Depending on a type of a scrap gun use weapon statistics in MF rulebook. But there are some changes:
Damage is one die type lower (for example 1d10 would be 1d8)
Range is only half of normal
Weight is 50% heavier than normal
It depends on the gun, but rarely self made guns have normal clip size. Roll for ammo capability % (1d10x10).
EXTRA RULE: UNRELIABLE
When rolling natural 1 on d20 when shooting or 1 on damage die, roll the following table for unreliable result.
1. Weapon jams. Repair as in technology roll rules.
2. Weapon breaks into parts. Cannot be used and must be literally assembled again from a scratch.
3. Doesn't shoot. Use 1 turn to remove wasted bullet.
4. Fires on random target - enemy or friend.
5. Accidentally fires twice. Roll again to hit -5 to d20.
6. Explodes. Weapon damage x2 on self. Cannot be repaired, parts and bullets destroyed.
|Pistol, black powder|
|Shotgun, from http://media.2oceansvibe.com|
|Sport rifle, from http://www.thehomegunsmith.com|