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I have some of the above bullets in .308. The SMKs I load with N140 just below max and as per Vihtavouri tables their max load produces 2712fps with N140 and 2750 with N540.
However for the Scenars their tables have 2648fps at max load of N140 but 2900fps with the max load of N540, a jump in MV of over 250fps.
If I want to use the Scenars for long range would I therefore be better to switch to N540 powder, as the 140 seems to produce a slower speed, even though it is a more "slippery" bullet?
Thanks for any advice.

I load 155 smks (both the 2155 & the 2156) and 155 Scenars with the same powder load of Tr140/Rs50. I achieve 2950fps easily with no pressure signs. The Viht Manuals loads are well known for being very conservative. When I used N140 the load to achieve 2950fps was considerably higher than any load in the Viht Manual.
The 2156 and the Scenars shoot to the same sight settings. The 2155s sight settings are slightly different.
All shot in a modern 4 lug target rifle with a 30" barrel.

Stick with the N140/TR140, RS50 I use and have done for many years, 46gr with no problems what so ever, pressure wise and achieve the velocities your looking for.
Bear in mind the N540 powder burns hotter and causes more barrel wear/ erosion

Thanks. I'll stick with the N140 and see how they compare.

Consider RS52. The FTR lot are shooting just this application and are raving about it.

Apparently it is producing the velocities while being less spikey. I am going to give it a try in my AR 77 loads (where again apparently it is shooting very well) and may even look at it for .303 down the line.

RS52 is a superb performer in 308 Win and similar applications. Think VarGet - if VarGet works well in a cartridge, then RS52 will work just as well or even better in some cases.

However, remember there are no free lunches in internal ballistics or thermodynamics, in fact usually diminishing returns for outputs v inputs. RS52 gets its high MVs partly because it is a 'high-energy' propellant, ie starts as a single-base extruded type and has nytroglycerin molecules infused into the nitrocellulose base. Same as the Viht N500 series - more than you might imagine in fact as Nitrochemie, Reload Swiss's manufacturer developed the infusion process and sold it to Vihtavuori to make its 500 series products many years ago. The other reason for very high velocities from Nitrochemie propellants is that the high-energy products also use the company's patented EI advanced infused deterrent process. This provides better and crucially longer effectiveness of the chemical 'deterrents' which slow the initial charge burn and pressure build-up. So, maximum peak pressure is reduced for a given charge weight allowing a higher energy input, and it is also maintained longer over a greater amount of bullet travel. In combination these effects allow startling MV increases from appropriate Nitrochemie powders- up to 200 fps in some combinations such as the Tubb 6XC with heavy bullets and RS60 / Alliant Re17 - but the overall heat input into the barrel chamber throat area increases substantially even if the pressures are safe. A longer deterrent effect also extends the physical throat area that peak pressure and heat are applied to as the peak period is extended over a greater length of bullet travel.

RS52's specific energy rating is slightly below that of N540 though at around 4,000 KJ / Kg (N540 is rated at 4,100) which reduces the heat input a bit. The untreated base nitrocellulose equivalents to RS52 and N540 are RS50 at 3,815 KJ/Kg and 3,720 for N140, very much cooler burning than the 'high-energy' numbers (and for that matter the ADI manufactured Hodgdon equivalents H4895 and VarGet which exceed the 4,000 mark by a large margin.) J = Joules of heat energy units.