50 Cent: Blood On The Sand

Due to the nature of this achievement; Solution for Millionaire In 50 Cent: Blood on the Sand it's a good idea to work on this achievement after having unlocked the Millionaire achievement, you can either play through the campaign naturally a bunch of times but a much quicker way is to farm money on Mission 4: Chapter 2, I won't go into detail though, the full explanation can be seen in the solution I linked above or the video I've linked below.

You need somewhere in the region of $1.5 million for all the upgrades, which isn't much more grinding so I just got that amount and then got all the unlockable achievements in one quick go!

Once you've farmed enough, find a telephone booth, they're scattered throughout nearly every level and use it, then from there go to the shop menu and just buy all the unlockables!

* ^{Credit to xCheexbrgr for the video}

Unlock means buy. Use a payphone once you've got some money together and buy all the pistols. Only 3 more to buy and pop, it's yours.