since the transfer is out of state, and they are peace officers out-of-state, they would be subject to the same rules as a citizen per ATF.
NV law requires a valid NV CCW to be NHP background-check-exempt along with a Nevada ID/DL. otherwise you have to submit to a NHP background check which is $25 and chances are you'll be denied with a CA DL since you don't meet the federal requirements to purchase.
ATF does not state anywhere that that a police I.D. is exempt from a NICS background check in any state that i know of. ATF also does not allow an FFL to sell a pistol to a non-resident, police officer or not.
ATF would not further allow an out-of-state (California) peace officer to purchase an AR-15 or other "non-civilian-legal" long arm, PD letter head or not. They could find the long arm they want at the show, and that dealer can send it to an in-state (CA) FFL and let that dealer handle the transfer.
don't know if this is accurate:
http://www.gunlaw.com/contraband.phtml
but it states that the only entity that can import high capacity magazines into CA are FFL's:
"Since January 1, 2000, high capacity magazines may not be transferred within California except to properly-licensed firearms dealers. The dealer may sell these devices out of state, or in the state for law enforcement use only. Importation or transfer of a high capacity magazine within California may be prosecuted as a felony."
i believe a CA FFL must have an additional state DOJ permit to sell or import hicap mags. a peace officer's ID would probably not serve as a legal exemption. imagine the instance where an off-duty cop is also a part time FFL and importing the mags into the state without a proper DOJ permit, it would be considered unfair to the FFL next door who doesn't have the permit but can't import them like the off duty cop can.
in reality, nobody is going to ask you questions when you purchase a high cap magazine. also, nobody is probably going to question an off-duty cop in CA with a hi-cap magazine. but technically, yes, it appears that an out-of-state police ID does not afford you any additional privileges that an ordinary CA citizen would have at the same gun show.
