In a sci fi setting, which gun would be the most realistic, effective,

>>54072425
Laser and gauss weapons require a lot of energy, which means heavy batter packs or shielded cables. So if you want "muh realism" portable weapons, they're right out.

>>54072425
Aimbot level enhancements connected to high tech scopes on Slav shit rifle that shoots explosive autocorrecting midflight rounds.

A taser that shoots a stream of nanites to a target and then a electrical current runs through that nanite stream for a split second to deliver shocks. Technically ammo based because the nanites that shoot out get zapped as well.

And I dunno, gun that shoots globs of sticky matter that expands and then hardens over whatever it hits to ensnare enemies or use it to seal holes in pipes.

>>54072425
Which situation? Ground or space? Against which target? With what logistical support? What minimum level of training? Not to get too /k, but you asked an extremely broad question.

Frankly bullets and missiles might be the most sensible choices for a long time to come, even in a 'sci fi' setting. Especially in space combat. Heat dissipation is a huge issue with laser/magnetic weaponry.

>>54072425
Whatever the simplest way to defeat/penetrate the most commonly-encountered personal defense tech the setting has, of course.

>>54072425
Depends on the situation.

Laser is extremely accurate, and easy to use for long distance shots. Gauss is good for anti-armor and cover, but is basically overkill for soft targets. Homing ballistic would most likely be used by law enforcement to minimize any casualties from crossfire, because it would have to take at the very least a couple seconds to acquire a target.

depends how far in the future. Laser and gauss weapons unfortunately do have somewhat of a hard limit for the forseeable future.

-Laser; finding out how to deliver enough energy with light alone through a medium like air.
-Gauss and laser both; tremendous energy requirements.

creatively applied tech on existing ballistic weapons such as electrothermal-chemical propellants, caseless ammunition, low-impulse high ROF weapons (extremely rapid burst fire fire weapons that have delayed recoil, or no moving parts at all) could be completely plausible for the future.

Five seven
Kek-9
Gold nova

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Whatever is cheap to mass produce.
There is a reason 40k favors the Lasgun, mostly because its not a logistical nightmare. Even if Stub Guns are easier to manufacture on lower tech levels.

Also never underestimate exotic weapons. Sure the basic weapon is some complex prone to break mass produced Gauss Sling. But that doesn't mean ribbon pistols, laser edges, flak pistols, or silly multi systems.
Honestly: Flavor and names are far more important than function, even more so once the exotic weapons shows up.
For fucks sake, 9mm pistols might still be common, because no civilians is wearing body armor.

>>54072425
Small arms: Firearms. Lasers and magnetic weapons have prohibitive energy requirements. Lasers also have the problem of being able to permanently blind anyone nearby, including the user.

Larger ground weapons: Missiles. Lots and lots of missiles. Firearms, but with room for exotic weapons like railguns or sonic weaponry in niche applications.

Space: Debatable, but either missiles, lasers or mass drivers will dominate, depending on a number of unknowable variables. (Personally I think a mix of all 3 is likely, but with the dominant spot going to missiles.)