All will easily kill elk size game at 100+ yards.
I would hope they will kill elk at 100+yrds!!!
Just messing with you, I know what your saying.
All have given some good info. The only factor I see hear is your desire to kill animals at 1600-1800 yards. That is going to require some major speed and energy to be retained at that distance. That puts you into the 330 Baer, 338 Allen Mag territory. Like Long Time Ranger stated, these are based on the 408 Cheytac case necked down to 338. Though the Lapua and Lapua improved will get you there, you are still going to need an extra push IMHO.
I just ran the numbers from the FTE Ballistics offered by apple for the iPod. At 1800 yards at 2950fps, at 1000ft elevation, 59 degrees, 78% Humidity, 28.857, you are going to be in the neighborhood of 1202fps, 962ft. lbs of energy w/ a come up of 61.48 MOA. The speed is well under the recommended fps and energy necessary for optimal performance from the 300SMK. This will vary due to temperature, humidity and most importantly your elevation.
Also there is a current thread where someone is trying to shoot 2285yrds. He figured that the 300SMK become unstable right at 1800fps. Though I believe Shawn Carlock shot a doe Whitetail at 1931yrds (if memory serves me) and got it on tape. He is a skilled marksman and very familiar with his equipment. He said it was one of those perfect scenarios that provided the ability to take the shot. So those are some things to think about.
The speed that I have posted is shot from a Custom 338 Lapua built on a 32" Douglas barrel. So the speeds you would be looking at for a 12 pound rifle would probably be more like 2850 to 2925 depending on length of barrel. Hope this helps. I think the Lapua is a great cartridge and would not hesitate to shoot it. I know from shooting it at 1070yrds, it sounds like a sledge hammer hitting the steel plate.