Ok, here we go. Get some 1/4" stud rod from Home Depot. If you have to make your flash hider, you'll want to get creative, if you have one... drill out 1/4 in, and recess the flash hider and gun body and do THIS:
5 min epoxy that to the gun...so its REALLY sturdy. Leave it alone. Let it cure for a day and don't mess with it. In that time, you can build your ODST Silencer. Make it so will slip over the flash hider you just epoxied on the gun, like THIS:
Get creative, i typically find PVC pipe that will fit over another piece snuggly, I would use styrene or similar to make the muzzle end of the silencer, cut out a circle the same size as your pvc silencer... then glue it on, sand it flush to the pvc pipe, and then drill a hole in the middle for the bullet path...
So now...I would slip on the silencer, make sure it fits, than find yourself a tap set at lowes/home depot for like 5 bucks, something like 1/32... small enough to not make a huge hole through everything, but be aware you're probably going to drill through that stud rod too... but offset the holes, tap them, and get corresponding screws of the same thread/depth and screw in the ODST silencer for added support.
Savvy??