So maybe I’m an idiot. I was out firing a Colt AR-15 HBAR with a Trijicon ACOG scope. I’d gone the extra step and drilled through the A2 carry handle on this otherwise pristine Colt so as to add the second mounting screw for the scope. The BDC (Bullet Drop Compensating) reticle has different crosshairs for elevation at different ranges (wind is of course still up to your doping skills). You zero at, say, 100 using the main crosshair, and your elevation is supposed to be correct at all the other indicated distances. One comment on that; it would be much better to refine your zero at greater distances, using that other crosshair, say, at 500 using the number 5 crosshair or etc.
Out in the real world though, your targets aren’t placed at nice, even, measured distances, so it gets just a little bit more complicated. I’d brought a laser with me to do range measurements. The laser registered a particular target at 385 yards. Said right there, so it couldn’t be wrong, “385 yd”. That’s close enough to 400 that I opt for the number 4 crosshair. Shot went high. “Not possible– I called that shot dead on.” Same thing again. Walking the shots onto the target, I find I have to hold halfway between the number 3 and 4 crosshairs*. “Crap. This shouldn’t be happening. I have nigh on three grand worth of equipment in top condition, the right ammo, and a standard length barrel. What the hell?”
Some of you will already have figured out the problem (I seem to recall something about an interplanetary probe oblitorating itself on Mars due to a similar error). The ACOG scope is calibrated in meters and the laser was set to display in yards. A yard is 0.9144 meters. In realistic rifle shooting distances, we can simplify that to either adding or subtracting 10% to do the conversion in our heads, and be close enough. At 385 yards I was rounding up to 400, which made sense, but I was still thinking all in yards. I didn’t convert. 385 – 10% (simplify further and subtract 38) = about 347 meters, or close enough to the 350 meter crosshair for this target. *Ah Hah!
Better yet would have been to take all of half a minute (only because I don’t mess with the settings much and I’d have had to take that long to figure it out) to set the laser to read in meters.
On a nice, relaxing day with a full belly and a Thermos-full of hot coffee (as backup this time) the sun shining and the birds chirping among the beautiful North Idaho scenery, this was more of an amusing lesson than anything serious. If there is ever a situation in which it really matters, you’ll want to be aware of these things in advance, and have taken the necessary steps already.
Part of my problem is that I fool around with so many different weapon systems, in addition to being an idiot. How does that saying go? “Beware the man with only one gun.” Something like that. He knows his weapon backwards and forwards, right and left, upside down and every which way, in the dark, summer and winter, and with one hand tied behind his back just to make if fair he’ll still kick your ass. Hmm. Maybe there’s a new IPSC stage in there somewhere.
Update: With the low recoil of the 5.56 round and a low power optic, you can usually spot your own hits even at longer distances. Take that for what it’s worth.