My Police Dept has a old MAC 10 .45acp suppressor. I have been tasked w/ finding out if this "Can" can be mounted on a Marlin Camp 9 , 9mm rifle. I understand the muzzle will need to be threaded/ modified to fit this suppressor , but will the sound level be reduced to a signifigant level. Is it safe? Thanks for all replys.

If it's an original MAC/Sionics .45 can, its value to a collector would be enough to pay for a new 9mm can.  To answer your question, no, it wouldn't work very well.  Those cans only suppressed .45 18dB or so, and it will be less efficient on a 9mm.  Totally not worth the trouble, IMHO.

Thanks for the help , Question : What is the best , cost not a concern, suppressor on the market for 9mm?   How much retail?

9mm what?  A Camp Carbine?  If so, you'll probably want to look into an integral design built on your rifle.  Sound Technology does one of the quietest.  

besides you would have a hard time keeping it from threading off while firing because of the long reach. and the fact that 45 is so much larger than the 9 it would allow ALOT of excess high pressure gas to blast out upon exit of the bullet.like quietshootr said it would be more effective and easier to get a 9mm suppressor and especially something with finer threads such as 28tpi or more.
Jeremy