According to JP wiki the SSR weapon will be Shikigami fire gun, a weapon with Assault++.

Just wondering which would be better for my fire team, having multiple copies of 0-star SSR weapons to start leveling their assault skill? Or MLBing one copy as far as I can? Currently running Andromeda, Svarog, Acala, Amon, and Agni right now. I do have D'art as a secondary hero so a maxed SSR fire gun does sound tempting. Thanks in advance.