ONE GUN?

[BullMagnet76]

7mm Remington Ultra mag, 300 RUM, or even better .338 RUM, the 300 has more factory ammo availiable

[Huntbear]

338 WM....  with 225 gr. Accubonds it shoots as flat as a 7mm mag with 175 gr. bullets and hits a hell of a lot harder.

[coachcw]

.300wsm with 190 grain accubonds of the three . that being said a 30-378 would be the best bet . I'm building a 6.5 x 284 and have the confidence that the 140 vld will get the job done .

[wraithen]

All of those rounds will do it. Sounds like you just want some justification so go huge 

[superdown]

325WSM 

[hub]

Thankfully, a one gun rule does not exist. If it did my pick would be the versatile,flat shooting,light recoiling, light weight venerable .270 Win.
From the list 300mag

[Biggerhammer]

300 Win Mag

[elkinrutdrivemenuts]

Wsm are for hipsters, cutting edge my butt!

Gimme a 300 wm anyday and I'll knock down anything in north America within 600 yds.  That's the go to gun.

[Biggerhammer]

.300wsm with 190 grain accubonds of the three . that being said a 30-378 would be the best bet . I'm building a 6.5 x 284 and have the confidence that the 140 vld will get the job done .

Ya big tease!!! I about fell out of the chair, when I read "190 Accubond". Can't seam to find one anywhere? 

[buckhorn2]

30-378 wby mag.

[mfswallace]

7MM RM

[netcoyote]

Geez...I thought you said a BIG gun....

.45-70

[Raul Duke]

Big game, one choice,  gotta go with my model 70, .308

[scottfrick]

300 RUM topped off with a Swarovski 4-12x50 Scope

[Jason]

300wm..I've been hunting with one for as long as I can remember and probably for a long time to come