Well, in theory anyways, all guns that are modern would not be obsolete within 5 years. They would have newer versions so like the M16A2 (Standard issue for Marines) would most likely be a M16A4 or 5. Along with some newer guns sure, but it takes a while for a gun to be developed, tested, and then sent to combat before it becomes standard issue. Even then it takes years for it to be phased in and other guns phased out.
I agree with you that they would be considered old guns but their newer variants would still be in use. specially if they are proficient with that weapon.