Are weapon upgrades 1 time use for only 1 weapon?

Upgrades are only per single weapon, so you hafta decide what weapon is the one you want to use most and upgrade that one.

There enough upgrades scattered throughout the game to full upgrade every weapon, but you might not find them all without help. You should, however, be able to full upgrade most weapons without issue. Definitely upgrade your handgun ASAP, because you always start with that after you die.

Each weapon can get one upgrade in spread, speed, power, and capacity. That's it, unfortunately.

weapons to upgrade
default pistol because it can't be removed from inventory and has higher power then silenced one
sniper rifle~ if you have the wii u-pad down it works like the carbine but has more power
crossbow~ the arrows can be retrieved,and if you have the pad up it works like a sniper
tactical shotgun~ best of the shotguns
magnum~ extremely powerful
SA80 assault rifle~generally a double tap on head kills
ump sub machine gun gained after the nursery mission~same as the above assault rifle

adding up upgrade totals
6-5 on fire rate i believe the tactical shotgun can take this upgrade but I'm unsure
6 on accurcy
5 on ammo capacity - cross bow cannot take a capacity upgrade
6 on damage

everything else just adds space and takes the same ammo
each gun above is the best of it's ammo type

as gained in game
-----melee-----
cricket bat~melee starter weapon
-----pistols-----
default pistol~earned by looting john doe's backpack in beginning
silenced pistol~found in the bathroom in buckingham palace
-----magnum pistol-----
magnum~found in a shack by scan hacking the door in the beaconfields
-----rifles-----
carbine~found in a security room near entrance to sewers
sniper rifle~found on docks near dead guy
-----shotguns-----
double barreled shotgun~found in boarded room in apartment mission
Model 870 shotgun~found by scan hacking the gate near gas station
tactical shotgun~found by scan hacking the gate in docks
-----submach-----
mp5~found in a safe during apartment mission
ump~in nursery
-----assasult-----
ak47~found in raven's of dee room during tower mission
SA80~found in stash room at the end of church mission
-----misc-----
gun turrets~1 just outside of safe house/2 near entrance to sewers
gernades~found through out game during and after buckingham palace mission
flame gernades~found during shopping mission and through out game
mine~found in safehouse after shortcut and through out game after
soda-juice-energy drinks~found through out game
cake~found through out game
medium medpack~found through out game
big medpack~found through out game
animal remains~found in c4 camp, apartment and dropped by animals
newspapers~found through out game laying around
queen's letters~found in important areas during game
letters of dee~found in verious areas after starting mission
invisable item~useless item found in containers
-----ammo-----
pistol ammo boxes-for defualt-silenced pistol
rifle bullet boxes-for carbine-sniper rifle
shotgun shell boxes-for all three shotguns
bolts-for crossbow
assault rifle ammo boxes-for both assault rifles
sub-machine gun ammo boxes-for both sub-mahine guns
magnum ammo boxes-for magnum