Im thinking about doing a 300 whisper can anyone give some advice dont know much about it im going to useing a mulitmount sup on it

im
interested in this also

Search for "300 AAC Blackout"

You can use any 308 rifle suppressor with it..
You can also use 9mm+ cans as long as it's only subsonic ammo.

300 AAC Blackout is the new standard for this round.  Check out this thread on the subject.

I have one I use a Hairtrigger Falcon 7.62 on and love it... get a 1x8" twist and you  can fire 220 and 240 gr subsonics. My AR has a 1x10 and handles 190 gr Hornadys great but keyholes with 200 gr nosler BT.  My 1x* bolt gun shoot anything!!   both are alot of fun and supper accurate and pleasent to fire.

If you're using a suppressor, you'll likely shoot mostly subsonic.  How much do you want to spend?  You can buy a short 300 BLK upper but it will cost you.  Alternately, you can buy a barrel and build it yourself.  You can contact Paladin Machine and they will sell you a barrel chambered in 300 BLK.  Price varies depending on what you want, ie stainless steel or not, adjustable gas block, etc.  Prices are very reasonable.

I do suggest getting a barrel with the gas block in the pistol position and an adjustable gas block.  It allows for a wider variety of loads to work.  You can also search on the quaterbore forums.  They have a dedicated 300 Whisper section.

I have a 10.5" barrel that I got from Paladin Machine and it runs great in semi and full auto with my SWR Trident and a 3 lug mount. It is probably my favotite upper, super quiet, very reliable and not too expensive to shoot with 147 grain FMJ's.

Another +1 for Paladin Machine, I have 2 of his barrels,  3 of his gas blocks. He can make you up anything you want and his prices and workmanship are excellent. Two of the gas blocks were custom made to order to my specific specs and he did not charge extra for the custom work.