barnes ttsx) i noticed it was seated with only one partial cannelure showing which told me it wasn't seated too long.

    that was when my fellow reloader acquaintance told me that he needed to switch bullets because that bullet created too much pressure in his gun. my shooter friend said that was the bullet he liked and wanted to stay with the barnes. so i told him to have corbon work up a new load that wasn't that hot.

    this was when the arguement began....as to which i didn't know much about the bullet shape. my first thoughts were, why switch bullets when you can probably solve the problem with reducing the amount of powder.

    maybe i'm wrong though.
  10. Jesse Heywood

    Jesse Heywood Well-Known Member

    One factor is the relationship between the bullet and bore size. A bullet too large for the bore will have higher pressure than one that fits the bore. Also, the distance between the bullet and the lands. If the bullet is too close, pressure will increase. I would suggest having the chamber cast to get the, but I didn't stay at a Holiday Inn last night, so you should rely on the opinion of someone who knows.
  11. 1SOW

    1SOW Well-Known Member

    243 Win XB @ Ants: +2

    All are jacketed, .355 dia, 124gr, 9mm:

    1. MG 'CMJ (copper base)'--4.3 grs to get 129 power factor--base doesn't expand

    2. PD 'FMJ' (lead base slightly recessed)- 3.8 grs to get 129 PF --base expands more

    3. MG 'JHP' (copper base) --3.8 grs to get 130+PF--base doesn't expand, but the short nose leaves more sidewall for rifling contact---seals better

    Barnes (I believe) offers a clad 124gr 'hollow base bullet' that's sort of a reversed hollow point, and I'll bet it expands fast and would require less powder for the same speed, but I haven't tried it.

    The type of bullet/shape does change the pressure build-up rate and bullet speed.
    Last edited: May 21, 2010
  12. Oyeboten

    Oyeboten Well-Known Member

    Hi chache,

    I will guess your friend's Rifle Ammunition issue, as you described, is more likely about other factors, than the shape of the Bullet.

    Headspace, weight of charge for the Powder kind with regard to the volume of the Cartridge, a clean or dirty Breech, too light a pressure being developed by the Powder/loading for Bullet kind or weight...Bullet too small for the Bore...Cartridge being driven forward by the firing pin, expanding and gripping there, then being driven back hard with a primer sticking out a little from the inside cartridge pressure before cartridge is driven back...not being driven back enough to re-seat how much the primer is pushed out from pressure in the Cartridge...all sorts of things could be going on.

    I am not in the front of the class on this, so, just speaking from the back row, anyway.
  13. Ol` Joe

    Ol` Joe Well-Known Member

    The shape affects how much bearing surface the bullet has on the bore and the friction of this is a big factor in pressures. It also will control how close to the lands the ogive is and a bullet touching the lands or in some instances very close to them will cause spikes.
    Jacket construction, core hardness, etc, are other factors but are not shape related.
  14. ranger335v

    ranger335v Well-Known Member

    "why switch bullets when you can probably solve the problem with reducing the amount of powder. maybe i'm wrong though."

    No, you are correct.

    Bullet contact surface obviously increases with the contact area, jacket and core composition and tightness of fit. BUT - by the time a bullet has traveled it's own length in the bore its diameter matches the bore, the tightness of the fit has reached neutral and from there on the original friction is of little difference. Bullets easily travel their own length prior to peak pressure so concerns about bullet shape (contact area) raising pressures significantly are largely irrelivant.

    If a load is producing excess pressure either reduce the charge or change the powder.

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