Hopefully this isn't too much of a noob question... Are the rail sizes different? I have a light that I use on M&P and Ruger Handguns and I tried to mount it on the rail on my AR. I couldn't get it to completely tighten down. Put it back on the handgun and it tightened down properly. I thought they were all the same? Thanks in advance....

Which light and which Ruger pistol?
The M&P has a picatinny rail, the SR9 has a universal/Glock rail which is a different size than picatinny.

AR Picatinny rails are 1913 Picatinny rails.
Many handguns use the same spec for their rails, but many do not.
The difference in your situation is the Ruger does not have 1913 Picatinny spec for that rail section.

Quality lights will come with different mounting keys, pieces that change out to accommodate different shaped rail slots.

Yeah, I went back to the original package and found the different rail mounting hardware. Once I put the correct mount on, it tightened right up. I was previously using it interchangeably between a M&P 22c and a Ruger SR22.