What is the dif between CCI 300 and CCI 350 primers? Can I use the 350 in .44 special cowboy loads?

300's are Large Pistol

350's are Large Pistol Magnum (more explosive to ignite the greater amount of powder)

Not sure if you can use them in the 44 Special though.

In practical terms, Ricci is pretty much right (although he failed to mention the price difference.)

You should have no problem in using a "magnum" pistol primer with any pistol load. As a matter of fact, I think they are of more benefit with light loads than will full-house loads. There is no pressure worry with the light loads. Watch it if switching primers in loads that are near the maximum working pressure.