From Shilen's website:
.308
- 8" for bullets heavier than 220gr.
- 10" for bullets up to 220gr.
- 12" for bullets up to 170gr.
- 13"*
Ratchet rifled 4 groove
- 14"* for bullets up to 168gr.
- 15"* for bullets up to 150gr.
- 17"* for bullets up to 125 gr.