AR hammers and triggers are made of 8620 steel, so yes they are case hardened. with the cost of an ar trigger (sear) at 10.00 ish if you do not do well the penelty is not very big. The Heat treat will penetrate more around the corner so the contact point of the hammer is deeper than the usual .015 depth of penetration.
Most times if the gun is doubling it is because the guy did not adjust the disconector to catch the hammer before it rests on the sear.
If you want to really try take a block of steel and drill 2 holes in it at the same spacing as your reciever and install the trigger pins. You can see outside the gun how it works and can play with the angle of the trigger face the depth of the hammer hook and by removing material from the front of the disconector change the contact amount. If you go to a show or buy a spare set you can work with them and always change the gun back if it does not work well.
To check the gun make sure the safety works, drop the bolt with the trigger back and forward and make sure it does not follow, (empty gun!!)
and with the trigger back drop the bolt, and slowly release the trigger, If the hammer slips off and does not catch the sear it is not safe.
Last point, Rock River has a trigger set that is very good also, but I think it sells for 120.00
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