Nope, it's an external one at 16mhz. I think the internal one on the chip can only get up to 8mhz, and is accurate to about +-5%

It's enough to work for most things. Only annoying thing is, if you use the ICSP header to set the clock to external, you need a working clock to even use ICSP. (the 328pb falls back to the internal clock in this case)