I took my trusty Remington 710 .30-06 to the range yesterday for some practice and “just me” time. Before I left, I bought a fresh box of ammo, but unfortunately I forgot to check the grain before buying it. (“Grain” is the measure of weight used for bullets. Heavier bullets have higher grain.) Generally, you can get 150gr or 180gr bullets for the .30-06. The 180gr bullets are good for larger prey like elk, while the 150gr bullets are better for smaller prey like deer and antelope. It’s trendy to hunt elk, but … there are various issues I won’t go into here. Bottom line, they’re more difficult to hunt, so I usually just hunt deer.
Because I hunt deer, my rifle’s sighted in for 150gr ammo, but I headed to the range with 180gr ammo yesterday out of carelessness. My scope is specifically adjusted to shoot 2″ high at 100 yards, and roughly right on at 200. To my chagrin yesterday, I shot right on at 100, and about 6″ low at 200. I was really, really puzzled by this until I thought to double-check the grain of my newly-acquired ammo. 180gr. Shit. Well that explains everything. Heavier bullets fall faster than lighter bullets.
As soon as that thought occurred to me, I heard (I swear!) Galileo laughing at me. Excluding air resistance, all objects fall at the same rate. The difference is in the rising part of the trajectory. Bullets don’t travel flat. Right out of the muzzle, they jump up just a bit because of the fresh kick from the exploding powder. I think that both the 150gr and 180gr Federal bullets that I buy contain the same amount of powder (chemical potential energy). The 180gr bullets don’t jump up as high as the 150gr bullets because they’re heavier. Both bullets fall the same amount over their trajectory, but the 150gr bullets start from just a bit higher up. Thus I shot low at both 100 and 200 yards.
That Galileo was one sweet smart motherfucker. And me? I just don’t always pay attention.
Update: A buddy of mine has pointed out that a 180gr bullet takes longer to reach the target than does a 150gr bullet, because of a slightly lower muzzle velocity owing to a heavier projectile. He’s absolutely right, and I missed that aspect of the problem completely. The rise difference is slight, and probably inconsequential. See Federal’s own data for the 150gr and 180gr bullets I was using if you’re interested.
Fuggin Bizzy 