In Borderlands 2, how high level does the Chulainn gun scale in the first playthrough?

The Chulainn gun in Borderlands 2 is a unique weapon that scales to level 50 in the first playthrough. This means that the gun will have a maximum level of 50 and will not scale higher than that until you enter True Vault Hunter Mode or Ultimate Vault Hunter Mode.

When is the optimal time to acquire the gun for maximum effectiveness in the first playthrough?

The optimal time to acquire the Chulainn gun in Borderlands 2 for maximum effectiveness in the first playthrough is around level 50. This is when the gun will be at its highest level and deal the most damage. Players can acquire the Chulainn gun by completing the quest "Clan War: Zafords vs. Hodunks" given by Ellie in The Dust area. It is recommended to wait until you are close to level 50 before completing this quest to ensure that you receive a level 50 version of the gun.