I rode motors early in my career for a few years in the warm months, when I did it we have the Kawasaki KZ1000 (Chips bike, mustache, does it get any better?).
You are thinking along the right lines when looking at possible issues when it comes to the vibration on the Harley as compared to other bikes like BMW's . I might suggest considering the Larue optic mounts with the locking throw lever, also I would suggest thread locking any screws on the mount and any others on the rifle as well (follow them up with a little paint for witness marks to give you a visual inspection reference to see if anything is coming loose).
When teaching Armorer courses, we always suggest that Officers shoot all of their carry ammunition (Rifle, shotgun, pistol) annually.
By shooting it annually, it gives a chance to:
1. Show that Officers have shot and qualified with their duty ammunition for court purposes
2. Show that the firearms are zeroed with the duty ammo, and that the firearm runs reliably on the duty ammunition.
3. Allows you an opportunity to shoot & replace everyone's duty/carry ammunition. This gets the old ammo off the streets, in theory old ammo could have been exposed to solvents, moisture, been deformed, etc, which could result in performance issues.
Carrying ammo too long in the field can result in catastrophic issues, where you could blow a gun up. As ammunition is carried, the gun powder (usually stick powder on .223/5.56 type ammo) will break down into smaller granules or the consistency of talcum powder, resulting in ammunition that will way over pressure and often times the gun will blow up at over 200,000-PSI . I would suggest replacing all ammo carried by your Motor Officers on a more often regular basis, as compared to Patrol Officers and the Office Dwellers whose guns are not as exposed to field conditions every day.
The other issued to consider is the the 11.5" barrel is way shorter as compared to 16"-20" variants. The shorter barrel means less velocity on the bullet being shot from it, which can result ballistic issues where barrier penetration and terminal performance is needed. Due to a shorter barrel, you will have a bullet moving slower, which can effect the terminal performance. If using a bonded bullet like the Federal 62gr Tactical Bonded, it may only perform on barriers out to 100yds, and after that it may not do so well. So you may consider using a dedicated SBR load like like the Speer Gold Dot 75gr, as it would probably perform better.
Greg Sullivan "Sully"