I find my loads are most accurate if loaded under the light of a full moon, sitting under a live oak three that's been struck by lightning, with a blue-eyed calico cat sitting on my left shoulder, the blood of a fresh-killed rooster sprinkled across the tops of my boots, and sipping Booker's bourbon from a Dixie cup I stole from Wayne Newton's dressing room.
Nick, without getting all Bill Nye, the Science Guy on you, let me say that you already know your rifle likes the 168-gr SMK. A 3-shot "one ragged hole" group with a .308 @100 yards means a group of, at most, 0.34-0.35 minutes of angle. For an untuned factory rifle, that's a keeper, by anybody's yardstick.
And there's nothing wrong with the 168-gr SMK (at least until you get beyond 600 yards or so). There's been many a target shooting championship won shooting it. If your gun is cozy with it (which it apparently is), and unless you have some unstated material need for a different bullet, I see no point in spending resources you could put toward other purposes in auditioning different bullets.
The key to not screwing up that level of precision at extended ranges will be learning to load with a very low standard deviation of muzzle velocities. With the 168-gr SMK, at 600 yards, a 50 fps variance in MV will net you a 5" difference in point of impact. In which case, even if the shooter does everything perfectly, those wonderful 1/3 MoA groups will blossom to better than 1.1 MoA. Your group size triples because your MVs were too inconsistent. Bottom line: at extended ranges, low SD is king.
So if holding down costs is a key concern, instead of fretting over what components to try to shrink what already is an impressively small 100-yard group, my suggestion is that you stick with what you already know works well: the 168 SMK. And pick a powder.
Any powder, really, so long as it's widely reputed to play nice with the 168-gr SMK. The .308 is such a time-tested round, it's pretty well documented which powders it responds to best. Then work on fine-tuning your load and your loading technique with the objective of maintaining those tiny groups to as great a range as possible.
A lot of people hang their hats on these replica recipes for military sniper/match loads. The US Army calls its 168-gr SMK match ammunition "M852." This is an image of the page from the Technical Manual for Army Ammunition, TM43-0001-27 (Apr 1994), detailing the characteristics of the M852 cartridge:
This one is a little odd because most military loads prescribe non-canister powders, which schlubs like me and you can't get unless we break into a DoD arsenal. But this one uses a canister-grade powder, IMR-4895, to be precise, 42.0 grains of it. Unless you're shooting Lake City brass, the charge weight might need a little tweaking. I see suggested charge weights for M852 with generic brass generally ranging from 41.5 grains to 42.2. The published max charge weight for this bullet/powder combination is 44 gr, minimum is 41 gr. So 42 gr might not be all you can be right out of the box, but it's a fair bet it won't be more than a few tenths of a grain from it. But since it also is fairly mild (~50,000 psi), you've got lots of wiggle room.
Me personally, I don't load anything in .308 (bolt guns only) except Varget. Not because I've tested it out against everything else, but just because I was satisfied with how it performed and went no further. I get groups more like 1/2" (@100 yards) with FGMM, so you're working with better base accuracy than I am, but with my Varget handloads I can match that 1/2", from a 155-gr Lapua Scenar up to a 178-gr A-Max.
Not that I consider this group representative of expert reloaders, but they do have a conveniently searchable online database. So for general information, out of 47 user-contributed 168-gr SMK loads at
The Reloader's Nest, nine are using Varget, seven for IMR 4064, six each for VV-N140 and VV-N150, three for IMR 4895, and the rest are a hodgepodge of two loads each or less. I could load .308 from now 'till kingdom come and not have need of a powder that isn't on that list.
Since I already mentioned the 155-gr Scenar, let me say that it is my bullet of choice if all I'm out to do is shoot pretty patterns on paper. Despite its lighter weight, it has a higher ballistic coefficient than the 168-gr SMK. So even at the same velocities, it shoots flatter and bucks the wind better than the 168 SMK by virtue of the higher BC. All that with less recoil, too. But because it's lighter, you also can drive it faster than the 168 SMK. Which only amplifies its superior wind-bucking and increased danger space. But it comes by that high BC by virtue of a very long taper and high ogive number (3.3 versus just 2.2 for the 168 SMK). Which makes it unusually long for its weight, and so harder to stabilize. Rule of thumb with shooters with 1:12 barrels is they like to drive it at to least 2900 fps to guarantee stability. If you've exhausted your supply of 168-gr SMKs and want to give it a whirl, with your 1:11.25 twist, stability should be no worry at all. But there is no free lunch, and Scenars are a little spendier than SMKs.
