Anything by Leonard B. Scott. He wrote several Vietnam war novels and the combat scenes are very intense.
Mathew Brennan wrote some Vietnam war historical accounts that are good. I especially Liked his "Headhunters" which is a collection of personal accounts of soldiers in the 1st of the 9th Air Cav.
Donald E. McQuinn wrote a series of three novels that have become my favorite post apocalyptic novels entitled "Warrior", "Wanderer" and "Witch". He also authored the "Honor Harrington" series.
Another Post Apocalyptic novel I like is "Wolf and Iron" by Gordon R. Dickson. Not as action packed as some, it's one that felt very real to me and is well written. I highly recommend it.
William C. Deitz has three novel series that are action packed. One is the "Imperial Bounty" series and another is the "Drifter" series. They are loads of fun, fast paced and laced with gun-play. The third is his "Legion of the Damned" series. It's a romanticized version of the "French Foreign Legion in Space". Plenty of military action. It also has cyborgs that are the brains of condemned criminals & hopelessly terminal patients in a box that can be plugged into big war machines.
If you like big, romping stomping robot war machines, look for Battletech, & Mechwarrior novels