fredj338:
It don't surprise me that your rifle that is throated for the 142 MK shoots the 85 grainer very well out of your 8 twist barrel. Both of my standard 260's are throated for the 142 MK's. I have never shot the light bullets through my F class match rifle, but I shoot them through my hunting 260, and they shoot superb seated at 2.710" O.A.L. I switched from the 85 gr. Sierra HP to the 90 gr. Speer TNT because of accuracy is as good as the Sierra, the B.C. is a little higher. I use 45.0 gr. of IMR 4064 in my rifle, in Rem. brass with a Federal 210M. The load develops 3460 fps avg. velocity from the #6 tappper 24" Krieger. That velocity was verified over two different chronographs. The largest 5 shot group I have shot with this load is .564", the smallest is .183". It is just very consistent from my rifle. I have seen no pressure signs at all. I don't know if you have tried them on coyotes or not but blows big holes in them.

As far as the 123's go I would try H4350, RL-19 or VithaVouri N140. A friend of mine uses VithaVouri N140 with good results out to 1000 yards at 2980 from his 26" barreled standard 260 F class rifle. It shoots good enough to shoot a clean score with a good X count if he does his job reading conditions. My match rifle just shoots so good with the 142's that I will stay with them. At the velocities you should be able to achieve with the 260 A.I. the 123 would be a great performer from the 1000 yard line.

Hope this helps some with the 123 Lapua.

RiverRat

Thanks RR for the input. I have tried RL19 a bit w/ the 123gr Lapua but accuracy is only so-so, right @ 1moa, but I haven't tried them out @ 300yet. Vel. is good though, running up near 3100fps. Like your .260, mine is shooting the 142grSMK right @ 1/2moa, going 2800fps+. Have you tried the 139grLapua in place of the SMK?