What I thought was a simple question turned out to be pretty complicated. It is difficult to say which one shoots the flattest until you look at the distance. Some go flatter on short distances and some go flatter at extreme ranges. This is because ballistic coefficient and muzzle velocity play a role in how flat a cartridge flies at what distance.
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The 338 370 Weatherby
In other words, the flattest shooting cartridge changes depending on the range. You would assume that every cartridge excels in its own field, but that is not exactly the case. But considering the question, the trajectory curve would be extremely important if you are shooting at long or even extreme distances. So, if you are looking for one that offers a relatively flat trajectory at all ranges, there is one to consider: The .338-378 Weatherby. Why?
First, we need to take a look at its main competitor: the .338 Lapua. It is popular because it was adopted by the military for long-range sniping. Being picked by the military immediately validates its quality and makes it incredibly popular. Other cartridges such as the .398, .300 Winchester, and the .50 BMG are popular because of that.
However, the .338-378 Weatherby deserves to be known, but it is not as popular not because it has done anything wrong. It just lacks that military association. Developed for long-range big game hunting, this cartridge is basically the .378 Weatherby necked down to house a .338 bullet, so you get a massive cartridge containing 114 grains of powder driving a smaller bullet.
The extra powder helps push the bullet harder, which translates into higher muzzle velocity, which means a flatter trajectory. Of course, that does not mean that you can keep loading up on powder and your bullet would fly straight on forever. There is a point of diminishing return here, but it can still push a bullet pretty fast. This is what you want since the faster the bullet gets to the target, the more energy you can dump and the more damage you do.
Muzzle Velocity
The .338 Lapua was developed to propel 250-grain bullets, but there is a shift toward 300-grain bullets recently. Federal’s .338 Lapua load comes with a 250-grain MatchKing bullet that offers a B.C (ballistic coefficient) of 0.606 and a muzzle velocity at 2,950 fps (feet per second). That is impressive, but the .338-378 Weatherby can do better.
According to Weatherby, you can get about 3,060 fps using 250-grain Nosler Partition load and this is confirmed by Hodgdon’s online handloading data. So, no false marketing there. Compared to any other cartridges with the same bullet weight and diameter, this is by far the fastest cartridge.
So how do these two compare in the field? Using a 250-yard zero, the .338-378 hits about 73 inches higher than the .338 Lapua at 1,500 yards thanks to that extra muzzle velocity. Simply put, the Weatherby has about 73 inches less bullet drop than the Lapua.
Another thing to consider is retained energy. Cartridges such as the .308 using 175-grain Sierra MatchKing bullet would only have roughly 340 foot-pounds of energy at 1,500 yards, which is about as powerful as a 9mm.
Energy
The .338 Lapua holds about 763 foot-pounds of energy at 1,500 yards whereas the .338-378 holds about 831 foot-pounds. At extreme ranges, you want to hold onto as much energy as possible, and the .338-378 Weatherby does just that.
At around 600 yards, the .338-378 Weatherby is going to be outclassed by other cartridges such as the 6.5 Creedmoor or the .220 Swift. But at 1,000 yards or higher, Weatherby triumphs. What is the downside of this cartridge? It only comes from Weatherby and they only make hunting-style ammo. There are three configurations to choose from.
First, you have one that uses a 200-grain Nosler Accubond bullet that delivers 3,380 fps muzzle velocity and a B.C. of 0.414. That is not too bad and can pull its weight for long-range shooting. Then, you have one that uses a 225-grain Barnes TSX bullet with a muzzle velocity of 3,180 and a B.C. of 0.482. Finally, the last load uses a 350-grain Nosler Partition bullet at 3,060 fps.
Conclusion
But wait, there’s more. If you want to squeeze out even more performance from this cartridge, you can handload it. This is pretty much a requirement if you are really serious about long-range precision shooting.
And there you have it, folks. The .338-378 Weatherby is, in my opinion, an overall flattest shooting cartridge. Of course, there is also the .50 BMG, but it is not fun to shoot for long periods of time with that recoil. It can offer an even flatter trajectory at even longer distances, but you would probably wish you picked up the Weatherby instead after a couple of shots. Smaller cartridges will obviously do better than the Weatherby at shorter distances, but that does not mean it is obsolete at that range either. It is middling in terms of performance, which makes it a perfect option.
