If you want a 5.56x45mm NATO semi-automatic rifle that is substantially shorter than either the AR-15 or the Ruger Mini-14, look no further than the IWI Tavor. This is a bullpup magazine located behind the trigger, making the rifle significantly shorter and more nimble for tight conditions. It also accepts AR-15 magazines, which are commonly available and further increases its overall versatility.
IWI Tavor (8/14)
Submitted to "The Best Semi-Automatic Rifles of all Time"