How do BDC reticles work?

[pbwalker]

Are the markers just 'rough estimates'? Do they assume a specific caliber?

[flintknapper]

Are the markers just 'rough estimates'? Do they assume a specific caliber?

Not "rough estimates", (but can be).

Some are caliber specific....but they can be used with good results if your projectile has a similar Muzzle Velocity and Ballistic Co-efficient as what the BDC feature was intended for.

In other words, if you have a BDC scope calibrated for a .223-5.56 at 3200 fps, it isn't going to work for your 30-30. Conversely, if you have a scope calibrated for a 150 gr. 308....you could use it on your 30-06, 7mm-08, etc...and be pretty darn close (assuming like MV and similar BC's).

"Caliber" (bullet diameter) really has little to do with anything. If your bullet leaves the pipe at about the same velocity as the BDC feature was intended to compensate for...AND the bullet maintains velocity (Co-efficient), it will have a trajectory very similar to what the engineers had in mind for the BDC.

[chabouk]

They assume a specific bullet drop, which is why they are "Bullet Drop Compensators".

The only way to assume a specific bullet drop, is to assume a specific bullet (by weight, shape, and ballistic coefficient), and specific loading (for muzzle velocity).

"Bullet drop" is the difference between point of aim and point of impact at a given range.

[smyrna]

Just trying to follow and understand the replies here...

I often see and hear the term ballistic coefficient when referring to bullets. What exactly is a ballistic coefficient?

[The Annoyed Man]

Just trying to follow and understand the replies here...

I often see and hear the term ballistic coefficient when referring to bullets. What exactly is a ballistic coefficient?

Linkey:

In ballistics, the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in² or kg/m². Normally BC's are stated in lb/in² by gun projectiles producers without referring to this unit.

Bottom line is that higher BCs are desirable for long range accuracy than low BCs. Also, keep in mind that a specific bullet's BC changes with velocity.

For example: a .308 diameter 175 grain Sierra MatchKing has a BC of .505 @ 2800 fps and above; .496 between 2800 and 1800 fps; and .485 @ 1800 fps and below. Therefore, to drive a 175 grain MatchKing with accuracy at 1,000 yards, you'll want a muzzle velocity of 2800 fps or better.

[The Annoyed Man]

Are the markers just 'rough estimates'? Do they assume a specific caliber?

One of my scopes is a Leupold VX-3 4.5-14x50 with a B&C bullet drop compensating reticle and it has caliber specific graduations. The markings are calibrated for a 168 gr Sierra Matchking in .308 Winchester, with the scope set at 10X. It is also calibrated for a different bullet weight/caliber, a .270 I think, but I don't own a rifle in the other chambering so I don't know how it works. But on a .308, with a 100 yard zero, elevating one stadia on the reticle puts you right on target at 200 yards, so it seems to work pretty well.