This is a very interesting subject and one that I get a lot of questions about.
What is the best distance to zero a combat rifle? To me a combat rifle is a rifle/carbine that is issued as a standard service weapon. In todays military in the US this would be either the M16 A4 or an M4 Carbine. Both are chambered in 5.56mm. The real difference in the two is the overall length and more importantly the length of the barrel. The M16 A4 comes standard with an 20" barrel and the M4 comes with a 14.5" barrel. The length of the barrel will directly effect effective range and accuracy. However, accuracy at long ranges is based more on the ability of the shooter than 5.5 inches of barrel length.
Ammunition selection is critical also for a couple of reasons. Reliability and performance. Military ammunition may not give you the performance of a match round for competitions but it will give you the most reliability during cycling on the battlefield. This is seen to be more important than performance for the Military.
Most manufactures of Military grade service weapons test their products using Milspec ammunition. The most common test rounds include but are not limited to M193 55gr ball, M855 62gr 'green tip' and Mk 262 77gr long range ammunition. Each one of these rounds perform differently in terms of ballistics.
In short M193 is more accurate than m855 but lacks the penetration. Mk 262 is more accurate over longer distances than M193 but again lacks the penetration capability of green tip.
Green tip was originally designed by the Belgians and designated at the time the SS 109. The projectile was designed to pierce Soviet Body Armor at 200m and through one side of a steel helmet at 600m in line with the specifications of an LMG (light Machine Gun). This was in preparation for a possible war with the former Soviet Union.
In order for the projectile to perform this task it needed to be stabilized for longer. This means make the projectile spin faster. So the twist rate of the M16 had to change from its original 1:12 of the Vietnam era or 1:14 from Stoners original design to 1:7 twist rate of the M16 A2 adopted by the Marine Corps and Army in 1982.
The performance of green tip ammunition is almost irrelevant on the modern battlefields of Iraq and Afghanistan. Our enemy is not technologically advanced by todays standards and is made up of militia members rather than enlisted men and unlike most first world militaries does not issue body armor to its soldiers. Therefore the M855 'penetrator' round is relegated to punching through mediums such as heavy cover like cinder block walls and vehicles.
When green tip strikes our enemy it penetrates 10-15 inches before it yaws, and separates into three pieces. The tip, core and sheath, creating three separate wound channels and thus incapacitating him. Unfortunately, the chest walls of our enemy are between 8-10 inches thick and green tip has exited before it can separate. When this occurs we generally see .22 calibre through and through wound ballistics that may not kill the enemy.
Mk 262 77 grain long range ammunition was designed to fill the ballistic gap of 5.56 and 7.62. It was specially made for Special Operations teams to use with their SPR rifles. After its successful incorporation into the SOF inventory Mk 262 became a heavy duty work horse for special operators throughout the Middle East.
The M 193 55 grain ball ammunition has a full copper alloy jacket and an antimony alloy core. M 193 is a light weight round that is generally used against personnel and unarmored targets. M 193 ball is a good training round that is extremely accurate over short ranges however it lacks the punch of 62 grain green tip.
When the question "What range do you recommend to zero a carbine?" comes up. I hear nearly every instructor say something like, "Well, what country are you deploying to, Afghanistan or Iraq?" And back it up by saying that if your going to Iraq you should have a 100m zero because of the combat ranges there. And a 300 meter zero for Afghanistan because of the extended ranges.
Well I call bullshit on all accounts.
In combat there isn't one specific range for shooting bad guys either in Afghanistan or Iraq. What is important is that you have a zero that best covers the ballistic profile of the projectile being fired.
This is why I advocate the 200 meter zero. A 200 meter zero is like a fire and forget system for the combat rifle from ranges between 0 - 250 meters.
When you zero your rifle for 200 meters you can either use a reduced target and shoot at 25 meters or shoot Point Of Aim/Point Of Impact at 50m. If you zero at 25, always confirm POA/POI at 50m as there will always be discrepancies when zeroing on a reduced target. A good shooter should be able to hold a 1" group at 25 meters with an inservice M4. (This is consistent with Military Specifications of the weapon being a 4" gun or holds a 4 inch group at 100m. A service rifle should hold a 2 MOA group and service ammunition should hold a 2 MOA group hence the 4 inch group.)
At 25 meter the projectile will be approximately 1" low and be POA?POI at 50 meters. At 100 meters, the projectile is traveling on its upward flight path and should be around 2 inches high at its culminating point and will intersect POA/POI at 200 meters. At 250 meters the projectile will be approximately 6 inches low of the POA and approximately 10 inches low at 300 meters. What this means to the operator is that he doesn't need to worry about 'hold overs' or 'hold unders' at all at any range between 0 - 250 meters based on the percentage of the target seen.
With a 100 meter zero the projectile will be in the parallax zone until about 87 meters then fly pretty straight through 100m which is its culminating point. At 200 meters the projectile will be approximately 6 inches low and 18 inches low at 300 meters.
The BZO sees the projectile intersect the POA? POI at 25 meters then fly's high, up to 10 inches at 150 meters which is it's culminating point. It then drops to intersect the POA/POI at 300 meters. The biggest issue with this zero is that the marksmanship standard in the Army for example is 1.5 inches at 25 meters. This extrapolates to a 6 inch group at 100 meters and 9 inches at 150 meters.
If a soldier puts his Aimpoint red dot onto a target at 150m with a rifle with a BZO his POI will be 19 inches higher than his POA less any shooter error.
This is one reason why our Military personnel are not killing enough bad guys!
An interesting fact to note is that m855 is not lethal beyond 200 meters and needs to be traveling at about 2700 f/p to be effective. This limits the effectiveness of SBRs in combat engagements beyond 100 meters and where shot placement becomes critical.