Quick load question

hardpan · Feb 7, 2009

Pop,
In general, does the pressure for a load decrease with lengthening the OAL assuming all the other factors remain the same (amount of powder, primer, bullet, rifle)?

Reason for the question is that i often find a longer OAL increases accuracy.

thanks,
Hardpan

Guest · Feb 7, 2009

OAL is a funny thing regarding pressure. It is such, as it impacts two key components of pressure generation. First, shorter OAL's shrink the internal case volume available for powder expansion by taking up more of it with the heel of the bullet. Longer OAL's can affect pressure by placing the bullet in closer proximity to the lands of the rifling, creating resistance sooner, and thus higher start pressures. In general, most rifle cases are not dramatically affected by deeper seating (shorter OAL's) as the case volume is such that only a small percentage is taken up by the bullet. In pistol cases (especially high performance ones) this effect can be catastrophic - consider the 40S&W as a good example of where seating depth is a key consideration. Longer OAL's can actually create catastrophic pressure spikes if the bullet is seated into the lands, as this creates markedly higher start pressures due to the increased initial resistance. It really all comes down to you knowing your rifle, and setting up seating depth with a careful eye to pressure signs. If you work up a load at a short COL, and then seat out to touch the lands, you'll have much higher pressures, potentially dangerously so. The fix? Just like with any significant change - drop back and work up to a safe max. If you've measured your rifle's throat length and know what OAL has bullets touching the lands, then you'll be fine as long as you stay off of them by a safe margin (like .015" or so). If you're working with one of the monolithic bullets (E-Tip, TSX, GMX, GS Custom) you'll need to heed the manufacturer's instructions as to distance from the lands. I know Barnes recommends a minimum of .050" for their stuff.

DrMike · Feb 7, 2009

In the scenario you envision, pressure will not be significantly affected by OAL, assuming that there is no significant engraving of the lands pre-ignition, that is that the pressure is allowed to move the bullet before encountering resistance from engaging the lands. The pressure measured is a function of volume of gas generated and resistance generated by the bullet as it moves down the barrel. For all practical purposes, if the bullet is a reasonable distance from the lands, the OAL will not be significantly affected by OAL of the bullet.

FOTIS · Feb 7, 2009

Let us see:

Cartridge : .308 Win.
Bullet : .308, 150, Hornady BTSP 3033
Cartridge O.A.L. L6: 2.800 inch or 71.12 mm
Barrel Length : 22.0 inch or 558.8 mm
Powder : Alliant Reloder-17

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms

-02.0 98 49.00 2760 2537 48585 8982 95.6 1.101

Cartridge : .308 Win.
Bullet : .308, 150, Hornady BTSP 3033
Cartridge O.A.L. L6: 2.650 inch or 67.31 mm
Barrel Length : 22.0 inch or 558.8 mm
Powder : Alliant Reloder-17

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms

-02.0 104 49.00 2841 2689 55259 8858 96.6 1.043 ! Near Maximum !