Sniper Central Letter: Ballistic Drop Compensators|
Ballistic Drop Compensators (BDC)
Most all tactical and long range shooters know the procedure of gathering data, logging it into your data book, and then consolidating it into a nice DOPE chart to be used for future engagements. Most of these charts have the MOA or MIL corrections that are needed for dialing in the elevation on a scope to engage a target at each respective distance. These charts then get tucked into shooting pouches, stock packs, taped on the inside of scope caps, or even taped on the side of the butt stocks of rifles. Anything to make them easily accessible while behind the rifle is considered feasible. But what about having that DOPE chart actually in the most useful place possible, on the elevation knob itself? Or what about somewhere even more convenient, making the actual reticle be the chart. That is essentially what a Ballistic Drop Compensator, also known as a BDC for short, does for the shooter. It is a very handy to use DOPE chart, but there are some limitations, as well as advantages, that come with BDCs. That is what this edition of the Sniper Central Letter will address.
Some History about BDCs
A BDC is a means by which a scope will automatically compensate for the drop of the bullet at various distances. Perhaps the most commonly known early BDC is the Auto Ranging Telescope (ART) system designed by James Leatherwood during the Vietnam War for the US Army. Leatherwood came up with a mounting system that itself would raise and lower the physical scope via a cam system as the scope was zoomed in from 3-9x. The operator simply had to zoom in until the hash marks covered a set distance which would estimate the range, but at the same time it would raise, or lower, the rear of the scope allowing it to compensate for the trajectory of the round. The system was purely mechanical and in theory it could work with any scope provided the reticle had the ranging hashes in them. For the US Army M21, it was combined and used with a Redfield 3-9x scope. While the system appears somewhat crude in todays world, it actually functioned well enough for its mission and the M21 served well during that conflict.
There are even modern versions of the Leatherwood ART system on the civilian market today. We at Sniper Central did a review on one of them a while back and you can read it here Leatherwood ART 2.5-10x44mm and while the scope quality itself was lacking, the system did work.
But the ART system is by no means the earliest use of the BDC, in fact, it is far from it. Sniper Central has a British No4Mk1(T) sniper rifle used during WWII and it has an elevation knob that is marked for range. The marked elevation drum is probably the most common style of BDC used on military sniper rifles and they have been in use for many years. If you wanted to get technical, you could even go all the way back to the Vernier style sights that were used on old target and buffalo rifles in the 1800's, as they too could be had marked for various yardages on the sights. These early scopes and open sights with yardage marks on them were likely not known as BDC's back when they were built, but that is exactly what they were. The ZF69 Hensoldt and Kahles scopes also used similar markings on the Steyr SSG69 and other sniper rifles during that era.
In the modern USA era, when the USMC looked to modernize their M40 rifles after the Vietnam War, the scope that they adopted to replace the Refields was a fixed 10x scope made by Unertl and which employed markings on the elevation knob to help shooters compensate for the M118 Special Ball ammunition out to 1000 meters. As was common on these military BDC scopes, instead of there just being marks for each MOA of adjustment, there were marks with a 1, 2, 3, ... 10, which corresponded to that many hundreds of yards or meters. So the adjustments were handled by the elevation adjustments of the scope instead of on the scope mount itself like with the M21 that the Army used.
US Army M3 knobs
When the US Army decided it wanted to get serious about their sniper program in the 1980's, they also adopted a fixed 10x scope with a BDC marked elevation knob that was calibrated for the M118 SB ammunition, but the Army elected to go with the Leupold Mark 4 Ultra scope. Again, in the 2000's, when the Army was deploying a semi-auto rifle once more, the M110 SASS, they chose another Leupold scope, this time a variable power 3.5-10 version and again it was topped with a knob with BDC markings on it, though this time it had BDC marks as well as marks for individual MOA clicks.
Having said all of that, it becomes evident there is certainly a long history of BDC's being used on military sniper rifles, but does that make it the ultimate or best solution? If so, why did the USMC not use BDCs on their most recent Schmidt & Bender/Premier Reticle M8541 SSDS scopes on their M40A5 sniper rifles from just a few years ago? Like most things in life, the answer is not black and white and everything is a compromise. As we dig in a bit deeper into the BDC question, we will need to talk a bit more about the different BDC options that are available.
Reticle vs. Knob
Currently there are two main styles of BDCs on the market today. There are the marked elevation knobs which we have discussed above and which were the traditional BDC, and then there are the ballistic reticles which have become very popular with scope manufacturers in more recent times.
The 223 or 556 knobs have become popular on scopes from Nikon, Burris, Leupold and others offering what they call AR scopes, or scopes designed specifically for the AR-15 platform. There are not a lot of scope manufacturers today that have other BDC knobs as a standard catalog item, but there are a few of them that do a 308 knob. Again Leupold and Nikon are two bigger names that come to mind. But what has become more popular is the ability to have a custom elevation knob with BDC markings made to order. In fact, several scope manufacturers are offering this as a feature, sometimes even for free. Vortex and the Leupold CDS (Custom Dial System) quickly come to mind. This service is often times offered through the scope manufacturers custom shop.
Kenton Industries is an aftermarket company that will make many different knobs for many different scope makes and for any custom load that a customer desires. We have used several of the Kenton knobs and they are high quality and they do a good job, in fact, Zeiss has even offered a coupon to get a set of Kenton knobs for your newly purchased Zeiss scope. So even if your scope manufacturer does not offer a BDC elevation knob for your scope, or you want a custom one made up for your very specific load, there are options available.
Nightforce 300WM A191 BDC (no longer available)
While the BDC knobs are not offered, or are available, from all scope makers, the BDC reticle just about is. The BDC reticles take on many different names, such as Ballistic Reticle, Ballistic Mildot, Accu-range, BDC Reticle, LR Duplex, Velocity 600, etc. The idea behind them all is very similar; the scope designers try to find a reticle that has marks on the vertical stadia that indicate where the user should be holding to effectively engage the target at that given range. Usually the center crosshairs are used for the 100 yard zero and then each hash mark below the horizontal zero is to be used for another 100 yards. There are variations on this theme, such as using the horizontal for bother the 100 and 200 yard zero, or there might be marks above the horizontal that are used for closer in ranges, and other minor differences, but the concepts are the same, there are marks for different ranges. We will discuss the advantages and limitations of this system later in this article, but for now, I just wanted to explain what is available out there.
When to Use a BDC
So when would a sniper or designated marksman want to use a BDC? That is a good question and one that can only ultimately be made by the individual team itself. But there are some things that should be considered. The beauty of the BDC is that they are very simple to use, fast, and they help remove many of the technical aspects of long range shooting when in a high stress environment. It becomes much easier to just move the dial to the "6" when the bullets are flying overhead than it is to look at your DOPE chart and then dial in 17 MOA of up. Simplicity usually wins out when in a combat situation. When a BDC is also combined with an elevation turret that needs fewer clicks, such as the Leupold M3 which itself has 1 MOA clicks, then the adjusting becomes even more simple and rapid. When externally exposed elevation knobs require multiple rotations of the knob to get to all of the distances within the capability of the rifle, then confusion can arise as to whether the shooter is a full rotation off, or not.
So when trying to determine if you and your team would like to adopt the use of a BDC, the advantages seem to lie in scenarios where there is:
- A need for rapid engagements (multiple targets)
- High stress, combat type of engagements
- Simplicity is desired
- Inexperienced operators
The list above seems to tailor itself well to a Designated Marksman role, or a sniper unit with a high operational count, where many mid range engagements may occur. Snipers and DM's within mechanized units would also seem to fit this role well, where they may find themselves doing a lot of overwatching operations or being very mobile and attached to traditional line units.
It does need to be pointed out that although the BDC is there, it by no means takes away from the capability of the weapon system and of course using the knob or reticle in the traditional method, not using the BDC, is perfectly fine and acceptable and in some regards, the BDC can be there purely for a backup or to be used just as reference points with fine tuning being done from there. This is how a majority of the military snipers operate when equipped with BDC's on the sniper rifles.
Burris Ballistic Mildot reticle
When not to Use a BDC
We have talked about when using a BDC might be desirable, but when would an operator not want to use a BDC? I would probably recommend that if the shooter desires the utmost in accuracy and time is not a factor, then a BDC is probably not the best choice. If the shooter has the time to spend examining the situation and then precisely calculate and look up the firing solution either in their log book or using their ballistics calculator on their hand held device, then the BDC simply becomes marks on the scope and are not critical.
Many teams may also memorize their data, or as we indicated earlier, annotate their ballistic data on a card or other chart that is easily referenced. Keeping in mind that the firing solutions change whenever any one of the many variables that effect bullet path change, so even these other methods will not be 100% accurate, in fact, even the best of the ballistic software packages out there are only going to get you close. Granted, the ability to take in to account many factors at least allows them to get 'closer'. Ultimately though, a marked BDC knob or reticle is only going to be accurate one very specific set of environment variables and is only then right for one very specific ammunition load. This makes all BDCís generally fairly crude.
Horus Style Reticles
What about the reticles available now that are marked in increments of MOA or MIL that can allow holding off in fine increments instead of dialing in those adjustments into the scope itself? These reticles do in fact work. One of the early ones we tested out, well before Horus and others made them popular, was the very nice scope from Holland Shooters Supply, the Holland Tactical. It used dots on the lower half of the stadia to mark out MOA in order to be used for holding off precisely. I would venture to say that this scope was a bit ahead of its time as we look back now and see that several of its features have now become popular and we were perhaps a bit slow to even recognize those features.
Horus H58 reticle
Now this style of reticle has become more common place, especially with the popularity of the Horus style of reticles. Their system does in fact work and can be effective at allowing a user to only use the reticle when holding off for different distances, weather conditions, wind etc. The thing I do not like about the Christmas Tree style reticles is that they can become very cluttered and can obscure the field of view to where some details of the target area that is being observed can be blocked out and missed by the operator. Horus, and others, offer many different options so there are some that are better than others and if one of these reticles is being considered, than looking through the various styles might be advised in order to find the one that works best for your team.
We have covered a decent amount of data with this newsletter including everything from some of the history of BDCs, their effectiveness, limitations, and even newer ideas that are becoming popular. To summarize, the rule of thumb for us here at Sniper Central is that if you need a weapons system setup for effective use in the field, a BDC is a good option as it allows for simplicity and can get you close to right on in a pinch, but if ultra precision is your desire, than a BDC is probably not a good fit. Most of us here were trained using BDCs in the various military schools we attended, so we have some bias, but we also recognize their limitations. We do tend to prefer the knob style BDC versus the reticles as they can be changed easily by ordering a custom knob and it can even be an option to have two or three knobs printed up for likely deployment areas or even have one made for general conditions for a new area your team will be operating in once orders are received. This allows for a bit more accuracy with the BDC. Of course, it is just nice to have so many options available today.