correct me if i am wrong but the gauge has to do with the thickness of the nail and the penny has to do with the length.
you have to be careful of the gauge because there are some guns that don't shoot different gauge nails.
good luck.
paul

there are 3 common sizes of finish nailers on the market. they are 18 ga,16 ca and 15 ga. the most common are 16 ga. which are larger brads "T" nails, usually 5/8 in to 2 in in length. they are used for trim,soffit, door casings, & etc. the 18 ga are smaller "T" or brads commonly known. the size are 5/8 to 1-1/4 in and are usually used in craft work. the most uncommon trimmer is a 15 ga nailer, which is known as an angle nailer, but is very good in many applications. overall, the 16 ga nailer is the most popular. I have all of these see my website www.toolcenrtal.com