You ever hear the Marine credo "every Marine is a rifleman"? The other branches have a similar sentiment.
(To be clear, never served in a military branch. I did work in federal law enforcement for just under a decade surrounded by a lot of veterans who did serve in a wide range of MOSs. I've always been more of a talker of the Columbo variety than doorkicker, so I learned a lot of people's stories).
In U.S. and many other militaries and law enforcement professions satisfactorily completing basic training means they met a qualifying marksmanship standard (I really don't know, leaning to no that the sharpshooter and expert distinctions beyond qualified/marksman would merit +1s since these are marksmanship tests on controlled ranges), plus I think some basic tactical orientation, at least fed law enforcement gets it. That wouldn't be equivalent to the "tactics" skill per se, it's more basic learning how to shoot and move so as to not get yourself or your team killed through If you land in a non combat arms MOS or branch and/or work largely a desk/admin type of assignment, you still have to meet that standard annually (I know fed law enforcement does it semiannually or quarterly depending on agency and field office resources). And between the two you get what I see as a Gun Combat of 0. So in Traveller lifepath terms, without an event that would push or penalize that score, they'd finish that first enlistment term with a Gun Combat skill of 0. Pushes to that in lifepath I'd say derive between specific training in combat arms (maybe you were infantry, maybe you tried out for special operations, maybe your culinary preparation equipment maintenance battalion was deploying to the equivalent of one of those big bases in Iraq/Afghanistan and some Sappers took pity upon you all and taught some drills based on what they've learned from roadside ambushes, etc.) or actual combat experience.
Gun combat 0 is pretty easy to maintain if achieved through proper training so the non combat arms enlistee can deliver that 0 about mustering out. Beyond 0, most folks from that world I used to work with would say the sort of skillset that snipers, special operators, Air Marshals have is a perishable skillset, which is why those professions spend so much of their non deployment time at ranges and shoot houses.
A-school and tech school are just that, schools after basic. Some MOS will give you some cool skills, some ... won't. I think the event table on the lifepath reflects this more than adequately. Not sure about SEAL but I know in Army and Air Force special operations while there is a "pipeline" for Special Forces, PJs, TACP or whatever they're calling it now, that's around two years or more, that pipeline is often interrupted because of lack of school availability or needs of the unit, so you might not be "fully" trained for 3-4 years (Special Forces I believe are supposed to go to both SCUBA and HALO but it often takes a while to get around to it, Air Force TACPs are more complicated because they have to go to everyone else's school on top of theirs). So the sort of patchwork approach in Traveller I still find pretty grounded.