I usually have 2 loads I use with a given bullet. For the 220SMK, I use either H4831sc, or H1000. The H4831sc is faster than H1000 in this application, and seems to tolerate deeper seated bullets better than the H1000. Therefore, my "fits in the magazine length" load is 68.4gr or H4831sc. The origional load I developed this with gave me 2738fps. I have since switched lots, and the new one gives me 2706fps, but a better ES so im happy. The H1000 lot I origionally worked up the load with gave me 2638fps. The new one gives me 2692fps. This H1000 load is seated out longer to .010 off the lands and is used in a singleshot format primarily for LR target shooting, or "hunting" (usually varmint shooting) where I need a minimum ES.
A recent attempt to replace the H1000 "long" load with a H4831sc load of the same length got 2741fps with 68.8gr. All of this is a bit deceving if you are looking for max velocity, since none of these are max loads. These loads could all be pumped up a good bit. You can find published data up to 78gr on the H1000, and 72gr on the H4381sc load.
That being said, you really don't get much less wind deflection, or flatter trajectory when you go to the max loads, jusr heavier recoil, and shorter case life.
If you want to push it, you can break 2800fps with H1000 if your rifle will allow.
One other thing, that 1350yd group was not shot with out of the box 220SMKs. These bullets, and all of my long range bullets these days are sorted into like bearing surface lengths, then into base to ogive lengths within a given BS group, and finially meplat uniformed. This process was implimented in order to try and reduce flyers at this distance caused by inconsistant BC, and to lower ES as much as possible. Its seems to work, since I usually get ES for 9-12 shots under 10 fps with the H1000 load. I usually list 16 fps for this loads ES since it is the maximum ES if I combine all of the rounds with this load combo ever shot over the chrony in a single day. ( thats 64 btw).
One more sidenote while im on a tangent, that H1000 is very temp stable. Very Very temp stable.