Ohh Crap, you got one of those reject flu collars!
Just teasing mate
The flu collar is 6" outside to outside (that's right), it may appear that your 6inch stove pipe simply cant fit over it
... but it does!
a couple things to do... get some emory cloth or steel wool or even your kitchen scotch brite pad and make extra sure the outside of that flu collar is smooth as a babys azz before trying to get the pipe over it.
Make sure when your trying to get the pipe over it that your feading it on perfectly strait on (no coming from an angle), wipe the outside of the flu collar with a rag and veggie oil to get every lil piece of dust of it, you may need to squeeze on the pipe a lil here or their as your trying to get it on... once you get it.... you got it! after its on you will think (OMG, I don't need to put a screw in this at all because its going nowhere
... put the screws in! and hope you have a removable baffle or clean out Tee!)