Recently purchased a new DPMS lower assy, and noticed that it had the best trigger pull and action I've ever seen on an "as issued" type trigger/hammer assy.  Inspecting the hammer, which is minus the cut out on top for that atrocious firing pin catch deal, I noticed that the shelf that the disconnector catches on is over teice as wide as I'd ever encountered before.

After thinking about it for awhile, I deduced that this piece was built from a burst fire hammer, and that's why the wide shelf.  Decided to see if I could find another of that type, to see if the improved pull was endemic to this style hammer, or just a lucky break for me on this one piece.

Went to the show last week, and called on my favorite parts guy.  While he did not have any AR15 hammers with this profile, he did have some M16 pieces with the wide shelf.  These hammers also have a different profile on the ser trip shoulder than I've seen before.

Anyway, bought one of the M16 jobs, took it home, ground off the sear trip, and installed it in another one of my ARs.  Sure enough, trigger pull dropped from 7 to 4.5 pounds, and overtravel is much less

It appears, from this limited test, that the burst fire hammers may have a slightly different geometry that gives a better semi auto pull.  

Anyone else have this sort of experience?

I thought having [b]any[/b] fire control parts in your AR15 was illegal unless it was a registered machine gun. I could be WAY off base on this one though.

That's what the Bushmaster catalog told me anyways.

The first hammer you describe was made from a newer 3 rnd burst M16 hammer.  The wider disconnector "shelf" is because there are two disconnectors in a 3 rnd burst mechanism.

Grinding off the auto sear spur makes a M16 hammer into a legal AR15 hammer.

Next time your order from your parts guy, ask HIM to grind the auto sear spur off before shipping to you.

Didn't order the hammer. bought it over the counter at a show.  If I was illegal, it was only for the 20 minutes it took me to get out my grinder and cut the rear spur off[:D].

BTW, I've been "illegal" like this often.  All my ARs have Mi6 bolt carriers that I have cut the shoulders back on, so they won't trip an auto sear, because I firmly believe that the lighter weight AR carriers, with the cutout undersides are hard on the innards of the rifle.  They recoil too fast.

Similarly, all my ARs, except for the new DPMS which came with an un-notched hammer, have been M16 hammers with the sear trip shoulders cut off. Can't stand that damned notch.

Really doesn't matter tho.  I've been the owner of a registered DIAS for nearly 20 years[:D] Use it in just one gun tho; the SP1 Colt.

Srill wondering whether the very good out of the box pull I've gotten w/both these hammers is the result of some geometry change in the new style part, or just luck twice in a row?

Having some M16 parts in your AR-15 is not illegal. Having all the parts is illegal, assembled or not.

Bushmaster does this as a CYA.  This was covered in a thread in the Legal forum a year or so ago (and it keeps poping up over there so it should be easy to find).  The law only indicates if they can make your rifle full-auto with the parts installed then you have violated the law.  Having 1 or more M16 parts is not necisarily illegal - its just not recommended as a CYA.

I recently purchased a DPMS lower and noticed the different hammer too.  It looks very similar to a KAC hammer.

I agree, the rounded M16 hammer, with the spur ground off, is much better than the notched hammers.  I have noticed many suppliers going back to un-notched hammers.