what kind of hamster needs a 4" dia tunnel? [:p]
Besides, for a propane-metered combustion you could always give the cover story of building a propane-injected pulse-jet engine for a combination AP Physics/Engineering project (that only works if you look like your in high-school)
Just say you don't need any help. If they try to kick you out because they think you are making a spudgun ask to speak to the manager and/or threaten an age discrimination lawsuit (if you are young.)
Unless you made known/given them a valid reason to kick you out of the store, (like say you are building a spudgun,) they cannot kick you out. (Unless they have posted, "we reserve the right to refuse service to anyone," but they still need some kind of reason, otherwise you can complain discrimination.)
If you don't want to go through the legal hastle of a lawsuit just talk to the manager and say you are going to register a complaint with the BBB (Better Business Bureau.)
Also, once you threaten any of these things they now have the right to kick you out and refuse to let you shop at that store ever again, so be careful of what you said.
Most of the time if you simply said that you don't need help you won't have any trouble. (Another option is to tell them that you are making a promotional t-shirt launcher but this only works with pneumatics.)
In short, do your research, know what you need to get before you go to the store.