Broadly the two currently available classes of motorcycle gun racks for patrol rifles fall into two categories:
Racks that go inside locking hard cases, usually the factory saddle bags are goods for bystander safety. However it requires a very small patrol rifle. Short barrel rifles and folding stocks pose certain issues (read more). Worse these racks require an officer to dismount, and open a locked box, with a key, before having access. This takes roughly 17 seconds… that's unacceptable.
The exterior racks usually allow full size rifles. However of the available models we surveyed, all required the officer to dismount to access the firearm. They also generally leave the actions exposed, and thus pose a public safety, and vandalism risk.
In 2013 an officer in California with one such racks was conducting a safety presentation at an elementary school. Unfortunately a child was able reach the action, and discharge the firearm. Three students were injured, a media firestorm erupted, and the city faced legal action from the families.
Disadvantages of compact rifles required for saddlebag storage.
They cost more. Being able to utilize an existing equipment inventory is a huge benefit, Unless your department has bins of compact AR pistols sitting around, you're going to have to buy them. With ever tightening budgets procurement for such speciality items can be a nightmare. Short barrel rifles also tend to be pricy items. An AR-15 that fits in a factory saddle bag, and is of the quality and design for effective law enforcement use can easily run $2,700 – $3,200.
They're less accurate. When you require accurate placement of high-power rounds at longer ranges a short barrel rifle should not be your go-to weapon. All things being equal, an 11″ barrel is less accurate than a 20″ barrel, and affords less stopping power. Shorter barrels also produce considerably more more felt-recoil. The short barrel also reduces the sight radius making aiming significantly more challenging, particularly in high-stress situations. Red dot sights can mitigate this to some extent, however they add expense, weight and complexity some forces don't want.
They're less reliable. An AR-15 with an 11″ barrel will generally have a gas system less than 5″ long. That means the operating pressure of the gas system will run roughly 28,000psi higher than a standard rifle length gas system. This also means the gases operating the action are significantly hotter, and have had less time to burn off contaminants. As a result short barrel rifles tend to experience significantly more fouling, and potentially higher failure rates and maintenance costs.