Why do 'Combo' weapons sometimes require only 1 weapon?

[V'ehxness]

Sorry, I don't wanna pile onto my other topic.

Basically, when I have the machine gun and the shotgun, the get fused to 1 single weapon.
But when I only have the machine gun without the shotgun, I can still create the combo weapon... making me effectively not lose a wepon, since I didn't have the second weapon to begin with.

[Follow The Faceless]

It's more like you slamming a new weapon from lvl-up onto the one you already have creating combo weapon. The caveat is that it has no related upgrades therefore underperforming in compare when you combine the 2 you have and fully upgraded.

Logically - it's a bug, But kinda "feature" as well.

[Relkin43]

I could be wrong but I think that is actually the achievement reward specific to each weapon for when it hits level 15