They are also referred to as socked-on-the-nose cancels. What are they? You have to go back before the current crop of spray-on cancels to understand them.
For almost 100 years, beginning in the late 1800s up until the 1990s, the USPS used machine cancels to cancel the mail. The circular date stamp (CDS) was about as tall as the average stamp. And the cancels had some wavy killer lines to cancel the stamp.
If the stamp was placed just right, or the canceling machine slipped, sometimes the CDS portion of the cancel would fall squarely onto the stamp. Yes, some collectors purposely placed their stamps so that they would receive a bullseye cancel. Collectors would ask the local postmaster to run a blank envelope through the canceling machine so that they could determine the exact location of the CDS. And then they would affix stamps on other blank envelopes to get a bullseye cancel.
Collecting bullseye cancels on smaller definitive stamps is more difficult because the margin of error was small. Collecting bullseye cancels on commemorative stamps is easier because there is room for the cancel to drift a little to the left or right and still fall on the stamp.
There are lots of different ways to collect bullseye cancels. Collect a bullseye cancel from as many different post offices that you can obtain. Collect bullseye cancels according to their date. Those are just two ways to collect.
There is a Bullseye Cancel Collectors Club (BCCC). If you are interested in learning more about bullseye cancels, check out the club. Itís a challenging area and if youíre looking for something new/interesting to try, it is worth considering.
Now you know what bullseye (or socked-on-the-nose) cancels are. And you may have found a new collecting interest.