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If you want a GOOD .50 cal automatic handgun, OP, you really should go with the Desert Eagle. .50 GI is crap. It's just a slightly scaled-up .45 ACP. Also, the only way to jam a deagle is if you use underpowered .357 or .44 ammo in it or never ever clean it (like US troops did with their "self-cleaning" M16s in Vietnam).
However, if you live in France, I'm pretty sure they (along with Italy, maybe Germany, and some other EU countries) have banned all .50 BMG and .50 AE weapons from civilian ownership as they use "military cartridges" (because teh army totally uses teh deagle brand deagle bro!!!!11).
I suggest you take a look at .50 Beowulf, .500 Phantom, .510 Whisper, .50 Peacekeeper, .416 Barrett (I know the M82A1 comes in .416), .460 Steyr, and .510 DTC rifles and .500 S&W and Linebaugh revolvers, and also old school African hunting rifles like .505 Gibbs and stuff.
Here's the kicker, however: any guns other than those using mainstream, common service weapon-based cartridges (besides magnums and .50cals, which can be pretty costly), are going to be expensive to shoot often unless you handload, with ammo being at least a couple dollars per round.
If you're really still interested in fiddy cow stuff, go read some stuff by JD Jones, John Linebaugh, and Teppo Jutsu.