Friday, April 30, 2010

Maximum Point Blank Range and the Battlesight Zero

The MPBR is the maximum range at which the bullet rise and drop stays within the vital area of your target. Anyone who has been in the U.S. Army or Marine Corps is familiar with a battlesight zero or improved battlesight zero (BZ0 or IBZ0). The concept for an MPBR or battlesight zero is pretty much the same: zero the rifle so that you get a point of aim that is effective over the longest range. The battlesight zero used by Marines when shooting the iron sight M16A2 is the 36/300 zero, meaning that the bullet will be on the sight line at 36 yards and again at 300 yards. The US Army uses what is referred to as an improved battlesight zero, which calibrates the rifle to be dead on at 50 and at 225 yards. The USMC also uses the 50/225 IBZ0 for M16A3 rifles equipped with Trijicon ACOG scopes.

The illustration above demonstrates how a battlesight zero works. The bullet is fired from the barrel and rises up to be exactly on the line of sight at 36 yards. It then continues to rise, topping out at 6"-7" depending on the round used and the barrel length of the rifle. It then descends until it is again exactly on the line of sight at 300 yards. This gives the Marine a good aiming point for a man sized target at any distance between 0 and just over 300 yards.

From the USMC manual:
If a rifle is zeroed for 300 yards, the bullet crosses the line of sight twice. It first crosses the line of sight on its upward path of trajectory at 36 yards, and again farther down range at 300 yards. Since a bullet crosses the line of sight at 36 yards and again at 300 yards when a rifle is zeroed, a rifle's zero may be established at a distance of 36 yards and the same zero will be effective at 300 yards. It is critical that a Marine fires tightly grouped shots directly on the point of aim when establishing a BZO at 36 yards because any error in shot placement at 36 yards will magnify as the bullet travels down range.

If the rifle is properly zeroed for 300 yards/meters, the trajectory (path of the bullet) will rise approximately 7 1/2 inches above the line of sight at a distance of approximately 175 yards/meters. At other distances, the strike of the bullet will be less than 7 1/2 inches above the point of aim. Only at 36 yards/30 meters and 300 yards/meters does the point of impact coincide with the point of aim. If only a portion of the target is visible (e.g., the head of an enemy soldier), the trajectory of the bullet may have to be taken into consideration when firing at a distance other than 300 yards/meters. If a Marine does not consider trajectory, he may shoot over the top of the target if the target is small and at a distance other than 300 yards/meters.

The 50/225 IBZ0 is useful as the bullet has much less rise at the midpoint of the trajectory. Its shorter effective range is more suited to urban and jungle warfare where visibility is limited and most engagements are at close range. The fact that the bullet rise is lower means that shots taken at ranges between 0 and 250 yards are much more accurate, with a bullet rise less than 2 inches at the midpoint of the trajectory.

The battlesight zero as a concept is very useful to hunters as well. When hunting deer, or any medium sized game, it is rare to know the exact distance that the quarry will be encountered at. Luckily, if your rifle is properly sighted in for its maximum point blank range (MPBR) you don't need to know the exact distance. While the ballistics vary from rifle to rifle, it is generally a simple matter using any number of online ballistic calculators to work out what the ideal zero for your rifle should be. The most critical calculation is your second zero. Based off of the size of the vital area of your target, you can compute the maximum rise and drop tolerable for your cartridge. Most white tail deer for example have a vital area that is generally 10 inches in diameter. Mule deer, elk, and moose have vital areas that are significantly larger. A large mule deer has a vital area around 12" in diameter, an average elk around 15", and a good sized moose nearly 21". A hit from a medium caliber rifle to this area will result in a quick kill. Therefore, if we are hunting white tailed deer, we can tolerate a maximum rise and drop of 5". Using this value, it is simple to calculate that the MPBR for a 180 grain Remington Core-Lokt .308 roundnose soft point cartridge in my trusty Remington 700 is 293 yards, with our second zero at 252 yards. With our rifle zeroed for these distances, we can be assured that a perfectly centered aim on a deer at any distance between 0 and nearly 300 yards will result in a hit in the vital area of our target.

The problem with zeroing your rifle for 293 yards in this case is that not many people have access to a 300 yard range. Not to worry, there are other ways to achieve the same zero for your hunting rifle. As it mentions in the USMC manual we referenced above, you can sight in your rifle at a closer range for the same result. In fact, if you have a good bench rest and a gridded target you can, with a little math, perfectly achieve a MPBR zero on your rifle at any range. Lets assume for this example that our range only has a 50 yard rifle range. We're shooting a Remington 700 chambered in .308 and plan to use the 180 grain Remington Core-Lokt mentioned above. By plugging in the information for that load, we can see that the bullet should hit 2.2" high at 50 yards (if you were at a 100 yard range, it would hit 4.45" high). Our first zero for this rifle and cartridge combination is actually just shy of 20 yards, and you can use that distance if that is the only range available at your local shooting gallery, but be aware that minute errors in measurement which may not be apparent at that close range will be magnified at longer distances, possibly throwing your shot off.

Remember: The Battlesight Zero and Improved Battlesight Zero discussed here only work on 5.56 M16 and AR-15 style rifles. You will need to find the maximum point blank range for your unique rifle, optic, and cartridge combination. Even differences such as the scope you have mounted on your particular rifle will change the MPBR and subsequent zero. Find the manufacturers information on your favorite rifle load, google up a ballistics calculator, and in just a few minutes after plugging in your date you'll have a good MPBR zero for your setup.

No comments:

Post a Comment