I dont remember what show it was, maybe Gangland or something, but they were saying that very organized street gangs like MS-13 and the MM do send members into the military to learn about weapons and combat.
I think there is probably a ton of gang members that have joined the military in hopes of having a better life and many of them probably do. A lot of them are just gonna end up right back in the hood but now have military training and probably have seen combat.