Can't you bring in hi-cap magazines if it's for a competition on private property/range? You just can't leave the hi-caps there, at least that's what I understand(no sale, export/import/lending of).
I will try and find the publicaiton on it. OK, I am not a legal expert and would call CA DOJ to be sure.
CA Firearms Law DOJ
• Penal Code section 12280(k) provides limited exceptions for registered assault
weapons and .50 BMG rifles while on a target range or while attending an
exhibition, display, or firearms educational project which is sponsored by law
enforcement or a nationally or state recognized entity that promotes firearm
proficiency or education.
• Penal Code section 12280(m) provides an exception for out-of-state residents who
bring assault weapons or .50 BMG rifles into the state if attending an organized
match or competition conducted on a target range, club, etc., for the purpose of
18 California Firearms Laws 2006
shooting targets.