You can tweak the AI precision and skill, in your ArmA 2 profile file.

It is also possible to change them in your difficulty settings, but this is usually less accurate.

class Difficulties
{
	class recruit
	{
		class Flags
		{
			Armor=1;
			FriendlyTag=1;
			EnemyTag=0;
			HUD=1;
			HUDPerm=1;
			HUDWp=1;
			HUDWpPerm=1;
			HUDGroupInfo=1;
			AutoSpot=1;
			Map=1;
			WeaponCursor=1;
			AutoGuideAT=0;
			ClockIndicator=1;
			3rdPersonView=1;
			Tracers=1;
			UltraAI=0;
			AutoAim=0;
			UnlimitedSaves=1;
			DeathMessages=1;
			NetStats=1;
			VonID=1;
		};
		skillFriendly=1;
		skillEnemy=0.55000001;
		precisionFriendly=1;
		precisionEnemy=0.55000001;
	};
	class regular
	{
		class Flags
		{
			Armor=1;
			FriendlyTag=1;
			EnemyTag=0;
			HUD=1;
			HUDPerm=1;
			HUDWp=1;
			HUDWpPerm=1;
			HUDGroupInfo=1;
			AutoSpot=1;
			Map=1;
			WeaponCursor=1;
			AutoGuideAT=1;
			ClockIndicator=1;
			3rdPersonView=1;
			Tracers=1;
			UltraAI=0;
			AutoAim=0;
			UnlimitedSaves=1;
			DeathMessages=1;
			NetStats=1;
			VonID=1;
		};
		skillFriendly=1;
		skillEnemy=0.69999999;
		precisionFriendly=1;
		precisionEnemy=0.69999999;
	};
	class veteran
	{
		class Flags
		{
			HUD=1;
			HUDGroupInfo=0;
			WeaponCursor=1;
			3rdPersonView=1;
			UltraAI=0;
			DeathMessages=1;
			NetStats=1;
			VonID=0;
		};
		skillFriendly=1;
		skillEnemy=0.89999998;
		precisionFriendly=1;
		precisionEnemy=0.89999998;
	};
	class mercenary
	{
		class Flags
		{
			UltraAI=0;
			DeathMessages=0;
			NetStats=0;
			VonID=0;
		};
		skillFriendly=1;
		skillEnemy=1;
		precisionFriendly=1;
		precisionEnemy=1;
	};
};