Well don't give up on Range 15.
It's one of the few last places you can shoot from all shooting positions at 300 meters. Just seems like you need to plan ahead.
As for the firearm registration issue.... I did call the Provost and the Firearm/Vehicle Registration
http://www.lewis.army.mil/des/le_veh_registration.htmAs of approx. a couple of months ago. All firearms needs to be registered "on post" and even though this part (Range 15 was not on the secured part of the post... it is still considered Federal property).
This is a change as of last year. Did the post have a change in command? You know new leadership... new rules.
Over the years with different situations the post has ramped up security from time to time. So I guess if you want to make this long journey to shoot - I say just shoot at Tacoma, Paul Bunyon, Renton Fish and Game and/or go up into the hills.
I used to hunt and register my firearm on post when it was Ft Lewis but that was over 2 decades ago. No way I'm going to jump through that many hoops to go shooting.