For a modern military I would have to go with the M-38 Swede, in 6.5x55, a fine rifle with an excellent cartridge. Very accurate, pleasant to shoot and a proven game dispatcher. I prefer the bolt gun for a hunting rifle and I do not like the weight or length of the larger military rifles, to lug around.

For the antique or BP guns, I would select the big Martini-Henry 577/450. I have watched them work in many BP matches and they are wonderful rifles, fast lock time and very accurate. All you need is a Zulu here or there to pot.

On the Martini, according to Wikipedia, the 101st captured one of them in Afghanistan in April 2011.
Ed