Long barrel's don't make a gun accurate
Not true. While the primary benefit of a longer barrel is to increase bullet velocity (which usually does improve accuracy as a faster bullet reaches the target with less drop and less wind deflection) it also allows for a longer sight radius with iron sights which does definitely improve accuracy (if using iron sights). There is also some harmonic frequency stuff with specific lengths, barrel weights, and such, but at that point you are in the realm of extreme bench rest shooters.
I'd say the guard DID stop the attack. His presence seems to have triggered the suicide. It is a repeated pattern with these recent shooters that they commit suicide once encountering resistance (here, newtown, one of the recent mall shootings). This is one of the central principles behind encouraging widespread concealed carry by civilians. ANY armed resistance seems beneficial.