I have yet to get my first AR-15 and I was just curious what to look for when buying a used model from someone (i.e. where to look for wear marks, is there anything that wear out especially fast, that sort of thing). Also if there ar any manufacturer specific problem you guys know of that I should look out for that would be great too. Thanks,

Luke

Some of the more readily seen stuff is whether or not parts match (wear or manufactures).  Pull the handguards off and see if there is some rust on the barrel (not a big deal, but guns should not be rusty if well maintained).  Look at the crown to see if the rifling is sharp and if there aren't any burrs or imperfections.

Check the chamber (again for rust...not all guns have chrome there, you know).  Look down the bore to see if it is clean and bright.  You can run a patch to see if there is rust.  Most used guns I've seen were not very well taken care of.

Take the bolt carrier group apart and check to make sure the bolt isn't pitted (bolt face) or cracked (cracks around the cam pin hole).  Make sure the firing pin isn't sharp at the end.  Perform a function check when rifle is assembled (unloaded, of course) by putting the rifle on safe and trying to pull the trigger, then set the safety to "fire" and pull the trigger, the hammer should fall.  Next hold down the trigger and charge the rifle.  When you let go of the trigger, you should hear the disconnector make a noise, then pull the trigger again.  The hammer should fall.

You can also ask round count, cleaning regimen, any malfunctions or jams and why, any modifications and why, if the rifle is factory or home built, etc.  All are things good to know.  If you purchase, strip the whole thing down and clean it and get familiar with the parts.  

If the gun was built from parts you could check the headspace.  I got an upper once and had its headspace checked and test fired at a gunsmith's for $5, but that's before I had my own gauges.

Anyone have anything else to add?

Not much more to add.  Make sure parts are from a well-reputed manufacturer.  check the bolt lugs for wear.  Obvious things, like the barrel shouldn't rotate in your hand-- and like "Z" said, the condition and cleanliness (bore, bolt, and inside the lower) of the rifle should give you some indication as to how well it was taken care of by its present owner.  

If you can test fire it, that is always a good option.
There are some things like out of spec parts or other issues that can cause malfunctions, that you might not see with the naked eye. Best bet when buying used  is to buy a 100% complete w/original parts rifle from a known maker like Colt, Bushmaster or Armalite. Be aware many rifles are "parts guns" of unknown origin built up on a name brand stripped receiver with no-name parts. Assume the worst (Chinese pot-metal parts) and pay accordingly in these cases, or better yet avoid these "mystery guns" altogether and buy only used rifles you can be sure are 100% original.

First things first, do you know how to field strip an AR15/ M16?

No I don't, if you have any advice I'd love to hear it. I do have a couple of the part charts so I know most of the parts you guys are talking about.  How do you strip it though?

Quoted: First things first, do you know how to field strip an AR15/ M16?

I was just thinking, does field stripping include removing hand guards?

Quoted: Quoted: First things first, do you know how to field strip an AR15/ M16? I was just thinking, does field stripping include removing hand guards?

In the Marine Corps,  when we field stripped our weapon, the handguards were the first thing to be removed. Some branches may not have taught the same thing though. Secondly, you pulled the rear pin out and tilted the upper receiver group upward to allow for removal of the charging handle and the bolt assembly. Next, you pulled the forward pin and seperated the upper and lower receiver groups. You then removed the buffer and buffer spring. afterwards, you carefully disassembled the bolt assembly (including the extractor). For the beginner, I would highly recommend having someone experienced to show you how. Pick up a good M16/AR15 manual to help. Good hunting for that first AR15.

Thanks alot for the info. Hopefully I get a chance to put it to good use soon.

Click on the Information tab at the top of the page here, and look under MANUALS, and download the TM9 (Operators) series.  Print it out.

It will give you similar info that I gave you, including parts nomenclature, what to inspect for, assembly/disassembly, cleaning and maintenance, and sighting in.  It's a very handy little book.

did not know you had to headspace an ar15 hmmm