What Mr. Eatman said: unless you're fishing in the W. P. Sea, you're unlikely to need anything like a 20mm gun, & even if you had it, times are different now[1] & it wouldn't do you much good.
Oddly enough, I've been thinking about your situation recently, since I'm probably going to have to retire someplace w/ a lower cost of living than where I [barely make enough to] live now in So.Cal., and the PI have been high on my list of places w/ potential [avid scuba diver, Jacques Costeau's favorite dive spots were off Palawan in that general region...
So anyway, the seriously outgunned Philippine fishermen being harassed by a Chinese capital ship can only surrender or be a Q-ship intending to lose [aka: floating bomb with a massive radio-jamming capability, so the Chinese ship ties up alongside & the crew evacs while jamming the Chinese shore contact. One less Chinese military yacht, with nothing but an oil slick and a hole in the ocean to indicate what happened.]
Meanwhile, back at the ranch [shoreside], you'll be much better off fighting the guerrilla war w/ a De Lisle carbine and some home-made mortars.
And despite what someone else said about China just leveling an island w/ artillery, w/ 1/4 the world's population, they want lebensraum
way more than Hitler ever did, so they'd probably invade rather than expend the ammunition on unidentifiable targets. But that prospect may still be a few years off [at least, let's hope so!]
And 50BMG is about the limit of man-portable weapons: you get up into the 20mm size & it truly becomes a crew-served weapon [heck, even the .55" Boyes rifle was considered a crew-served weapon.]
Best of luck to you, and just remember, against an overwhelming occupying force, you'll be better served by a silenced .22 than a recoilless 20mm. Altho' in an imperfect world, a junk w/ an Oerlikon sure looks like a nice way to travel...
[1]
http://www.imdb.com/title/tt0048640/trivia?ref_=tt_trv_trv