I spent a month in Mexico two years ago and had no trouble staying 'current' with my shooting skills. For a nominal fee (say, ten or so bills US on the desk of the head LEO) I was provided with a permit (he called it a writ of exemption), directions to where I could go shoot with out being harassed, and the names of several people who would 'rent' me some toys to play with as well as some ammo. For another $750 I managed a machine pistol and 200 rounds. I had my choice of a selection of weapons, some of which I've never seen in person. One guy even claimed to have a G11 but no ammo (I'd guess casless 4.7mm is kinda hard to find in mexico...<sarcasm>). Given the Latino male's sence of bravado and braggadocio, I'd take his claim with a little skepticism.
Was it all legal? Most likely not. Was I alone when I went shooting? Does anyone remember the IRA training camp in the Libyan desert in "Patriot Games" ? (thats a no) Did I see any AR's down there? Yeah, but most were M-16s.
Just remember, we here in the US are particularly 'prudish' when it comes to obeying laws, especially in comparison to our South American brethern.