In a perfect world we would have all the time to train on the skills we need to go to war, with all the resources needed to conduct world class training, and no training distractions to pull people from training. Here in reality that isn't the case. And then there are "nebulous" training requirements that mean all things to all people. One of those nebulous requirements is that of the Squad Designated Marksman.
Every manual out there says you should train them, every manual out there says this is what they should be able to do, but in the end it is a unit responsibility to ensure that SDM's can actually put steel on target at the ranges they need to. I have yet to see a live fire training exercise where the SDM was actually given a target to engage at distance. I'm sure someone somewhere has done it, but it was probably an afterthought as the ranges set up by the Army to support the Army never had to play with an SDM when they were being built back in WWII.
So training SDMs is a problem, but we can adapt our current facilities to meet the training needs, it just takes a bit of flexible thinking. The other side of the house is equipping our SDMs. The answer to that has been debated for a damn long time.
The best answer in the current inventory is an M16A4 with an ACOG, or barring that an M4 with an ACOG. Why? Because in the hands of a trained marksman even with M855 it isn't difficult to make hits out to 600 meters. On the other hand everyone wants M14s or M110s so that they have a 7.62 caliber rifle at the squad. There is nothing wrong with that except now your ammo is incompatible with everyone elses and you increased your SDM soldier load by 15 pounds at the lightest.
The solution to the problem really falls down into one of training. We can't get enough experienced NCOs to actually be competent marksmen that they can turn SPC Joe Snuffy into a competent marksman in order to have a cadre of competent SDM's who aren't immediately promoted to Team Leader or pulled away by the Snipers.
If you'll notice, the SDM issue is an Army issue. The Marine Corps maintained their tradition of "Every Marine a Rifleman" and so they don't have SDMs. This is why I'm convinced it is a training problem. Even with crappy M855 the Marines train to shoot 500.
Anyways, what proceeded before is a long backstory to the debates over what is better, M855A1, Mk262, Mk318 SOST, or the various 7.62 solutions for equipping SDMs. The Army tests ammo for suitability by "mean radius" which uses some statistical manipulations to calculate where bullets fall within a given shot group. The Navy tests by Extreme Spread. Both of these methods require shots to be put on paper, and the both require every shot to be plotted to provide the calculations for accepting a lot of ammunition into the inventory.
Extreme spread is a good way to get a quick look at your maximum effective range for a one shot hit on target. As long as your rounds always land between the extreme spread you can easily calculate your max distance for a head shot or torso shot. On the other hand "Mean Radius" is useful for describing the cone of fire that a properly stabilized machine gun will put on target. The Army doctrinally likes machine guns more than snipers (dating back to our doctrine of fire and maneuver relying on machine guns to fix an enemy while a maneuver element flanks to close with and destroy the enemy) so the Army accepts ammunition tested by "mean radius".
The navy on the other hand puts hard and fast MOA limits at distance on ammunition. Both Mk262 and Mk318 are in spec only if they are 2 minutes or less at 300 meters. At that distance you can expect a similar sized group of M855 to be around 12 to 14 inches. By the time you get to 600 meters M855 falls all over the place, usually around a 28 inch circle.
Time and time again Army units have noted that the Navy spec'd ammunition produces tighter groups (the AMU used Mk262 ammo when I went through SDM training) and recommend that SDMs be issued Mk262 ammunition. The biggest problem for me is that I can't get any for SDMs to actually do any sort of training on match grade ammo since I'm tied to the Army supply system right now.
Now the Army has accepted M855A1 as the new "green round" and it is supposedly more accurate than M855 even though it has the exact same accuracy standards for acceptance (machine gun level accuracy). Supposedly this stuff will shoot under 2 MOA at 600, which I really hope is true. If it is the case I will be able to train SDMs on ammunition that doesn't suck balls for accuracy at distance. The debate on lethality of M855A1, Mk318 and Mk262 don't really bother me, at this point I just want PVT Snuffy to be able to hit the target with 90% chance of making contact with the first shot.