Can you tell me why AR manufactures have gone to notched hammers. I did a quick search and found this on M4carbine.net

Quote Originally Posted by XXXXXXX  View Post
The notched hammer was introduced by Colt in the semi-auto version of their rifle. It is billed as a safety feature, catching on the bolt carrier to prevent out of battery or slam firing.

The notched hammer actually "catches" the firing pin collar on a ramped style AR15 carrier not the carrier itself.

It's not to prevent out of battery firing as that is completely impossible on the AR platform as it is. Nor is it designed to prevent slam firing which is also impossible on the AR platform assuming a quality gun and good ammo.

The notch is designed to be a fail safe in the event of disconnecter failure where the hammer fails to remain cocked as the bolt returns to battery.

Sorry ...tek forum .

They work with both style carriers so you do not loose the function they provide if you are using an older SP1 style carrier.

Notched hammers are harder to find than the standard type. Also there are more than one style of notched hammer. One is not as tall as a standard style hammer and likely slightly lighter in weight. Have no idea why they dropped them down in height. The original notched hammer in my older Colt was as tall as a standard hammer.

I found a defect in one of my lowers once because I was using a notched hammer. The trigger pin hole was drilled too low in the lower. A notched hammer does not get pushed down as far as a standard hammer because of the notch. With the trigger setting to low if I held the trigger pulled too long when firing the carrier would not push the hammer down far enough to catch the disconnector that was setting too low. It caused the gun to double from hammer follow. The manufacturer replaced the lower for me.

I believe it was because colt decided to start putting those sear block abominations inside their rifles and the original hammer wouldn't clear it. I don't have one though so can't confirm.

I thought the notched hammer went with the 'civilian style' bolt carrier?

Colt still uses them, my 6920s have them even though it accomplishes nothing with a m16 bcg. On my sp1 if I mimic a hammer follow by pulling the bolt to the rear, slightly releasing and pulling the trigger the hammer notch will catch the firing pin collar. Supposedly if you were firing and the disconnector failed and your hammer continued to follow the bolt this should prevent a runaway. Haven't actually had it happen and today with m16 bolt carriers used the notch doesn't mean anything.

They are interoperable with each other. The above post is correct in that the notch catches the firing pin collar. If your firing pin is sheilded then it will never be able to catch the collar so a notched hammer wont allow this to happen.

I have seen this feature "work" first hand on a malf rifle.

It eventually bends the firing pin as well.

Yep, and the firing pin retaining pin gets mangled too.

Yup!!!  Utter abortions, those damn things!!!!  I switch to fully shrouded/full circle carriers when I encounter them on the older Ponies.