If you mean the rifles...
The M40A3 (currently fielded by USMC Scout/Snipers) fires a .308 Winchester, aka 7.62x54mm, and, when outfitted with the now defunct Unertl 10x, weighed in at 19.5 lbs. It is a rifle capable of holding a minute of angle (MOA) which means it's group size (per 5 rounds) is one inch per 100 yards of distance. That means every round in a 3 inch circle at 300 yards, 7 inches at 700 yards, and 10 inches at 1000 yards.
However, around 800 yards the .308 drops below the speed of sound (appx 1100-1200 fps) and the passing of the round through the sound barrier shockwave can sometimes have unpredictable results. Depending on specific atmospheric conditions, this may, or may not, be a terribly mute point.
Despite popular belief, the official rifles cannot be purchased because they are not built by a company. USMC match grade armorers (MOS 2112) get all the parts and each rifle is hand built and tuned. However, many companies, such as H-S Precision, offer replicas to civilians.
It should be noted that these rifles are inherently capable of better than 1 MOA accuracy, but the fault lies with less than stellar military match grade ammunition. There is civilian match grade ammunition available that is more accurate, but they differ from company to company and rifle to rifle, which makes it impossible to determine a "best". It's a simple fact that different rifles and ammunition work better together than others. Handloads are a snipers best bet. Many recognize military grade brass as being the single greatest ever, but utilize primers, bullets, and powder from other sources.
The M82A3 is made by Barrett Rifles Mfg. fires a 12.7x99mm round (commonly denoted .50 caliber, but is actually .510). What most people don't understand is that this is not actually a sniper rifle. To be considered a sniper rifle, it must be able to place every single round inside 1 inch at 100 yards (or, as you read above, be MOA accurate). Hence the designation "Special Application Scoped Rifle" or SASR (pronounced "sasser"). Since this beast fires a round that already creates half inch holes, keeping the holes inside an inch is tough.
It is also important to note that group size regulations differ through the community. Many measure from inside edge to edge, whereas the military requires the entire hole be inside an inch. Therefore, by USMC Scout/Sniper standards, the SASR is actually a 2 MOA weapon (give or take, some rifles have seen more abuse than others). In regards to the round fired, the SASR doesn't typically shoot match-grade ammunition in combat. It uses Roufoss rounds. Follow this link if you care...
The SASR is capable of hitting man sized targets, first round, out to 1200 meters or so. In many cases, this is used as a anti-material weapon (trucks, tanks, radars, SCUDs, etc.) which can extend it's useful effective range to 2000 meters. The price you pay for hauling this bad-boy around is a 32 lb boomstick. Add a quality optic (S&B or Leupold 20x) for 3 or 4 lbs. Ammunition is about 4 lbs per 10-rounds in a magazine. So a combat ready rifle is about 40 lbs. But you'll need more than 10 rounds...
Which brings us to the glorious Accuracy International Arctic Warrior firing a .338 Lapua Magnum.
The full kit (folding stock, scope, bipods, loaded magazine) weighs about the same as a M40A3. However, the single most important factor due to the success of AI is the durability of the rifles and scope mount together. AI's utilize a rigid all metal frame and the plastic stock is simply bolted on. The scopes are also single point mounted and can be removed and reattached without the loss of zero. A common field problem for both the SASR and M40 is bumping the scope off zero. Watch the YouTube video I posted above to see what I mean.
The accuracy of the AI has much to do with the round it fires. Engineered to maintain accuracy and velocity at greater ranges allows this weapon to fulfill the needs of armor penetration (filled by the SASR) and terminal ballistics on human flesh (filled by the M40) all in one weapon system, and one round. The .338 LM is rated to point targets (body shots) at 1300 meters (which is about 1430 yards). However, at 1500 meters, the bullet is still traveling at appx 3,000 fps which is fast enough to penetrate 5 layers of Level IV personal body armor (also level 8 glass, the highest). That extends the anti-material ability out to about the 2000 yard range. The sound barrier tumble never comes into play creating a possibility of greater ranged shots.
So, in my personal, experienced, and slightly humble opinion...
The greatest rifle is the Accuracy International Arctic Warrior series due to it's durability and reliability.
The greatest all-purpose round for active military engagement purposes is the .338 Lapua Magnum because of it's velocity and accuracy.
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