All classes can become Profiencient while paladins, rangers and multiclassed fighters can achieve Specialized. Only single or dual classed fighters can achieve all five levels and it is possible to become a Master in more than one weapon. At first level, the maximum number of slots that can be put into one weapon is two, limiting the warrior to Specialized. This means that a fighter must be at least 9th level before having sufficient slots to become a Grand Master. Specialization imparts additional bonuses when the warrior attacks with that weapon as outlined in the table below. The extra attacks are only for melee weapons and are in addition to those of the warriors' level.

From Play It Hardcore (says manual contains errors):

		Bonuses
Level	# Slots	To Hit	Damage	# Attacks	Speed
Proficient	1	normal
Specialized	2	+1	+2	1/2	0
Mastery	3	+2	+2	1/2	0
High Mastery	4	+2	+3	1/2	-1
Grand Mastery	5	+2	+4	1/2	-3

From Play It Hardcore, BG2 Tweaks Grand Master component:

		Bonuses
Level	# Slots	To Hit	Damage	# Attacks	Speed
Proficient	1	normal
Specialized	2	+1	+2	1/2	0
Mastery	3	+3	+3	1/2	0
High Mastery	4	+3	+4	1/2	-1
Grand Mastery	5	+3	+5	3/2	-3

From Planet BG:

		Bonuses
Level	# Slots	To Hit	Damage	# Attacks
Proficient	1	normal
Specialized	2	+1	+2	3/2
Mastery	3	+3	+3	3/2
High Mastery	4	+3	+5	3/2
Grand Mastery	5	+3	+5	2

There are discrepancies between this table and the pdf manual, and BG2 Tweaks ups the bonuses for Grand Master. So need to determine what the table really looks like and then adjust for BG2 GM tweak.