http://www.speer-bullets.com/education/primers.aspx

There are three primary chemical components that create a modern primer. The initiator is a percussion-sensitive primary explosive that will detonate when struck sharply. The fuel provides gas pressure and sustained heat, and the oxidizer provides extra oxygen to support burning in the closed case.

The explosive mix in the earliest primers was mercury fulminate.

The mercury residue in these primers attacked cartridge brass and made it brittle, resulting in case failures.

This situation was remedied in the late 1920's when lead styphnate priming was developed. This primary explosive left no corrosive residue, and meant that shooters no longer had to wash their guns. Today, styphnate priming is nearly universal.

Which primer is "hotter"....R209 or CCI209?

The CCI209 is hotter...

Hull: STS
Wad: Rem Fig8
Powder: 21.0 gr Green Dot
Primer: R209....8,800 psi
Primer: C209....9,700 psi

No, the R209 is hotter...

Hull: STS
Wad: Rem Fig8
Powder: 17.5 gr Clay Dot
Primer: R209....10,270 psi
primer: C209....9,020 psi

No, really, the CCI209 is hotter...

Hull: STS
Wad: Rem Fig8
Powder: 19.3 gr Clays
Primer: R209....9,000 psi
Primer: C209....11,400 psi