I recently purchased a bolt-action rifle chambered in .243 Winchester. Knowing that .308 Winchester brass could be die formed to 243, I bypassed buying any brass as I have plenty of 308 brass on hand.
While studying my various manuals and referring to my QuickLOAD program to decide on a powder and charge to start with in the 243, it occurred to me to determine the difference of the suggested loads between 243 and 308. I cannot think of a useable application of the knowledge from this investigation, but it interested me, so I set out to find some answers.
As 308 cases can be made into 243 cases with no more modifications than simply reshaping the case (I.E., no change of length), the volume of the two cases must be very close, if any difference at all. From that, I concluded the case volume would have no bearing on the chamber pressure differences.
I decided to use my QuickLOAD program to run some calculations, so I started with bullet choices. .243″ diameter bullets are available in weights ranging from 55 grains to 107 grains. The lightest weight bullet I could find in .308 caliber is 85 grains, the next up being 110 grains. I chose 85 grains as the bullet weight as both bullets need to be the same weight for an ‘apples to apples’ comparison.
There are myriad powders that can be used in both calibers. Other than I have IMR 3031 on hand and am familiar with it, I have no reason to choose it over any other.
Using QuickLOAD, I determined that 43.2 grains of 3031 is a 99.9% charge density in 308. Applying that charge to 243, the program showed that charge to be 98.0%. I’m assuming the difference is in bullet seating depth, the seating depth for 308 being a bit more than for 243. But, again for the sake of comparison, I chose to keep the same charge for both calibers rather than increase the charge in 243 to match charge density in the 308.
All my load manuals indicate large rifle primers for both calibers. QuickLOAD does not incorporate primer brisance differences, so there’s really nothing to separate between the calibers with respect to primers.
While entering data in QuickLOAD, I was careful to enter the same barrel length in both calculations.
So, we have two different calibers with cases almost identical in volume, the same bullet weight, the same powder, the same charge and the same ignition brisance.
The only thing I know of at this point that remains different (and unchangeable) is bore diameter and area. This is where the pool goes from three feet to eight feet.
The bore diameter of the 308 is 1.267 times larger in diameter than the 243. The cross-sectional area of the 308 is 1.606 times larger than the 243. Let’s approach this from the opposite direction. The bore diameter of the 243 is 85% of the diameter of the 308. The cross-sectional area of the 243 is only 62% of that of the 308.
I spent 30 years as an industrial pipefitter. I learned that twice the pipe diameter does NOT deliver twice the flow in volume, it delivers FOUR times the flow, assuming the pressure is the same. Inversely, 1/2 the diameter only delivers 1/4 the volume.
Let’s apply that to gas flow through rifle barrels. 1/2 of .308 is .154″. If the volume of flow is X @ .308″ diameter, the flow will be 25% of X @ .154″ diameter. That means the flow will be reduced by four percent for every one percent of diameter reduction.
.308/100 = .00308
243 is 79% of .308
100 – 79 = 21
21 X 4 = 84
So, the gas flow in a .243 barrel is 84% less than the gas flow in a .308 barrel. That explains why you can’t use 308 powder charge values in a 243.
Wanna’ see some more convincing numbers? Let’s compare chamber pressures.
85 gr. bullet
43.2 grs. IMR 3031
99.9% charge density
85 gr. bullet
43.2 grs. IMR 3031 (same charge as 243)
98% charge density
Oddly enough, the velocity of the 243 load only went up 9%. But the chamber pressure of the 243 load went up 248%!
In order to get the chamber pressure of the 243 down to the 308 chamber pressure, the load would have to be reduced to 30.8 grains, a 29% reduction. That would result in an estimated velocity of 2555, a 23% reduction.
So, what have I learned from all this? All components and values being equal, reducing the bore diameter drives the chamber pressure through the roof exponentially, not equally.