Re: 155 Scenars 18" Barrel
I had hell with my 21" Noveske. It didn't like any of the 30ish different combo's of powder and seating depth that I tried for the 155 Lapua, or the 15ish I tried with the 168 SMK. Best accuracy I was getting was around 1.25 MOA and that was with IMR 8208 XBR. I could safely squeeze 2900 out of Varget in the 155, but grouping didn't work for me. I would assume with an 18" you will be around 2800 with a hot load of Varget.
Powders I tried with the 155:
Hogdon Varget, Benchmark, H414, BL-C(2)
IMR 8208, 4895, 3031
Accurate 2230
Ramshot TAC
Fastest: Varget, 3031
Most accurate: 8208 XBR, 3031
I have never worked so hard trying to get a gun to shoot, and ultimately failed. I hope that I just had a lemon barrel and you have better luck.