There are rumors that they have put a Mini gun somewhere but so far I havnt found it. There arnt as many weapon pick up as on the last GTA. When you do the bank job getting weapons will become much easier. You'll know what I mean when you see the money you get from it
Goto the TW@ internet Cafe, go on one of the terminals and go to www.whattheydonotwantyoutoknow.com - apparently that is an easter egg site giving you maps to secret places and weapons that rockstar put it - apparently
you may already of known this but if you wanted a desert eagle early on in the game before it become available to buy then just get a pistol with as much ammo as you can , go to the gun dealer and simply kill him , it may be the gun shop on the third island , when you kill him he should drop the eagle and your glock ammo transfers when you pick it up. hope you find this helpful - the game is f##king amazing aint it!!!!!!!!!!!!!!!!!
There is a pistol located at the top of the Fire escape stairs crosswalk on top of the little theatre building (where all the cabarets are held) that's just around the corner (not literally) from where your first safehouse is at the start of the game. I only found it by chance but you can keep going back there as it constantly reappears!
I have the guide to go with it, and it mentions nothing about any secret weapons. I havent used the cheats, but i have heard that they only unlock the usual weapons, 2 pistols, 1 rocket launcher, no minigun or heat seeking rocket launcher that we saw on San Andreas. I'm pretty sure there is no chainsaw/katana etc, perhaps a bit disappointing but it'll be an addition perhaps in future games or in the expansion content due out on the 360.
Not the greatest of weapons but opposite the first house you get in the game there is baseball bat hiding in a dark corner. Also this is quite hard to explain as i have no pictures to show but there is a secret/hidden car on the last part of liberty you unlock, if you look at the top of the island there is two roads linking off from the main road into the middle of no where follow the road on the left and it takes you to a deserted house, go behind the garage and there is a car hiding in the bushes, happy driving :) and batting :S lol