Couldn't tell you because there is no way to know what kind of metal or material the cap is made of or whether there is anything else coated on it that could be harmful. Maybe it is safe but ghetto smoking devices have always been a bitch to me, makes it harsher than it has to be and usually wastes smoke. A metal or glass pipe and other smoking devices are usually available at head shops, flee markets, gas stations...CVS sells corn cob pipes for like $5 that can fit a lot of weed, hit smooth and taste decent.

Here are some other suggestions: http://www.overgrow.com/growfaq/1454