Re: Help with the AR-15 Rifle
Even if you don't intend building your own rifle from the ground up, I'd recommend you get a copy of "The AR-15 Complete Assembly Guide" by Kuleck & McKee and also a copy of the M16 Technical Manual (Army TM 9-1005-319-23&P).
Also look at some of the on-line animations of how the M16/AR15 family of rifles work.
This will help you understand the firearm you are buying a lot better and hopefully make it a safer proposition for you.
The upper and lower receiver assemblies slot together and are locked in place by two pins. Depending on the tolerances of the upper and lower the fit may or may not be tight/snug. If there is some slop between the two and this bothers you, there are ways to reduce it using products like Accuwedge, matched upper/lowers from guys like Mega Arms that have a locking screw or just use a chunk cut off a kids eraser.....
It is possible to completely assemble M16/AR15 rifles with a few simple tools. And many people do succesfully build their own rifles from individual components.
Whether the end result is as good/relaible/safe/accurate as buying a complete gun from a recognised manugfacturer or smith will, I suspect, depend greatly on the competence of the assembler, the quality of the components used and their ability to resolve any issues they may encounter in the tolerances/fit of any out-of-spec parts.
No offence intended, but I would urge extreme caution in the case of someone who doesn't really seem to understand the basics of the major assemblies let alone the mechanics of the system and the smaller components.
Go and buy a complete, tested rifle from a recognised retail outlet.
It's not worth risking your life trying to build a rifle you don't really know anything about.......