Research by Chantox.

Big endian for Wii U, little endian for 3ds.
Weapon data is stored in the executable.

struct WeaponData_MH3U {
	int16 model;
	byte rarity;
	byte weaponVar;
	int32 price;
	byte Sharpness1;
	byte Sharpness2;
	int16 damage;
	byte defense;
	byte affinity;
	byte elementalType;
	byte elementalDamage;
	byte unk1[2];
	byte ailmentType;
	byte ailmentDamage;
	byte slots;
	byte unk2;
	byte unk3;
	byte sortOrder;
	byte unk4;
	byte unk5[3];
}

• Model: Weapon model.
• Rarity: Rarity level of the weapon, starts at 00 meaning Rare-1.
• WeaponVar: Weapon Variable value, controls Gunlance shelling, Hunting Horn notes and Switch Axe phials. Check below for a table.
• Price: Price in zenny of upgrading towards this weapon. Creation cost (where applicable) is 1.5x price.
• Sharpness: Seems to be split between two bytes, as this field is identical in Wii U and 3ds, in spite of the opposite endianness.
• Damage: True raw, value shown in-game is multiplied by a weapon-specific bloat value.
• Defense: Extra defense given by the weapon.
• Affinity: Weapon base affinity, signed.
• ElementalType: Element ID. Check below for a table.
• ElementalDamage: True element, value shown in-game is multiplied by 10. Signed, with the element being hidden for negative values.
• unk1: Always 0, so far...
• AilmentType: Ailment / Status ID. Check below for a table.
• AilmentDamage: True status, value show in-game is multiplied by 10. Signed, with the ailment being hidden for negative values.
• Slots: Weapon slots, in range {0,3}. Higher values are untested.
• unk2: -
• unk3: -
• SortOrder: Untested, increases sequentially through upgrade trees.
• unk4: Probably weapon color, untested.
• unk5: Always 0, so far...

Variable values

Switch Axe

Value	Phial Type
0	Power
1	Element
2	Paralysis
3	Dragon
4	Exhaust
5	Poison

Gunlance

Value	Shelling Type
0	Normal 1
1	Wide 1
2	Long 1
3	Normal 2
4	Wide 2
5	Long 2
6	Normal 3
7	Wide 3
8	Long 3
9	Normal 4
10	Wide 4
11	Long 4

Note: Not all the shelling values have been tested

Elements

Value	Element
1	Fire
2	Water
3	Thunder
4	Dragon
5	Ice

Ailments

Value	Ailment
1	Poison
2	Paralysis
3	Sleep
4	Slime