A 3-9X40 scope has variable magnification from 3 to 9 times, and a 40 mm objective lens.
For quick, short distance use, you want low magnification.
For aimed fire at longer distances in darker conditions, you want a larger objective.
A quick check at Natchez Shooters Supply turned up the following options.
Weaver 1.5-4.5X24 Classic Extreme Riflescope Ill. German #4 Reticle Matte
Weaver 2.5-10X50 Classic Extreme Riflescope Ill. German #4 Reticle Matte
Burris 3-9x40 Fullfield 30 European Riflescope 3P#4 LRS Electro-Dot Reticle Gloss
Burris Fullfield TAC30 1-4x24 Tactical Scope Illuminated Ballistic CQ 5.56 Reticle Matte
I haven't used any of these scopes, but I have used other Burris and Weaver models. Any of these scopes is likely to work very well, and will be a big change from iron sights.