I'm shooting a .308 Remington 700 SPS varmint in an AX stock, 26" barrel, 1:12 twist off of a bi-pod and rear bag (photo below).

The rifle seems to have quite a long throat, I've measured it at approx 2.94" using an empty casing with a bullet head pushed loosely into it (I've read that Remington .308 rifles generally have long leades) so I've assumed that loading close to the lands is going to be problematic.
I currently only get to shoot centrefire at Bisley - mainly 100M & 200M on Siberia and 600y on Century. I don't yet have enough confidence/skill to shoot any further!
Varget
I started by using the Lee manuals min recommended load for Varget (42g) with .168g SMK, PPU brass and CCI LR primers. I made up a batch at SAMMI length of 2.80" and went from there. I've experimented with different varget powder weights out to 45g and different lengths out to 2.83" and shot these at 100M @ 200M (no signs of pressure on any of these) . The best combination @ 100M so far has been 42.7g @ 2.82" (see below - I pulled one shot waiting for the stupid wind to die down! - squares on target = 1")

Vit 550
I couldn't get hold of any Varget, so I went halfs on a 1KG tub of N550 with a mate from my rifle club. I used it with some .168g SMK and .168g Hornady A-Max heads, PPU brass and CCI LR primers @ 2.82". I guessed that 42.7g of Varget was giving me approx 2557fps (I don't have access to a chrono) so I worked out that approx 44.5g of N550 should give me roughly similar performance @ 2554fps (using the figures in the Lee manual). I used these on a club visit to Short Siberia 100M range and both the SMK's and the A-Max bullets shot very well. If fact the center of the target got shot out and the markers had to put up white patches on a different part of the target to shoot at?!
IMR3031
I bought 1lb of this simply because my local RFD didn't have anything else in stock! I loaded most of my SMK .168g with 39.2g @ 2.82" & 2.83" to give me approx 2521fps. I also loaded a few more in batches up to 41.5g. I stuck the rest of the powder behind some .155g Palma match I'd bought to try. Results with the .168g were okay, the best seemed to be at the lower end of the scale, around 39.2g @ 2.82". It didn't like the .155g at all and failed to shoot any type of group.

Summary
I've stuck by and large to using .168g bullets, (mainly SMK's) PPU brass and CCI primers. I've read that .168g bullets should be okay out to 600 yards, although I have bought some 175g to try. I've now run out of CCI primers and could only get Federal as a replacement; I have however bought 1000 of these, so I should be okay for a while.
My main problem has been with getting enough powder to work up loads and experiment with. As of today none of my local dealers have any Varget in (I see Kranks have some back in stock, but it's very expensive once you've added the transport costs). I'd love to be able to just buy a load of Varget and carry on, but apart from the steep price of the stuff the supply of it into the UK seems a little patchy?!
There's a tiny bit of N550 about, however I've read that using it can reduce the life of your barrel, so am debating whether to get any more. I wasn't really that impressed with the IMR3031 (and my local store has run out any way!) .
I've tried to find some N140 but no-one local has any. I've contacted the suppliers of TR140 and am contemplating getting 2KG of this next time I head down to Bisley. In the mean time I've bought 1KG of N150 to try with .168g & .175g bullets.
I realize that this is all a bit random at the moment, but unless I can find a decent supply of powder I'm forced to buy whatevers available at the time. I'm hoping I can sort out a decent supply of TR140 that can keep me going through the summer. I've read some encouraging reports about the stuff and so hopefully I can find a load that my rifle will like.
Questions
Although some of the loads detailed above shoot well at 100M, I've found that they can be a bit hit and miss at longer distances. I've noticed that the necks on the last lot of cases I've used were a little 'sooty' and have read that this could be a sign of low pressure? Do I need to increase my powder weights for longer distances and to increase the pressure?

I've tried to stay at least .5 grains below max load for safety's sake, although when I've shot at 100M the bullets with more powder in had a larger spread so I've generally stuck with lower loads as it produces better accuracy at this distance.

None of my loads have shown any real signs of pressure, although I had a slightly stiffer bolt when using the higher loads of IMR3031.
My groups got worse as soon as I increased the length from 2.82" to 2.83". Do you think it's worth me experimenting with 2.84" and above (as long as they'll fit in the magazine!) to try and get the bullet closer to the lands?