What's the odds of concealing the mp5
Machine pistols have +2 modifier to all concealability tests
So anyone trying to spot it will make perception test of (Intuition plus perception) +2 - standard guards have about 6DP on that tests, so they are guaranteed to spot it (having 2 hits on buy), and they are usually not alone, making it even easier for them to spot it.
you can improve your chances by actively hiding it (the lined coat would give you +2 to conceal IIRC).
So you make an opposed test of Palming + Agility (you have no palming skills o this would be -1 for deafulting) - giving you 4 DP (1 hit on average).
all of the above didn't even touched yet the metal detectors at the mall entrances and all sort of other security measures.
To cut things short. in your case, carrying it around and trying to hide is similar to waving it (obvious for anyone trained). I highly recommend you leaving it in car.