5.56er
First, what kind of metal are you blueing?
Second ....
I have had occasion to use cold blue (Birchwood Casey - Super Blue) and while cold blueing is certainly the least durable of the blueing options out there, it can give you a decent finish.
The trick for me is in the preparation. When sanding with wet/dry sandpaper, I went up to 1,000 grit (400 - 600 grit may be enough), then degreased the metal three times. Pay special attention to the part that tells you NOT to touch the metal with your hands after that - any oils (from your hands or otherwise) will cause the blueing to not take like it should.
I have not tried the Perma Blue yet (I still have it sitting with my supplies), so can offer no experiences with it, but in general would say pay close attention to the preparation phases.