Second Best Options...

Let's face it.  The human body doesn't care about pistol rounds.  The ballistics from the subsonic standard 9 mil has been proven in both the lab and on the street that the human body does not stop and bow immediately to the pistol caliber, and that a rifle caliber is the preferred round when you need to stop the attacker immediately.  I'm not saying that the ballistics from the latest 9 mm defensive loads won't do the job, but when it comes to "He needs to be stopped right now", nothing says hold up a minute like a high velocity rifle round.
And then there's the stability the rifle platform provides and the added long-range accuracy as well as quicker follow up shots.  The long gun has it going on!    
But for the Motorman, a long gun isn't always an available option.
Face it.  As Motor Officers, we are everywhere.  We're in the school zones.  We're in the busy shopping areas.  We're running escorts and road blockages on the high-volume downtown events, and large-scale sporting events.  In other words, we are already present at what are deemed to be "Soft Targets" by evil people so inclined.  And because of that we need to always find ways to be as tactically prepared as possible, even given our space limitations on two wheels.
The reality is that not every Motor Squad gets the approval to carry the long gun on patrol.  Maybe the brass doesn't want to throw any love from the budget our way to upfit the bikes with the required equipment.  Maybe the brass doesn't want their friendly motor cops to look like they are ready to go to battle.  Unfortunately, we can't always rely on the Fire Department to "start extrication" to drag the bosses’ heads out of their respective asses, and even when they do policy moves slowly in most departments. 
But in the modern world we still have options! 
If we are dealing with a lack of space, three options come to mind.  The first and probably best option is basically a SBR chambered in 5.56 mm.  I’m thinking of a rifle caliber pistol, sub 9-inch barrel, with a folding stock or folding pistol brace.  That will bring serious firepower to the fight in a package that will fit in the smallest saddlebag!  But check with your Rangemaster about the legality of a personally owned stamped SBR on patrol!  Many departments prohibit officers from using personally owned SBRs on duty! 
The second option is straying into the pistol caliber carbine, specifically one with a short barrel (sub 9 inches) with a folding pistol brace that can provide stability like a long gun.  These Pistol Caliber Carbines are purpose built to provide good firepower in a compact package and while it may not be a rifle grade caliber multiple rounds of modern 9 mm all going where you want them because of the stable platform is definitely an option to look at.  I’m thinking PCC’s like the Kuna and CZ Scorpion as an example.  They aren’t cheap but they work very well!  With a longer barrel than your standard pistol they get a slight bump in velocity and add to the power of the 9 mm.
The third option and probably least thought of is the pistol chassis systems that are out there.   These options are super interesting to me as a motor man who might not want to throw a grand or more at a PCC.  Many of these chassis systems can be had for only a couple of hundred bucks and will use your pistol! I took an old Gen 4 Glock 17 that I retired years ago and threw it into the MX1 chassis system.  Now I have a great handling platform that is capable of accurately hitting targets at 50 yards or more with ease as compared to the pistol itself.  A pistol in a chassis system such as the MX1 or Flex system (many more on the market to look into) is a great way to make your pistol more effective at range with the ability to put multiple repeated rounds into a smaller target at longer ranges!
Do a little research, talk to your gun guys and gun dealers, and find out what options you have that will help you keep yourself and your community safe and give you the tactical advantage!
Ride Safe and Train Often!!

Joe