Does pointing your gun up or down affect its accuracy? The answer is “maybe” according to a recent US Army Armament Development, Research and Engineering Center (ARDEC) presentation released at the 2017 National Defense Industry Association small arms conference. The presentation covers a study conducted by ARDEC which examined the effect of ullage (free space in a loaded round) and orientation on the consistency and velocity of small arms ammunition.

A slide from the ARDEC presentation showing x-ray images of .50 caliber ammunition in different orientations. The red arrows point to the free space in the round; note how the position of the free space changes depending on the round’s orientation.

The findings of the study may not be exceptionally surprising – it turns out that rounds with more ullage are less consistent – but they are remarkable in how detailed they are. The ARDEC team testing followed standard EPVAT procedure, accounting for pressure, velocity, and and action time (the time between ignition of the primer and the bullet leaving the barrel):

A slide from the ARDEC presentation, showing the test data for 5.56mm M855 ball and M856 tracer ammunition. Note the low difference between results at different orientations; this is due to the fact that both M855 and M856 have very low ullage.