I’d have to say the Barret M-99.

The reason is not because of the rifle itself but rather the caliber it fires.

The first real development of an ultra long range sniper cartridge was the .408 Cheytac.

Cheytac M-200 Rifle chambered in .408 cheytac (above)

Barret M-99 rifle chambered in .416 Barrett (above)

Because the .408 was first to really dive head first into the ultra long range category it has developed a kind of mythical status, shows like future weapons and and the movie shooter added to this reputation. The .408 and the rifle that fired it are an outstanding combination. However, in the years since the .408’s development some competitors have overtaken it. Even if only by a slight margin.

The most notable is from Cheytac themselves. They further necked down the cartridge case to .375 caliber to produce the .375 Cheytac. It is unequivocally ballistically superior to its bigger older brother.

We can thank California for the .416 Barret. Cali outlawed the .50 bmg cartridge so Barret necked down the .50 bmg case and loaded a .41 caliber projectile in it in order to comply with California laws. It only took 3 years to develop a gunpowder compound capable of taking advantage of the combination. Despite having significantly less powder and recoil than the big 50 the more aerodynamic 41 caliber bullets were capable of staying supersonic to incredible ranges (Around 2200 yards). The .416 has been shown to make first round hits out to 2500 yards or so. Compare this to the .50 that goes subsonic around 1500.

An indicator of the effectiveness of the .416 is to look at the what the pros use. There is a marksmanship competition called “The King of Two Mile.” That’s correct, 2 miles. Below is a list of what the top 10 competitors used.

Two out of the top three were the .416 Barret. The number 2 slot is a modified .375 Cheytac case. The .408 is not on the list.

The .375 and .416 are absolute bruisers as you can see from the picture. They are not for the faint of heart and require more than the average marksman is trained to do.

In order to take advantage of their maximum ranges you need very high quality range finders and ballistic computers with the ability to accurately determine the correct environmental conditions. At those ranges a miscalculation in your density altitude can make a big difference in whether or not you hit the target.

They also require the use of projectiles with little to no variability in weight, length and shape.

At any rate, if I had one cartridge to pick for an ultra long range shot and had to do it with a factory rifle it would be an M-99 firing the .416 Barret.

EDIT:

Wow! 100,000 views. For an average Joe like me it’s a little humbling to know that many people have seen something I wrote. The original answer was meant to be a somewhat cursory response, not getting too deep into the details. Considering the level of interest I feel compelled to provide some more.

I selected the Barrett M-99 chambered in the .416 Barrett cartridge as the rifle with the most capability for striking targets at the longest possible range. This would not indicate that the M-99 is always or even ever the best option for a sniper to carry. The .416 Barrett cartridge is an absolute beast. It’s performance is incredible but as with all engineering endeavors there is always a give and take.

This is a formula one race car. Arguably one of the highest performing vehicles on the planet. It’s also extremely technical to drive, requires incredible maintenance, a very well trained operator and probably costs more than the average home. It’s a very purpose built machine.

The .416 cartridge is similar in some ways. It’s performance is incredible. In order to take advantage of that performance potential the cartridges it fires must be loaded with very precise measurements of powder with a very specific burn rate. The projectiles must be weighed to very close tolerances and seated in the case to exacting depths. The operator must be flawless and they are very expensive to operate. Some of these problems are common to any cartridge if you are attempting to shoot at extended ranges. The problem of encountering ballistic inconsistencies at the ranges the .416 is capable of shooting are intolerable compared to most other target engagement distances. The difference between 1,000 and 3,000 meters is not three fold in complexity. The increase in difficulty is exponential. Take the .308 for example, with an effective range of around 1,000 yards. Some small variation in chamber pressure may not manifest in a miss. At 3,000 yards even with a .416 a minor variation in chamber pressure may manifest in missing by meters. The fact that anyone can do it, let alone in adverse combat conditions is more than impressive.

So, to explain why I chose the .416 in a little more detail.

In order to hit a target at extreme ranges you have to be able to accurately predict the flight path of the bullet. In any long range shot the rifle is not pointing at what you are aiming at. The barrel has to be aimed above the target. This results in a parabolic flight path to the target. Much like throwing a football.

You must have an accurate ballistic coefficient in order to predict how high above your target you need to point the muzzle.

Ballistic coefficient is defined as “a constant in ballistics that represents the efficiency of a projectile in overcoming air resistance.” The instant a bullet leaves the muzzle it is acted upon by gravity. If you fired one bullet on a perfectly level trajectory at ten feet above the ground, and simultaneously dropped a bullet from a height of ten feet they would both hit the ground at the same time. The bullet fired would have just travelled some distance. Gravity is constant. If rifle bullets were capable of maintaining a constant velocity the math would be fairly simple. Bullets however immediately begin to decelerate due to atmospheric friction. The energy required to move air molecules out of the way slowly robs the bullet of energy. The .416 leaves the muzzle at roughly 3,300 feet per second/ 1,006 meters per second, or Mach 3. At these speeds air almost acts like a fluid. A bullet with poor aerodynamic qualities would decelerate extremely fast at those speeds.

Ballistic coefficients are determined by measuring their aerodynamic efficiency against a standard model. For modern rifle shaped bullets the most accurate drag model is referred to as the G-7. This model is produced and extensively tested. Then when a manufacturer makes a new bullet it’s ballistic coefficient can be calculated mathematically by comparing its shape to the model.

Standard G-7 ballistic model.

This calculation is only a starting point. Every rifle is slightly different. Even two rifles produced by the same manufacturer may produce slightly different velocities or characteristics due to almost imperceptible variations. While a calculated ballistic coefficient will get you very close, you must fire your desired rifle/ammunition combination and observe it’s real world performance in order to develop a truly accurate ballistic coefficient.

For more details on ballistic coefficients and drag models:

There are many factors acting on a bullet besides drag and gravity but a bullet with a high ballistic coefficient coupled with a high muzzle velocity minimizes the the work the shooter has to do in order to compensate for these factors.

A more aerodynamic bullet retains its velocity better, which means it takes less time to get to the target. This in turn means that gravity has less time to work on it. So the shooter doesn’t have to adjust for as much elevation. Wind also does not have as much time to deflect the bullet resulting in less lateral correction.

Stability is also an important factor. Modern rifle bullets don’t naturally want to fly pointy end forward. The nature of their shape places their center of gravity closer to the tail end of the bullet. This inherent instability is addressed by imparting spin on the bullet by cutting rifling into the barrel. A series of grooves are cut into the inside of the barrel causing the bullet to spin at a prescribed rate

The twist rate is determined by many factors but essentially the longer the projectile and the smaller the diameter, the faster the twist needs to be. The bullet is a gyroscope in flight and the angular momentum from rotation is greater than it’s inherent desire to want to fly heavy side first.

A bullets rotational speed is directly connected to its forward speed. A twist rate of one revolution for every foot travelled would be listed as 1:12 or (one revolution for every 12 inches). This may not sound like that much spin until you convert it to RPM’s. The 416 for example would require a 1:9 twist for a 500 grain bullet that will be stable through transonic speeds. That comes out to 1.25 revolutions for every foot travelled. The 416 leaves the muzzle at about 3,300 feet per second, that’s 247,500 rpm’s. This is more RPM’s than is required to stabilize the projectile but it is needed so that the bullet will remain stable at range when it has significantly slowed down. Imagine a spinning top on a table. When it slows down and it’s angular momentum becomes less than gravity it will start to wobble and then fall over, a bullet will do pretty much the same and when you reach that distance all accuracy is lost

So again, a bullet with a high ballistic coefficient will retain its speed and therefore its stability for a longer distance.

Ballistic coefficient is only one factor in a long list of what makes an accurate long range bullet. It is one of the most important though and has a huge impact on many of the other factors.

I will compare the 416 Barrett cartridge to the .375 Cheytac (which I consider the very close second). The .375 can on the high end fire a 400 grain (25.92 grams) projectile at around 2850 feet per second. The 416 can fire a 500 grain (32.39 grams) projectile at 3,300 fps.

I picked one bullet manufacturer that makes exceptionally aerodynamic bullets for each bore diameter. The company is called Cutting Edge bullets. They have a product line called the lazer. The bullets are the same form factor for each bore diameter.

The 400 grain .375 caliber Cutting Edge lazer bullet lists a G-7 ballistic coefficient of .445

The 500 grain .416 caliber Cutting Edge lazer bullet list a G-7 Ballistic coefficient of .491.

500 grain .416 caliber Cutting Edge Lazer bullet.

While the .375 is an AMAZING cartridge and is capable of remarkable things. The higher ballistic coefficient coupled with significantly higher muzzle velocity of the .416 Barrett makes it the winner in my mind, if ultra long range hit probability was the primary deciding criteria.

Thanks