big, heavy
loaded
enemy
They succeeded in silencing the enemy guns.

anti-aircraft, anti-tank, artillery, field, hand (usually handgun), machine, semi-automatic
a.50-calibre/.50-caliber machine gun

.45-calibre/.45-caliber, .50-calibre/.50-caliber, etc.
stun, taser, tranquillizer/tranquilizer
laser, radar (= used by the police to check the speed of cars)
harpoon, spear (usually speargun)
glue (AmE), nail, pellet, spray, staple
fake, replica, toy
air
squirt, water (both AmE) (water pistol in BrE)
paintball

be armed with, carry, have, tote (informal, esp. AmE)
Look out! He has a gun!

load, reload
I loaded the gun with my last two bullets.

draw, produce, pull, pull out
He pulled a gun from his pocket.

brandish, wave, wave around, wield
She brandished the gun menacingly.

aim, hold, point, raise
He raised his gun, aimed and fired.

cock
He grinned and cocked the gun with his thumb.

man
Enemy ship approaching! Man the guns!

handle, use
fire, shoot
drop, lay down, lower, put down
holster (esp. AmE)
Jorge quickly holstered his gun.

silence

blaze, fire, go off
Guns were firing and grenades going off all around.

aim at sb/sth, point at sb/sth
shoot sth
This new gun shoots a laser beam at the target.

jam
be mounted
There was a machine gun mounted on the back of the Jeep.

control (esp. AmE), law
a state that has strict gun control laws
a gun control advocate

crime, violence
Anyone can be affected by gun violence.

culture
These kids have grown up in a drugs and gun culture.

safety (esp. AmE)
My big brother taught me about gun safety.

ban (esp. AmE)
amnesty (BrE)
The police pointed to the success of a gun amnesty earlier this year.

lobby
the powerful gun lobby in the US

shop, store (both esp. AmE)
dealer, maker, manufacturer
owner
ownership, possession
enthusiast, nut (esp. AmE)
club
show (AmE)
attack, battle
crew
emplacement, position
turret
carriage (esp. BrE)
barrel
rack
belt
They grudgingly took off their gun belts and holsters.

the barrel of a gun
I found myself looking down the barrel of a gun.

guns and ammunition
We're very short of guns and ammunition.

hold a gun on sb, hold a gun to sb's head
Two armed men held a gun to his head and made him empty the safe.

a/the smoking gun (= something that shows that sb has done sth wrong or illegal)
There's no smoking gun that they have found to date.

turn a gun on yourself
The gunman turned the gun on himself.

Gun is used with these nouns as the object:
engine