I have never been a fan of bullet drop compensating (BDC) scope sight reticles, and therefore have had no experience with using them. Recently, however, I had the opportunity. My experience may be something that many people would describe as, “Everyone knows that,” but perhaps if nothing else this discussion will be a reminder, if not any sort of revelation.

A friend recently acquired an ACOG sight with 5.56mm BDC reticle, and asked for assistance in getting it zeroed. I was able to determine that for the drop compensating stadia lines to be accurate at longer distances, the sight needed to be zeroed at 100 meters (not yards). That we did with a 55 grain M193-type FMJ load. I then recommended trying the BDC feature at 200 and 500 yards where I’d positioned steel targets, a 7×12 inch much-reduced IPSC silhouette and a more or less full-sized head and torso target at the longer range.

Using the 200 meter stadia for aiming resulted in a consistent series of hits on the small 200 yard plate, so on to the 500 yard silhouette: Not so good.

Using the 500 meter stadia line on the 500 yard target resulted in shots not just hitting high, but impacting high on the berm behind. So, why’s that?

Now you’re saying, “The reason was obvious; 500 yards isn’t 500 meters.” Well, I knew that, but didn’t anticipate that the difference in trajectories would be so different. After all, 223/5.56 trajectories are really flat, no?

Plus there was another factor that I considered but had also just dismissed as not being that important.

In discussing the experience with another friend, he mentioned our altitude at 10,000+ feet above sea level which was the other factor. I still thought it couldn’t have made that much difference, but his question prompted me to actually research the question and not continue making assumptions.

So, although altitude differences won’t matter in any significant way to the vast majority of shooters, it was a large part of why we didn’t get hits at 500 yards using the 500 meter line. Just using the horizontal drop values for M193 ammunition at 500 yards the difference between my elevation and sea level was about 12 inches: –68 inches versus –80". And, assuming I have the ACOG drop calibration correct, using the 500 meter stadia line raised the point of aim about 103 inches, or 35(!) inches more than necessary.

What’s more significant for most shooters, though, was the fact that we were trying to hit a 500 yard target with a 500 meter aiming aid. 500 meters is about 550 yards, and that 50 yard difference mattered much more than I anticipated.

If we fire a M193 bullet at 3040 fps muzzle velocity horizontally with respect to the earth under sea level “standard” atmospheric conditions, it will drop about 0.5 inch in the first 50 yards to a target. Under the same conditions, however, in the 50 yards from 500 to 550 yards, it will drop over 22(!) inches. That means if we use the 500 meter stadia line in the BDC reticle for a 500 yard target, it will be aiming about 22 inches high, and more than enough to cause a miss of my steel silhouette.

The point of all this which is obvious in retrospect (even to me) is that although BDC reticles and other preset aiming aids such as calibrated riflescope elevation dials can be comforting because they reduce the amount of thinking involved when engaging long range targets, they must match not only the dope of the specific ammunition being used, but also the shooting conditions, especially the distance to the target. Whether a deer is at 200 or 220 yards with something like a Hornady 168 grain bullet and the difference in drop is a couple of inches, that probably won’t matter in most situations. If, however, we make a range estimation error of 50 yards between 500 and 550 yards and the drop difference is 18 inches, that’s not even a close miss.

And as a last point about BDC reticles, even when everything is Goldilocks perfect, what if the target is at 450 yards rather than 400 or 500? Hint: The aiming hold is not half way between the two marks.

Anyway, this isn’t about what’s best for engaging long range targets. That’s for the individual to decide, but is just another example of how it’s never too late to learn (and truly understand) something new. In this case it was to truly understand (and confirm) the ballistics of your aiming aids, and don’t just assume that those reticle marks are going to be accurate for every situation.

Note that the above discussion relied upon some inexact figures, so your calculations may not yield the same results, but I’m confident they’re close for my purposes.

This is a whole area of knowledge I aspired to know more about, but right now knoe so little. I really appreciate the insights here.

If you are shooting M193 out of a 16" gas gun, you probably aren't actually getting 3040 FPS out of it either....


Perhaps, but probably not.

I like a BDC for quick and dirty firing solutions. They can work very well, but they are far from goof-proof.


Also, not all BDCs are created equal. The good ones are cut for real world velocity out of real world guns. The lazy ones are cut for the numbers that ammunition companies put on their ammo boxes. Those numbers come from Universal Receivers and 24+ inch barrels. You will never get those velocities out of a gas gun.

Re is also the distinct possibility that the acog ( which really didn’t hit mainstream use until long after the M16A2/M4 was adopted) had a bdc reticle calibrated for M855 slower heavier bullets. I bet this could be a significant factor as well

Thanks for sharing. It reminds me that I need to go to the range and gather more DOPE.

BDC reticles work well when:
- The actual bullet's ballistic coefficient and muzzle velocity are very close to the ones used to calibrate the BDC.
- The actual atmospheric conditions are very close to the ones used to calibrate the BDC. I consider this the air Density Altitude.
- The targets are large enough to allow for some aiming errors, distance to target calculation errors, and DOPE errors.

BDC reticles rarely work for me because:
- I use different loads than what they are normally calibrated with. I use heavier match bullets, with better Ballistic Coefficients, with lower Muzzle Velocities.
- I almost always shoot at higher altitudes than what the BDC reticles are calculated for. Meaning that my air Density Altitude is higher.
- I generally demand greater precision than what is afforded by a BDC reticle.
- I generally shoot at smaller targets than what the BDC reticles are designed for.
- I shoot at a boatload more target distances that what the BDC reticles have subtensions for.

quote:

Originally posted by IndianaBoy:
If you are shooting M193 out of a 16" gas gun, you probably aren't actually getting 3040 FPS out of it either....

You are of course right about the ballistic variables that may not be what they’re supposed to be, but FWIW the last time I measured the velocity of M193 from a Colt LE6920 the average of 10 shots was 3039 fps.

But that time I’m not even certain it was actual M193, and may have been someone’s lower velocity 223. A lower velocity, though, would have resulted in the POI also being lower, not higher than expected.