What Are Bullet Drop Compensation Scopes and How Do They Work?

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Bullet drop compensation scopes are one of those rifle scope topics that seems to be filled with a marvelous combination of advertising misinformation, horror stories, topped off with a spring of magic. Ask ten different shooters to provide their opinion of a bullet drop compensation scope (which is more commonly called a BDC scope), and you’ll most likely see a mixed bag of views. Two or three shooters will swear by them, two or three won’t or don’t understand how they work, and the remaining two or three won’t touch one with a 10-foot pole.

Understanding Bullet Drop Compensation Scopes

The truth about BDC scopes probably lies somewhere in between, and I’m going to do my best to provide an unbiased opinion of their functionality, effectiveness, and limitations.

Defining Bullet Drop Compensating Scopes

A bullet drop compensating scope is a scope that either uses the reticle or specialized turrets to pre-calculate the distance that a bullet drops over specific distances.

When a bullet exits a rifle barrel, the force of gravity immediately begins to take effect on the round as it moves towards the target. This effect causes the bullet in altitude to drop as it moves downrange, with the amount of drop increasing as the distance from the rifle barrel increases.

To hit a target at longer distances, a shooter has to compensate for this drop by aiming the scope at a higher point than the target. This technique is called a holdover, and to successfully calculate how much to holdover requires knowing several factors, such as:

  • Bullet caliber
  • Bullet Speed
  • Bullet Coefficient
  • Distance to target
  • True power magnification range

This data has to be entered into a formula that takes all that data into account and provides an amount (usually in inches) to hold the reticle over the desired target.

To use a more real-world example with data: The average 40 grain 22Lr bullet drops about 8 inches at 100 yards (on average). With a 22 rifle that is sighted in for 50 yards, you would have to aim the rifle scope reticle several inches over the 100-yard target in order to hit the target. If you aimed dead-on, the round would impact 8-10 inches lower than you were aiming.

With a BDC scope that was built for a 22LR and properly configured, the BDC reticle or BDC turret would pre-calculate the amount of reticle holdover needed to hit the 100-yard target. Now, that’s an over-simplified example minus the math involved, but the idea is to provide an easy-to-understand approach to BDC function.

How do Bullet Drop Compensator scopes or BDC Scopes Work?

Most BDC scope functionality is done one of two ways:

  • BDC Reticle
  • BDC turret

Let’s talk about each approach in more detail:

BDC Reticle

Riflescope BDC Reticle
Weaver V24 with 223 Reticle

A BDC reticle is different from a traditional duplex reticle. The BDC reticle features easy-to-see marks that start below the crosshairs and extend down the vertical crosshair post. These marks come in several shapes, including:

  • Micro-dots
  • Open circles (which is popular with Nikon)
  • Hash marks
  • Diamond-shaped marks

Some rifle scope brands also include a secondary mark between each of the primary marks in order to offer more in-depth BDC sighting options.

Each BDC-based sighting mark corresponds to a specific shooting distance. For example, the first mark down from the crosshair may be for 100 yards; the 2nd BDC sighting mark may be for 200 yards, etc. Depending on the bullet caliber and scope brand/model, BDC scopes may offer a sighting solution that extends out to distances as far as 800 yards for a centerfire rifle.

BDC Turret

Riflescope BDC Turret
BSA Sweet 22 Scope with 36 grain BDC turret

While the BDC reticle is the more common and more popular option for bullet drop compensation, another method involves the use of a specialized BDC turret.

Like the BDC reticle option, most BDC turrets are also caliber specific and offer a bullet drop compensating option for shooters who prefer to turn the turret versus using the reticle. A BDC turret features distinct markings that denote specific shooting distances.

Once a scope with a BDC turret is configured correctly and sighted in, the shooter can turn the BDC turret to the pre-set distance. The ballistic math calculates the necessary bullet drop caliber for that specific distance is already built into the turret.

For example, going back to my previous example of a 22LR shot at a 100-yard target:  A shooter using a 22LR scope with BDC turrets would turn the turret to the designated 100-yard mark and then shoot on the target while holding the reticle dead-on. No holdover is needed as it’s pre-configured into the turret.

 Which BDC Method is Better?

I’ve used scopes with both types of BDC functionality, and both work well (when properly configured). The configuration is a vital part as that’s where most shooters run into issues with BDC scopes. Most shooters don’t take the time to run through the entire configuration process, and even less take the time to test the BDC accuracy on the range.

While I’ve used scopes with employed both types of BDC functionality, I prefer the BDC reticle option as it offers more flexibility should the shooting distance change and is more simple to set-up.

The turret option requires you to take your eye out of the glass to make turret adjustments for distance, while the reticle option allows the shooter to stay in the glass to make distance changes.

BDC Scope Limitations

Detractors of a BDC scope typically base their dislike of the platform on several limitations that can be encountered with this type of scope. Here are some of the more common complaints about BDC rifle scopes:

Designed with Restrictive Requirements

This complaint centers around the fact that most all BDC-based scopes are caliber specific, and most all are built so that the BDC functionality only works with a single, specific bullet weight moving at one particular bullet speed.

For example, I use to have a Nikon M223 scope that featured an integrated BDC reticle built for a 223/556 AR platform. The BDC capability was created to work with a 55-grain bullet with a bullet speed of 3240 feet per second. Nikon selected those parameters because the 55-grain bullet is the most common and most popular bullet weight for that caliber, and the 3240 feet per second number is based on the average speed of a 55-grain bullet exiting an AR-15 with an 18-inch barrel.

The BDC function of that M223 scope worked surprisingly well, but only with those specific ammo and bullet speed parameters. But what happens if my AR-15 shows the best accuracy with a 62-grain bullet (which it did)? Now the BDC functionality isn’t exactly on, so to speak, as the different grain weight throws a proverbial wrench in the BDC reticle accuracy.

Bullet speed is the other data point that can cause issues with the BDC function. As bullet speed is necessary for the bullet drop calculations, nearly every BDC-based rifle scope comes with a pre-determined bullet speed need for the bullet drop configuration to function correctly. As I mentioned before, these bullet speeds are typically based on averages for the most popular bullet configuration. However, what happens if your specific rifle ends having a bullet speed that is lower or higher than the pre-configured BDC bullet speed? The BDC calculations built into the reticle or turret are no longer exactly accurate.

There are ways to work around the bullet weight and bullet speed variances, but those corrections may require either a slightly different approach or some testing at the range.

Having to adhere to the specific caliber data points (bullet grain weight and bullet speed) can be off-putting for some shooters.

Built with Restrictive Range Requirements

Most BDC reticles and turrets are built for “normal” pre-determined distances like 100 yards, 200 yards, 300 yards, etc. With the proper ammo and sight-in configuration, most BDC set-ups perform reasonably well.

However, what happens when you need to shoot at 250 yards versus 200, and the BDC reticle you are using does not offer a half mark between the 200 yard and 300-yard BDC aiming point? Now you have to do your best to aim in between those two BDC points, which may be challenging. This can be especially challenging if you are using a BDC-based rifle scope in a hunting scenario and are trying to shoot a trophy Mule Deer that is on the move.

Those reticle gaps are another reason that some shooters do not like to rely on a BDC-based reticle.

Require Range Testing and Tuning for Optimal Accuracy

This is really more of a complaint than a limitation, but it’s worth discussing. Most BDC scopes (be it with a reticle or a turret) need to be fully range tested before being officially used. Given the BDC data’s generalized nature, these types of scopes most likely need to be thoroughly tested and tweaked at the range before they are considered to be completely sighted in.

While that sounds like common sense, you’d be surprised at how many customers I’ve seen at my day job that buy a BDC scope, take it to range and sight it in at 100 yards per the manual, and then don’t test the accuracy out to distances beyond 100 yards.

If you’re planning on shooting a scope with built-in bullet drop compensation, then you should consider spending some range time testing the BDC function out to extended distances.

How to Address BDC Scope Limitations

Typically there are three potential ways to address the common limitations found with caliber specific BDC built rifle scopes:

  • Brand Based Ballistic Calculation Programs
  • Old Fashion Trial and Error Testing at the Range
  • Custom Made BDC Turrets
  • Phone-Based BDC Applications

Let’s discuss each in more detail:

Brand Based Ballistic Calculation Programs

Most rifle scope makers offer a ballistics calculation application that works in conjunction with a BDC rifle scope reticle or even BDC turrets (in some cases).

If you run into an issue with either the bullet weight or bullet speed configuration, you can use one of these applications to “re-configure” your BDC functionality to work with variations in bullet weight or bullet speed.

Here’s an example of that re-configuration using a real-world example. Going back to my experiences with a Nikon M223 scope above, my AR-15 showed a definite preference for a specific 62-grain ammunition that was chronograph tested to be traveling an average of 3050 feet per second out of an 18-inch barrel.

Using the Nikon Spot-On application, I determined that the first holdover point down worked for 188 yards (versus the default 200 yards), the second holdover point worked for 282 yards (versus 300 yards), etc.

Using this data, I effectively extended the BDC functionality of the AR out to just under 600 yards on 12X power.

Old Fashion Trial and Error Testing at the Range

Another potential option for overcoming BDC scope limitations is to address the problem the old fashion way: shooting on the range until you figure out the impact points for each BDC holdover point at certain distances on a specific scope power range.

Sighting in a BDC scope at the Range

This approach’s downside is the amount of time and ammunition it will most likely take to verify the impact points.

While I find this approach an interesting way to proceed, I much prefer to use technology to my advantage.

Custom Made BDC Turrets

If you prefer not to invest the time to learn the factory BDC reticle or BDC turret, another potential option would be to invest in a custom-made BDC turret built for the specific caliber, ammunition, and bullet speed of your particular rifle.

A suggestion if you are thinking about going down this path: Take the time to shoot a few groups of the rifle through a chronograph to determine the average bullet speed of the particular ammo that you want to use. Relying on estimated bullet speed data from the rifle maker or ammunition maker is the single biggest mistake I see with this type of turret.

The concept behind this approach is relatively simple: You provide the custom turret maker all the necessary data (caliber, ammunition, bullet speed, sight-in distance, etc.), and they create a custom turret that mounts on your scope with the BDC functionality built-in for specific distance markings on the turret.

While this is a more custom option than what you see with a more generic caliber-based BDC set-up, it tends to be highly accurate. If you provide the correct data points, the BDC functionality is spot-on.

The downsides with this approach as follows:

  • Not every rifle scope brand offers scopes designed to support an aftermarket BDC turret.
  • The costs associated with the custom turrets.

Phone-Based BDC Applications

If your BDC reticle doesn’t work for your specific rifle set-up, the final option is a phone-based BDC application.

This type of application lets you enter all the necessary data points for bullet drop compensation and then uses your specific BDC reticle to do all the ballistic math based on the distance to the target that you select.

I’ve used this type of set-up to create a dope card of sorts that is mounted on the stock for quick reference.

There are several different makers of this type of ballistic application, but my personal favorite is an iPhone version called iStrelok. I purchased the upgraded version, which lets me set-up several different rifle profiles within the application.

FAQS

Here are some of the more frequently asked questions that I see and am asked in regards to bullet drop compensating rifle scopes:

What’s the difference between a bullet drop compensator scope and a BDC scope?

Nothing as they are both terms used to describe the same type of rifle scope. BDC is a commonly used acronym for “Bullet Drop Compensator” or “Bullet Drop Compensating.” Within the rifle scope industry, all three of those terms are used interchangeably.

Are BDC scopes the same as a Mil-Dot scope?

While a BDC scope and a mil-dot can be similar, they are not the same thing. A BDC riflescope is equipped with a reticle or turret that offers built-in bullet drop calculations. The more common type of BDC operation is done via a reticle.

A mil-dot scope is a scope that is equipped with a mil-dot reticle. These two reticles are slightly different as a BDC reticle typically only features BDC aiming points on the lower portion of the reticle’s vertical post. A mil-dot reticle has mil-dots all along the vertical and horizontal posts of the reticle.

Mil-Dot Reticle Nikon BDC Reticle
Mil-dot Reticle BDC Reticle

Most BDC reticles only allow for bullet drop adjustments, while a mil-dot reticle allows for bullet drop adjustments as well as left or right adjustments for wind drift.

How accurate are BDC scopes?

If the shooter sets up all the necessary BDC data points correctly and accurately, BDC-based rifle scopes can be extremely accurate.

Several factors go into that statement about the accuracy, such as the rifle’s accuracy coupled with the shooter’s level of skill. But, if properly configured, a BDC reticle or BDC turret can offer excellent accuracy in both the field and on the range.

What does BDC mean on Nikon scopes?

Although Nikon is no longer producing rifle scopes, they use to offer several rifle scopes models that were equipped with different styles of BDC reticles.

Nikon used the BDC acronym and a number to identify the farthest effective distance for the BDC function to operate. For example, a Nikon scope with a BDC 600 reticle meant that the BDC reticle was designed to offer bullet drop capability out to 600 yards. Nikon’s BDC 300 reticle was designed for 300 yards, and so and so forth.

Hopefully, you found some of this information helpful, and I’ll update this page as I come across new information.