looks like you got room for longer bullets. I doubt the 140 GR stuff will fit but a little heaver bullet looks doable. even if you had to seat them deep you likely would be ok. that round has more case capicity than the 260 REM so you should have case capacity to spare especialy in the lighter pressure loads. those 100GR'ers should to really move velocity wise. and that bullet certainly will work on deer.
Im researching bullets now for my 260 barrels for hunting and there is a pretty good selection.
Congrads on getting it to cycle and fire. what was your port size just for the record?