Ok here are my thoughts.

I am thinking of having one of my AR barrels threaded to standard colt threading to accept a gemtech 223 pirhana.  

my delimma is, that i have also wanted to suppress my ruger 22 pistol for a LONG time...much longer than i have even owned the AR.  

So my question is...is there anything keeping my from having BOTH my AR barrel AND my ruger barrel threaded to the same standard 1/2x28 pitch, and swapping the 223 suppressor between the two guns???

Obviously the 223 suppressor will not be AS efficient as a true 22 rimfire suppressor on the 22 pistol...But i can imagine that it would still work pretty darn good, as 22 isnt super loud to begin with.

has anyone ever done this??  Is there any reason that i CANT do this??

Good question.  I would be interested to see if anyone has done this too.  I would think the .22lr would tend to GUNK UP a good suppressor.

well, i would also assume this...but if im gonna spend the money on the can, i will be pretty dilligent in the cleaning and maintenance of it.  Even if it gets gunked by the 22 crap, a bath in solvent would fix it right up wouldnt it??  

I know next to nothing regarding the cleaning and maintnance of suppressor cans, so i am just guessing...but i assume that regular cleaning will keep it working correctly.

It will work fine.  You are correct however that the suppressor will not be as quiet as a good 22 can.  The baffle stack is setup for 223 pressures (most 223 cans today are pressure-driven cans) and a 22lr just won't be able to make it happen.  It'll still be quiet though.  Also, you would probably want to dip it more often.  223 cans are almost all sealed cans so the only way to clean them is dip them insolvents or use an ultrasonic cleaner.  I just went ahead and bought different cans (heck, I own eight).  Went with an AAC M4-2000 QD can for the M16's and an AAC Pilot for the 22's.  A 22 with a good can is movie quiet.  The sound of the bullet hitting the target is louder than the gun!  Really cool!

if you have the resources, would you mind swapping one of your 223 cans onto one of your 22lrs and report your findings to us???