When I am training, the one thing that I see with AR15 format shooters is trouble with mechanical sight offset (the problem created by the sights of an AR-15 being roughly two inches above the bore).
Taking it slow and having multiple rounds, most guys move onto target fine, but break out the stress box for timed trials—and limit the rounds fired—and I regularly see some fine shooters fall victim to the vagaries of sight offset.
Therefore, I pose a different problem to sight offset. To qualify as passing in our most basic carbine course, the shooter must be able to go to both superior side prone (strong side up) as well as inferior side prone (strong side down) and produce solid center-ofmass hits.
If you are not familiar with side prone, let me just say that it is as the name would indicate. The tactical uses of the side prone positions and the canted rifle are numerous.
As for the side prone, instead of being prone facing the target, the shooter is on the side essentially paralleling the target. The position is great if going standard prone would give up the shooter's location or expose the lower legs to fire. While an ankle shot is imminently survivable, it is generally a career ender, plus if you cannot move in the fight, your chances of survival diminish. Additionally, some guys use a 90-degree canted rifle for popping up from behind cover, and the scuba prone calls for an equally canted weapon. These are all fine shooting tactics, but you have to train on them—the skill set will not simply descend on you because you own a rifle.
The problem of sight offset, whether combined with speed or need for precision, is already tough enough. As the AR is doped in, it is done with the barrel and the sight in the same vertical plane. That is to say that the sighting tools, whether iron or optic, are straight up and down over the barrel. When you move to side prone, which necessitates canting the weapon 90 degrees, the formerly vertical protocol shifts to a horizontal arrangement.
What this means to you is that no matter now meticulously you sighted in that weapon, that Battle Sight Zero (BZO) no longer applies. That round is no longer in the precise parabolic arc that you established; rather the effects of air resistance and gravity take root with some immediacy. As you sighted in, you arced the trajectory of the round to intersect the sight plane at a specified distance.
I am not going to get into the argument as to what distance is correct, but I believe that 50 yards is the zero distance for irons and non-magnifying optics. Depending on the height of the sights over the bore, in the end quite a bit of loft can be put on the round.
Now that you have moved to side prone, you have a round that is vectoring like a curve ball right or left, depending on which side you are lying on, coupled with no gravity compensating immediate onset to the vertical zero. Physically, you are in a stable shooting position, presumably well concealed, but what you once knew as a point of aim/point of impact relationship no longer exists. If you can learn something about it here, you will come out better than learning it while taking fire.
When teaching the side prone, I got more misses than in any other position, even if I allowed the students to shoot supported. What I never realized was that I had already learned to compensate, so I had to start articulating that compensation to the people entrusting me with their instruction.
While I am going to share my findings with you, this is not the Holy Grail. I am letting you know that this is a situation warranting your attention, but I cannot give you any absolutes. I fired four quality weapons in my tests with four quality sighting systems and got four separate, but problematic, results. I suspect that I could rotate the optics around and get another four different outcomes.
For this experiment, we used a 20inch Colt A-2 with iron sights, a Colt 10inch with an EOTech, a 14.5-inch upper with an Aimpoint on what would normally be a co-witnessing mount with A.R.M.S. SIR rail system, and an ACOG 4X32 on a 16-inch barreled upper.
The above is the gear that I see most in training. I use gear that I see used because I want to understand the problems with the gear coupled with the best use of the gear. Using some esoteric trinket that removes me from understanding the pitfalls of regular gear would give me more the mindset of a politician than a trainer. Politicians can afford to be removed from the plight of the regular guy—I can't.
I do not consider it my job to induce underpaid guys to spend their hard-earned dollars on gear, but rather to help them use the kit that they have. I would rather see them spend $1,000 training with basic gear than spend that $1,000 on better gear that would result in their having no funds for training. I still give equipment advice, but it is generally concerning practicality and simplicity.
Further, I remove a vast preponderance of cheap stuff off front rail systems with the advice that it not be reinstalled. I'd rather have a tritium front sight post on my AR than Trashcanistanmade half-generation night vision gear. My equipment mantra is simple: Find something basic that works, and know how it works intimately. Do not change it until enough money is available for a sweeping upgrade, along with the money to properly train with the new gear.
As for the inherent weaknesses in the gear I use, in the end some of the weaknesses become strengths, while the reverse also happens.
I love iron sights—no switches, no batteries and no gimmicks! While a shooter can be fast on irons, the fact is that you must get in position and get the back and front sights properly aligned to deliver precision rounds on target. By definition that will not be as efficient as lining up a single red dot, no matter where your head is positioned relative to the red dot sight.
The EOTech is great and it is hands down my favorite reticle, but I find the operation sometimes cumbersome. In addition, that low mount gets uncomfortable when I use it for prolonged periods.
The Aimpoint is fast and the operation is simple. The lens covers are effective and easy to operate, and the sight is high and comfortable for long periods of time. The downside is that the red dot in the Aimpoint is large and it is all but impossible to shoot a tight group, particularly because in my southern environment I have to run the Aimpoint as big and bright as Betelgeuse. That is great for fast hits but lacking for precision shots.
The Trijicon ACOG 4X32 is probably my favorite all-around optic, but for CQB use the mind had better be focused and properly compensating for the sight offset and magnification. Plenty of times I have seen guys use these and deliver beautiful pinpoint shots on the wrong paper target. In the real world this is a nightmare.
In looking for answers here, the variable of sight height above barrel is the key element, but barrel length, bullet weight and that elusive barrel twist factor probably come into play. If 100 of you try this experiment, we may end up with 100 different results, but I will wager that the "error" movement is in the same direction.
As mentioned earlier, I use a 50-yard zero. I cannot explain it, in that I thought there would be more variation, especially judging from all the misses, but 90 degrees cant right or left produced for me a localized center-of-mass pattern at 50 yards. The widest variation of pattern was, as expected, with the higher mounted Aimpoint with the bright red dot. From the 50-yard line, weapons were fired unsupported. At 100 yards they were fired supported. Winchester range ammo was used. (I used to call it cheap ammo until I had to get a second mortgage to buy more.)
Moving back to the 100-yard line, the zero moved as I had hypothesized. Starting with the iron sights on the A-2, the shots were high, as was to be expected from a 50-yard zero shooting from the 100-yard line. When I moved to the left side prone, the pattern moved to nearly off the body and most certainly into a zone of causing questionably sufficient harm to stop the threat. The patterns started high, as the impact should have for 50-yard dope, and moved almost identically off axis while dropping lower.
With the Aimpoint, the side prone rounds were completely off paper. I am confident that the higher mounting on the A.R.M.S. SIR rail compounded this outcome. The Aimpoint is a fine, dependable optic and the SIR rail is quite functional, so this outcome should not reflect badly on either. You simply have to be in touch with the operational parameters of your equipment.
The clean reticle of the EOTech on the 10-inch barrel allowed me to shoot an acceptable group, and the drop from the 10-inch barrel was not any farther than the drop from the other length barrels. However, I shot it suppressed (I find the muzzle blast from the short barrel unpleasant at best), and the increased velocity could have kept the rounds from dropping more than they did. Alas, another variable to be isolated and defined.
The ACOG, like the EOTech and iron sights, moved into a questionable hit region when canted. The ACOG allows for tight groups. Tight groups are nice, but in the real world and the matter of tightly grouped misses, I would much prefer sloppy hits as long as they are in kill zones.
I hope you believed me when I said earlier that I was not offering answers. If you didn't believe me, then by now you are disappointed. There is an offset problem I wanted to bring to your attention, and now it is up to you. I cannot tell you precisely how your equipment will function, and I am not going to put ideas in your head as to what your results might be.
I see some students with an optic mounted so far above the barrel I need a stepladder just to help them adjust it. I am not going to venture a guess as to how far out that shoots when canted. Find out what your weapon will do. A 90-degree cant can be a very effective tool, but it is only a tool if you develop it.
No amount of my telling you this and no amount of your knowing it can make up for training and learning from experience. Get out to the range and see what your personal results will be!