New HBAR 20" A2 only fired first of attempted 6 rounds of Federal XM193. Firing pin strike on primers seemed faint. When second round failed to fire, used fwd assist to ensure bolt was closed. Tried some Win Q3131A rounds, same results.

However, fired Am Eagle 223 rounds fine.

Friend's DPMS fired the Q3131A and XM193 rounds OK. Compared hammer tension between the ARs, DPMS seemed much harder to cock. Also compared hammers and bolt carriers...DPMS seemed to have longer opening cut on the underside of the carrier, and firing pin more exposed, OLY firing pin enclosed right up to the tail end and underside cutout in the carrier was smaller. Hammers were the same shape, straight front edge and top curve, no M16 tail hook, although looks like the hooks were machined off on both OLY and DPMS hammers.

Wondering if lighter feel of OLY hammer spring is causing light primer strikes, or if the OLY just won't take 556. Trigger pull on OLY also seems much lighter than on the DPMS.

Would stronger hammer/trigger assembly springs solve the OLYs problem? Appreciate any feedback. TIA.

Compare both firing pin lengths.

Hammer springs may be too light.

Could switch firing pins and if you get the same result, most likely weak hammer springs.

Trigger springs shouldn't matter.

I'm no expert and others may have a better idea than I. You could also swap the carrier group and see if it still happens.

It's a long shot but...

Make sure the ends of the hammer spring are resting on the trigger pins.

Some people cut off one leg of the hammer spring to make a lighter trigger pull. Check to make sure it hasn't been modified.

Examine the bolt carrier, notice that the firing pin can only protrude from the end of the bolt when the bolt is rotated into the locked position. You might compare firing pin protrusion on the two bolts.

I had the same problem a few months ago.  I grabbed a friends new rig during a shooting session; new A-2 standard rifle.  Dropped in a mag and preceded to have light primer strikes.
Thinking that he know how to build the rifles, since he has watched me built up a hand full of rifles for him and asked many questions, I never dawned on me to pull the rig apart.  Well after going threw a few different ammo types, and all having problems, I started to pull the rifle apart.

What I had found was that he had tried to lighten the trigger by bending the hammer spring legs, then installed the legs below the trigger pin.  Also, he had not checked the bolt firing pin entry lip and firing pin collar.  all three of which, including the spring leg location, had contributed to the problem.

The firing pin was spun to clean the burs off the collar contact face(400 grit). The firing pin entry channel lip on the bolt was de-burred by using 400 wet/dry sandpaper with my thumb to apply pressure while rotated. And, the FCG was reinstalled the correct way.  
The rifle was cured in less that a few minutes.

When you check your FCG, make sure that the pins and opening on the parts are lubed and bur-free.  Sometimes when the parts are factory installed, they can be a little gritty and have some rough edges at the pin holes. This can slow the hammer down enough to cause light hammer strikes too.

P.S. The reason that some of the rounds will fire, while other types will not, was due to the hardness of the primer cups.

Hope this helps.

Really appreciate all advice/info...will check it out. Happy New Year to all!

Let us know what you find.

Found the problem. Everything OK except hammer spring ... was in backwards. Hammer much harder to cock now.

Weather looks bad for this weekend, may not get chance to range-test, but will advise results as soon as I can.

Thanx again to all for their assistance.

Well, that's different! Glad you found the problem and it wasn't serious. An easy fix.