I recently went shooting with my FIL, BIL, and some nephews at the local (outdoor) shooting range.  We mostly shot various .22lr rifles, but I brought along my AR and a loaded magazine for some variety.

Well, while shooting my AR it bumpfired on me at least twice, shooting two shots each time.  When my FIL tried a few shots it bumpfired on him once as well.  Both of us were seated with the buttstock pulled into our shoulders.  Needless to say I was concerned not just for the rifle but potential legal issues because it was a city managed supervised range.  I didn't want anyone reporting an automatic firearm.  Fortunately the range supervisor was very understanding about it.

Anyway, some details about the AR.  It's a CavArms lower with a stock DPMS trigger assembly.  The upper is a complete CMMG midlength upper, including bolt carrier group.  The trigger has not been altered in any way from its original installation.

What more, I've fired this AR once before without incident, the only thing different was I previously used a C Products aluminum magazine and this recent time I used a Magpul magazine.  Both times involved brass cased domestic manufactured ammo (Winchester or Federal).

I hadn't added any lube to it since I last cleaned it two or three months ago (though I had lubed it at the time I cleaned it).  I don't know if that could be a contributing factor.

Anything particular I should look for before I take it to a gunsmith to check?

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Thanks for all the input and help.  While locking back the hammer I noticed that it was locking with a slight tilt.  Turns out that the hammer pin had "walked in" on the left side so the left side of the hammer was "floating" though the right side of the pin is still holding.

That explains the problem.  Now I have to look into what it's going to take to get it fixed.  I had a friend do the LPK install and I don't have the know-how to take it apart and try to fix it myself.

Function check the FCG per the TM.
The usual cause is the disconnector, disconnector spring, or improper assembly.

The first thing I would do is check the disconnect.  Not sure how familiar you are with the trigger assembly on your AR.  I am assuming it has a "Stock" AR style trigger.  The disconnect keeps the rifle "semi-automatic".  How to check the disconnect.  Seperate the upper from the lower.  The disconnect is The piece right above the trigger.  Release the hammer to go up into the "fired" position.  Pull the trigger and hold it.  When you lower the hammer it should catch on the disconnect.  When you release the trigger the disconnect should "let go" and the hammer will "click" into position and be ready to fire.  Repeat this quite a few times and see if the disconnect fails to catch the trigger.  If the disconnect fails to catch the trigger at any time this is most likely the source of your "bump" firing.  You can get a new disconnect and replace it yourself quite easily.

It could be your secondary sear. If they get a little short, this will happen.

Great walkthrough.  When I get home I'll to through these steps to see if the disconnect is having issues.  Thanks for the suggestions so far.

Just wanted to emphasize again that the first trip to the range a few months back had no malfunctions.  However, something may have come loose or perhaps some debris got into the trigger group before my recent range trip.  I'll report back what I find when I check the disconnect.

I had a lower that would do this as well.  It was not the fire control group however that was the problem.  The buffer retainer hole was 'egged' out, and the buffer spring? Buffer? ... whatever, was causing the gun to fire multiple shots.

Inspect for physical damage to areas in/around the firecontrol group.  I had another gun try this trick when the trigger/hammer pin (forgot which) was walking out.

If you do not see anything obviously defective, it is easier to replace the whole FCG group than to try and nickle and dime your way through them all looking for the culprit.

TRG

How is that even possible?  The buffer retaining pin wouldnt have made the hammer hit the firing pin.

I dunno.  It happened w my very first AR, circa 1999.  Traded the rifle to another user, he knew of the issue and knew how to repair the lower.

TRG

Make sure your pins aren't walking out of the trigger assembly. It happened to me and I had a 2 shot burst automatic machine gun for awhile.

Bump for update?

Sorry about that.  Getting ready for a big trip (family reunion) and didn't get around to checking the AR.  My bad.

The disconnector hook is either damaged or worn.  
The disconnector ledge on the hammer may be worn or broken.  
The disconnector spring may be weak or broken.

If all of these are correct then you need to file a small amount off the front of the disco where it touches the top of the trigger.  This will increase the amount of disco engagement which should cause the hammer to be more reliably held back for proper reset.

bump with update in OP

Yup, same thing I had going on. Get on youtube and search for ar15 and there are some good vids on there about assembly. It's easy to fix, the spring is probably backwards and not holding the pin in. Common mistake.

Definitely check this^^^^^.

Also, look for pieces of primer, crud, metal, etc etc, under the trigger; I've had some pieces of primer blow out and wedge themselves under there enough for the it to throw everything out of alignment and the disco would barely catch the hammer sometimes and the recoil would knock it loose causing double/triple taps.

Oh, it's definately the hammer pin walking on the left side of the hammer.