Such an open ended question...
What caliber and ammo type (handload, factory or match?
Semi-Auto, Revolver or single shot (single shot could be breech, bolt, etc...)?
Who is shooting it (accuracy depends on the shooter, not so much the gun)?
Is it free standing or bench (bench in hand or bench in vise)?
Scope or Iron sights?
I would tend to go with a single shot, 8" plus barrel, scoped handgun in a bench vise, shoot indoors at room temp being the more accurate