Surfing ebay I came across this upper.I have to admit it looks pretty damn cool.
[img]http://ebay2.ipixmedia.com/abc/M28/_EBAY_86466d1189e2c09234389ee109d4b8da/i-1.JPG[/img]
I know the can is fake, but it looks cool regardless.

Does the fake can have to be welded on or what?I've been trying to find some info on google regarding this but can't find any.

Here is the auctions description:
THIS IS A BRAND NEW DPMS 11.5 MACHINE GUN UPPER WITH A 5.5 INCH FAKE SILENCER ATTACHED. YOU CAN HAVE IS WITH A BAYONET LUG OR WITHOUT WHICH MAKES IT POST BAN LEGAL. IT HAS THE BIG M4 HANDGUARDS AND COMES WITH THE CHARGING HANDLE BUT NO BOLT. IT IS A CHROME MOLY BARREL WITH A 1 X 9 TWIST. NEW UNFIRED. BUYER PAYS 12.00 S/H I ACCEPT PAYPAL AND MONEY ORDERS ONLY. WILL SHIP TO USA ONLY! GUN CASE IS NOT INCLUDED.

I believe without lug it's OK, so long as fake supressor is welded, making it also comply with Short Barrled Rifle regs.  

if it has a threaded barrel, then it will have to be pinned / welded.

i thought i read where ATF says that a suppressor is a flash suppressor also.

would this apply to a fake suppressor? hmmm...

be cautious on this one.

If it is a fake can then it has no flash hiding capabilities. If it was permanently attached & no threads you are ok. The bayonet lug would have to be ground off though.

Most fake cans now are just solid aluminum stock that has a bore slightly larger than the firearm.  They do not significantly reduce flash or noise, nor can they be easily modified to do so.  I think some early fake cans were basically the real deal without baffles.  Since they were big expansion chambers, they did help hide the flash and reduce muzzle signature by a few dB.  BATF said that was a no-no, if you're interested in the interpretation of law by tax agents.