BDC stands for bullet drop compensation. BDC reticles include hash marks or dots that correspond to a bullet’s drop at a known distance. It’s a neat reticle concept that I got excited about when I first learned about these types of reticles for a number of different reasons. The primary reason for my excitement was the idea that it would reduce the mental gymnastics of converting the targets distance to the correct MOA or MRAD holdover (or turret adjustment) making engaging multiple targets and different distances faster. Sounds too good to be true right?
As it turns out the promise of rapid target engagement is true, but it comes with a lot of fine print. Consider the Vortex AR-BDC3 reticle (pictured above) for a minute. The major hashmarks on the center post have been calibrated for 300, 400, 500, and 600 yards specifically for 5.56mm (or .223) cartridges with projectiles weighing between 55 to 77 grains with a muzzle velocity of 2700 to 3000 FPS. The dots correspond to the windage corrections at the corresponding distances for the same projectiles when the crosswind measures 5, 10, or 15 MPH. Still sounds promising right? I think so.
So, why don’t I like BDC reticles? Let’s look at some external ballistics which I think will help illustrate my reasons.
|600 Yards||2700 FPS||3000 FPS||Difference|
|15 MPH Wind Drift||106.82″||92.18″||14.64″|
I’ll be the first to admit that there may be something wrong with my assumptions and logic here. In fact, I wouldn’t be surprised to be corrected by a much more experienced precision rifle shooter. However, the drop and drift differences seem to indicate a pretty significant margin of error. That margin of error is almost the same size as an official IDPA target which is much larger than the size of a vital zone target (and that’s not even accounting for the combined rifle and ammo margin of error). In essence, the promise of rapid target engagement is starting to seem like it comes at the cost of precision to me especially at longer distances.
Here is another way to look at the cost of precision. Each hash mark on the reticle, has a corresponding MOA subtension. This means the hash mark is equivalent to a specific MOA hold over and the reticle has made an assumption of what the MOA come up should be for the rifle and ammo combination at that distance. The problem is that there will be a difference between the assumed hold over and the actual hold over needed for the rifle and ammo combination. The next table highlights this difference for projectile drop.
|Distance||Reticle MOA Subtension||Actual MOA Drop||Difference|
|300 yards||2.4||3.27||2.61″ Low|
|400 yards||5.6||7.65||8.2″ Low|
|500 yards||9.5||13.28||18.9″ Low|
|600 yards||14.6||20.45||35.1″ Low|
Based on the differences in the table above, we can see the difference under 300 yards doesn’t make much of a difference on a high thoracic vital zone hit which is a target area of about 8 inches. However, it could mean missing on a smaller target area (like the head box) depending on the precision of the shooter, rifle, and ammunition combination. At 400 yards, the difference is large enough that it means likely missing any vital zone target without a hold outside the actual vital zone target area. Beyond 500 yards, the difference is so large that, in my opinion, makes the BDC reticle useless from a precision shooting perspective, at least with this particular rifle and ammo combination.
At one point as I was developing my opinion about BDC reticles, I thought maybe it wouldn’t be so bad if I worked backwards from the known external ballistics of my ammunition and rifle combination to determine a more accurate distance approximation for the hash harks. This can be done by looking up the reticle subtensions and corresponding distances on a ballistics chart. The following table enumerates what the distances would look like for the Vortex AR-BDC3 reticle.
|Hash||MOA Subtension||2700 FPS||3000 FPS|
|3||2.4||277 Yards||296 Yards|
|4||5.6||357 Yards||392 Yards|
|5||9.5||436 Yards||485 Yards|
|6||14.6||520 Yards||580 Yards|
While I could do the same thing for the windage dots, I’m not going to because it just makes things messier. The point of the BDC reticle, in my opinion, is to allow faster target engagement by reducing mental gymnastics. I’m not willing to trade precision for speed since I firmly believe that a miss is an unacceptable hit. In order to regain that precision I would have to keep several distance and wind speed numbers in my head that aren’t that easy to remember. Not to mention that targets between those distances would require quite a bit of slow educated guessing before having enough confidence to pull the trigger and making a good hit.
The only way I can see a BDC reticle actually allowing fast target engagement without sacrificing precision would be to find the right rifle and ammunition combination where the external ballistics line up with the reticle subtensions. In other words, if the ballistic data shows that the come up for the projectile is 2.4 MOA at 300 yards, 5.6 MOA at 400 yards, 9.5 MOA at 500 yards, and 14.6 MOA at 600 yards. I just don’t know an easy way for the average shooter to accomplish this.
I was able to generate a ballistic chart that gets pretty close to this using the data for a cartridge using a 68 grain projectile with 0.355 G1 BC and an initial 2760 FPS muzzle velocity. This means that it might be possible to find the right combination of a factory load and correct rifle barrel length, but that requires having the means to try different barrel lengths. It might also be possible to develop a hand load to get really close ballistic performance of a projectile out of a rifle, but this approach requires the know-how and equipment required to develop custom hand loads. Either way, this last table shows just how close the right ammunition load and rifle can get to a BDC reticle.
|Distance||Reticle MOA Subtension||Actual MOA Drop||Difference|
|300 yards||2.4||2.62||0.66″ Low|
|400 yards||5.6||5.92||1.28″ Low|
|500 yards||9.5||9.89||1.95″ Low|
|600 yards||14.6||14.6||0″ Low|
Again, I may be completely wrong in my understanding and assumptions about BDC reticles. As such, anyone reading this should take everything that I’ve said here with a grain of salt. If you are reading this and happen to know that I am wrong, then please correct me.