I just picked up my AAC 7.62SD this week(under 90 days
I was thinking of getting my PTR-91 threaded but was wondering how well the HK-91 platform preformed suppressed.
Any advise?

I don't have personal experience, but I've thought about doing the same with my CETME. People have told me that you might need to get bigger diameter locking rollers or whatever they are called (the cylinders on the sides of the bolt). They keep the rifle locked for a longer period of time, so more gas will exit the front of the barrel instead of blowing back in your face.

Other than that it should run fine.

I have been told that you should not suppress a rifle caliber gun with a fluted chamber.  I have seen it done, and it worked, so I do not know what the problem is or if its just an urban legend.

dunno the logic behind that.  It may damage the brass more since more force is pushing gas back into the action (they sure get dirty quick).

usually you'd use a diff. locking piece for suppressed fire, assuming one is available for your gun type.

a report I read stated the HK91 worked well silenced, i.e. not loud

You can suppress a rifle with a fluted chamber, but you should not shoot subsonic rounds in a rifle with a fluted chamber.  

You may have to change the locking piece, it depends what is in the rifle now.  The rollers have nothing to do with the amount of time the action stays locked, but they are directly related to the amount of head space your rifle has.  So if you change the locking piece before you shoot your gun check the head space.

the rollers are meant to stay locked during the peak of pressure, then to gradually move into the bolt head and allow the bolt carrier system to go into recoil.  The angles on the forward “wedge” of the locking piece dictate the timing of this event.  These are keyed to certain cartridge pressures and certain configurations of firearms.  All cartridges are not created equal, and any variation to the system as a whole may affect the timing.

Also when the locking piece is changed and the rollers a worn (made smaller in diameter from use) the bolt head will move forward giving the rifle smaller head space.  

The two common locking pices for a G3 are 45° and 50°.  The 45° locking piece is the standard and is probably what you have the 50° locking piece which is marked 18 is used in the short barrel G3K.  All this being said you will probably be ok shooting your rifle suppressed in its current configuration.

Why no subsonic rounds through a fluted chamber?  This is a build I'm interested in but would really appreciate knowing why I'm not doing something.

A fluted chamber allows some of the propellent gasses to escape out of the breech, thus allowing the possibility of a bullet lodging in the barrel.  Since subsonic ammo is radically reduced power ammo (10gr of powder vs. 40gr or more for standard) the round needs every little bit of gas and pressure.

Thanks for the info guys.
Here is a pic of the can anyway.

Anyone done this yet?
I put this on the back burner for 4 months and have not seen an HK 91 pattern rifle with a can yet.
Don't make me try it first

I have a 7.62SD and a mount for my HK, but haven't shot the combo yet.

is'nt the MP5 a fluted roller locked gun? Don't they work fine with sub-sonic ammo and supressors?

Have one for a few years. Worked fine for the last several hundred rounds.
To me it is not much dirtier than normal. Given the iron sights the POI shift in not worth adjusting for.