LC,
Many cartridges will get you to 1K.
I primarily use 1-8 twist barrel for 6mm and 6.5, and 1-9 twist for 7mm.
I use more 6.5's and 7's than anything for LR shooting and hunting.
A number of cartridges I use are not ideal for Encores, but I have used the 243 Win, 260 Rem, 6mm-284, and the 6.5-284 for shooting out to 1K and beyond.
I use the 6.5-284 more than of the others, with the 6-284 being second.
The 6mm-6.5x47 Lapua, 6.5x47 Lapua, and the 6.5x47 Lapua Improved (6.5 Long Dasher) will all do the trick also.
They will shine well in 15"barrels-They seem to out perform themselves.
These chambering are in MOA's and XP's so I will not give specific loads.