You can make changes to your save file using gibbed me3 save editor download here so that the collector rifle is available in single player too:
open a save file with editor
select the raw tab
navigate down to / - 4. Plot /Player variables, and click little square button ... on the right of (Collection). a new window will pop-up.
press Add. look to the right at the table with two rows.
paste this into the Name field: SFXGameContent.SFXWeapon_AssaultRifle_Collector
change the nubmer zero in Value field to: 1
press Add to add one more entry
paste this into the Name field:
SFXGameContent.SFXWeapon_AssaultRifle_Collector.Fl
ags
change the nubmer zero in Value field to: 1
click OK and the window closes
save the changes to a new savegamefile by clicking save at the top icon bar.
Launch game, load the new save and play with collector!