For 3Gun, CRC has the longest local shots, out to about 200. Weld a little less, Pike's peak less and Lake George less. Raton is pseudo local, and they might stretch to 400 or 500.

Irrespective, for the vast majority of shots in all of 3Gun, a 16" only hurts a tad more than an 18" if you don't know your data past 200 yards. If you want to build a 16" mid-length gas gun, you will be fine.