As mentioned in Hugh's post there is no U.S. registration requirements of firearms. In fact, the NICS laws specifically forbid the U.S. from requiring registration that is why there is a time limit set as to how long the FBI can retain firearm transaction records. However, there is no such restriction placed on the individual states. You can bet that the State of Wisconsin keeps a permanent record of the sale of handguns. Is that considered registration?
Because of all this gobbledy gook it is extremely important that when a firearm sale takes place between two private individuals that the seller fills out a very detailed receipt, including, the make, model, caliber, type action, serial number, date (and even time) as well as the name and address of the buyer.