Went to the range today to zero my AR. Ammo was LC M855.  Went through 90 rounds but 6 were duds.  Is this typical for surplus LC ammo?

If it has gotten wet,yes. If not,no it is not common.

M855 is built waterproof, so "getting wet" isn't really an issue.  But if it is actually surplus rather than XM855, there's a reason it's surplus.  (Actually, since the Clinton White House banned sale of surplus US ammunition, I'm putting money on it not being surplus but XM).

Every manufactured item has a failure rate, and even the best mass produced ammunition will have some failures.  But I'd be interested in whether or not your rifle had something to do with it.  Did the 6 that didn't fire have good, deep firing pin impressions?  Do you keep your rifle, and particularly your bolt and firing pin, good and clean?

The only duds I ever recall having where with LC. I don't remember if it was with m192 or m855 or both.  Accordingly I'm not a big LC fan, high priced but low quality.

Several lots of M855 were pulled due to the ammo not being sealed as it should have been. I've seen it and the primers are dead due to excess moisture migrating into the case.
This ammo has been sold as "surplus",how it got there is anyone's guess. The brass may be slightly discolored. A local EOD squad gave my club a ammo can of this ammo and duds were many. At least 15%.
As long as you get no squib loads I'd try to shoot it and salvage the bullets from the duds.

I doubt it was my rifle.  Only 30 rounds were put through the rifle since my last cleaning and the other 84 rounds I fired today all went off.  The 6 that didn't go off all I heard was a click and no bang.  

Primer looked okay but didn't look all that closely.  Ammo stored in ammo can with desiccant.  Just surprised to see that many rounds (6.6%) failed.

So, OP was this truly "Surplus" LC M855 (as in previously delivered to a .mil or .gov organization) or is this the XM855 sold commercially? I think we would all like to know as 6% is an extremely high (and unacceptable) failure rate. Do you have a LOT# and packaging description you could post as well?

OP:
Before we all start making all sorts of guesses, you need to provide more information:

-You do need to look at the primers.  Look at them and tell us if the were struck or not.

-Where did you get the ammunition?  A loose random bag at a flea market or what?

Telling us the primer "looked ok" tells us nothing.  

Thank you but all I wanted to know if this is a typical or excessive failure rate for commercial XM855 (yes it was XM855 not M855).  I bought 1000 rds in bulk off the internet.  Rounds have been disposed of so I cannot tell if the primer were struck.  Next time I have a dud I'll be sure to look for this!

How hard is the primer dent ? what hammer spring ? is the hammer spring weak or broken ? These are questions to ask, Those round will have slightly harder primers

Just a technical issue here: it is virtually impossible for moisture by itself to affect a primer, especially a military primer.  The priming compound is placed in the cup, a foil disk is placed over that, lacquer is applied on the disk to seal it into the cup and then the anvil is inserted...the dimensions of the cup and the foil, and the use of lacquer seal the priming compound very effectively.  I've tried to kill primers by soaking them in water, oil, WD40, etc. and it just doesn't work because the primer is so darn well sealed.

Now powder CAN get wet and that CAN keep it from going off, usually by making it clump near the flash hole and fouling it.  But "moisture" getting into a case will almost certainly NOT kill the primer.  Those lots of GI ammunition that were pulled were faulty for other reasons, not the least being that they did not meet MIL-SPECs for the round - all GI ammunition must have the primers sealed with a visible sealant (more lacquer) and must pass a waterproofing test.

No, it is not normal at all.