Shopper uses a set of rules commonly known as the PACK rules. Where PACK stands for:

Pile (In order to hurt the player who is last with a weapon in your inventory, damage must be spread to at least 1 other team. Generally the Pile rule is combined with ABL or KTL these days.)
ABL (Injuring any team with a weapon in your inventory. Do NOT injure the last team unless the Pile rule is used and you spread damage to multiple teams with a weapon in your inventory.)
CBA (Collect a Crate at the start of each turn Before Attacking with a weapon from your inventory)
KTC (Kill The Cow or person who is breaking the games rules.)

The ABL rule is sometimes replaced by KTL:

KTL (Injuring the 'leader' with a weapon in your inventory. Damage can be spread amongst other teams so long as the leader is injured in the blast.)

Around 2000, a new rule which originated from the Roper scheme made its entrance into Shopper:

AFR (Attacking from the rope when it is possible with the chosen weapon.)

The combination of AFR and the PACK rules then became known as the proPACK rules. Around 2003, the AFR rule got so integrated into Shopper, that the difference between PACK and proPACK was forgotten and AFR became a standard Shopper rule.

Source: http://worms2d.info/Shopper