Reasons for AR15 v. M16 hammer differences?

[NeoWeird]

Ok, this is just a hypothetical, I don't need help or advice or anything like, I am posting it in the gunsmithing section in hopes I can get a more detailed answer than a two or three word response - an actual functional response. To the topic~~~~

Now I've never been in the military or had the luxury of holding an NFA item, but having a heavy interest in firearms and the mechanical aspect of things, as well as a healthy curiosity of the way things work, I've figured out in my mind the main function of parts on the M16 fire control group. I just can't seem to figure out why there are the differences in the M16 and AR15 hammer.

I understand the bobbed tail and the slightly different disconnector hook, but what about the other differences? The elongated hammer face? The wider body? The longer slopping edge towards the pivot pin? The only reasons I can think of is possibly more mass to slow hammer response once the auto sear releases it to give the bolt that much more time to engage in the barrel extension (or vice versa - remove un-needed material for a light faster ignition on the semi-auto 'sporting' AR) or possibly to give the hammer just that much more downward travel when the carrier moves to the rear. That still doesn't explain all the differences though; on the rest of the fire control group the changes are so simple that it's easy to tell their purpose, but on the hammer it has 3-4 differnces that just don't make sense to me (yet).

Anyone care to explain it to me? I spent all day at work thinking about it and my mind is going in circles now.

[bobfried]

I find this website helps:

http://www.quarterbore.com/nfa/registeredreceiver.htm

It's hard to explain until you see the full FCG together and in operation.

[NeoWeird]

I find this website helps:

http://www.quarterbore.com/nfa/registeredreceiver.htm

It's hard to explain until you see the full FCG together and in operation.

Thanks for the information but that didn't really help. My mind works very well at piecing parts together in my head, and working in three dimenions, so I understand the way the trigger goes together and functions I just can't figure out the reasons for the differences in the two hammers.

If you look at the diagram onthat page you can see that most of the parts have one or two changes that are easily discernable. The hammer however has several changes that don't seem necessary in my mind and I can't figure out why.

I mean, if you took a hypothetical AR-15 hammer and added the bobbed tail and the extra material on the disconnector hook, why wouldn't it function as an M16 hammer? And if it would function, why was the M16 hammer designed differently to begin with?

[StraightShooter]

What is a sear block? What function does it perform in a semiautomatic AR.

[Sgt Raven]

What is a sear block? What function does it perform in a semiautomatic AR.

Back when Colt was the only game in town they started putting the sear block in AR15s so people couldn't convert them to full auto.

ETA spelling

[StraightShooter]

Thanks.... and sorry Neo for the thread jack.

[NeoWeird]

It's ok StraightShooter, I would have given you the same info if Sgt Raven hadn't given it to you first.

Still looking for the answer to my question though.

[Army]

The main changes were made so an AR hammer cannot be used in an M16. The vice-versa, is changes to the M16 hammer to not function correctly in an AR15.

The upper "hook" on a 16 hammer, is to engage the full-auto sear assembly, which is operated by the bolt carrier. The carrier on a 16 has more surface on the bottom than a 15. Look at your 15 carrier, see the area where the hammer passes through? There should be about 1/2 to 1" of metal in a "band" at the end that contacts the buffer. Look carefully, and you will see on the carrier where extra material was removed (look for the machine marks). A 16 carrier will have about 2 1/2" of "band" there, which bumps the FA sear as it goes forward, releasing the hammer to fire. All the trigger does at this point, is stand by for you to release it. Rolling the selector to semi, will push the FA sear out of engagement, and bring the disconnector into play.

The hammers must be different, so as to not allow these parts to work together. Modified 16 hammers (bob the "tail") are known to slip from the 15's disconnector, and even not engage it whatsoever, following the bolt and carrier and producing a slam fire, and/or out of battery fire, as it pushes the firing pin forward. 15 and 16 fring pins are also slightly dimensionally different at the rear flange.

Basically speaking...don't use M16 firing componants in an AR15.

[NeoWeird]

The main changes were made so an AR hammer cannot be used in an M16. The vice-versa, is changes to the M16 hammer to not function correctly in an AR15.

The upper "hook" on a 16 hammer, is to engage the full-auto sear assembly, which is operated by the bolt carrier. The carrier on a 16 has more surface on the bottom than a 15. Look at your 15 carrier, see the area where the hammer passes through? There should be about 1/2 to 1" of metal in a "band" at the end that contacts the buffer. Look carefully, and you will see on the carrier where extra material was removed (look for the machine marks). A 16 carrier will have about 2 1/2" of "band" there, which bumps the FA sear as it goes forward, releasing the hammer to fire. All the trigger does at this point, is stand by for you to release it. Rolling the selector to semi, will push the FA sear out of engagement, and bring the disconnector into play.

The hammers must be different, so as to not allow these parts to work together. Modified 16 hammers (bob the "tail") are known to slip from the 15's disconnector, and even not engage it whatsoever, following the bolt and carrier and producing a slam fire, and/or out of battery fire, as it pushes the firing pin forward. 15 and 16 fring pins are also slightly dimensionally different at the rear flange.

Basically speaking...don't use M16 firing componants in an AR15.

So are you saying the M16 hammer is wider than an AR15 hammer to prevent it from being used in an AR15 receiver? That makes sense, a sort of forced safety feature. But that doesn't explain the larger hamer face, particularl at the upper most portion of the hammer.

Like I said, I understand mechanically how the FCG works, but I can't figure out WHY there are those changes. The bob tail makes sense, the disconnector hook makes sense, if the wider body is to prevent interchanging them in receivers then that makes sense, but what about the upper top of the hammer face and the tail that trails from the hammer face towards the pivot pin?

Sorry if I sound ungrateful, because I am grateful, but it's kind of frustrating on my end because I don't know how to make the question more clear and at the same time I know I'm not being as clear as I need to be.

Maybe someone else can explain it better than I can...

[superhondaz50]

The hammer differences arent as perplexing as some may think. One must realize that there are many different versions of hammers, both semi and full auto. Yes there are a few differences between the m16 hammer and the more "common" AR15 hammer. Of course the AR hammer is missing the auto trip hook. The disconector hook can vary on different AR hammers, for example Stag hammers have the skinny disconector hook, while on DPMS AR hammers have the wide disconnector hook, the same size as a M16 hammer. The wide disconector is only needed for 3 round burst models. As for the cut out portion on the top of the Stag AR hammer (and colt and most other companies) this is to prevent a slam fire effect, or in other words this is to prevent Joe Blow from pulling out his disconector in hopes of his gun slam firng full auto. Basically how it works is it keeps the hammer from following the bolt closed and striking the firing pin. If the notched hammer is following the bolt home it will actually bind up on the bolt and prevent the hammer from striking the firing pin. In fact try it for yourself, take you gun (empty of course) and rack it, let the bolt ride all the way home. next grab the charging handle and very slowly pull it back until you feel it contact the cocked hammer, dont move! pull the trigger it will release the hammer so it rests against the bolt, now slowly let the bolt ride home, you will notice it will stop about an inch from closing, wallah! Thee hammer is now bound against to bolt preventing the slam fire, you now have to charge the gun to un bind everything. The DPMS hammer does not have this feature, in fact the DPMS hammer is basically a bobbed m16 hammer. Also M16 hammers do fit and function in AR receivers. you can use them all you want as long as you bob the hook off rendering the part useless for FA, there is no problems with disconector engagement as AR and M16 disconnectors are dimensionally equivalent with regards that the AR disconector has had its "tail" bobbed........ oh yeah please excuse spelling

[4D5auto]

In the early years of the AR's till about the first part of the 90's. 15 and 16 parts were fully interchangeable, 16 parts will function flawless in a 15. Back then, they just hacked off here, added there to make them 15 parts, this is what made them for civilian use with the AR 15 designation/Semi Auto. In the beginning years of the AR 15, only parts you could get were modified M16 parts so they would be legal for use in the 15. It wasn't till the early 90's if I recall, changes were made in the lower receiver where as more material was left so you couldn't drop in a 16 auto sear. Then came the advent of the 3 shot burst which added the cogs on the hammer and other changes with associated parts. The early years of the 15 were very easy to convert to a 16, all you needed was hammer, trigger, disconnect, carrier and either a M16 auto sear, which were and still are available at gun shows, or a drop in sear, or the home made version.

[NeoWeird]

I knew that older AR parts were modified M16 parts; and I know of several places that still sell those parts. I just couldn't figure out the reason for all the hamer changes; but it does make sense that the uper step is to prevent slam fires. Here is a picture I made from the of so common AR diagram.



1. Auto sear bob tail to engage auto sear when in selector is FA position. As bolt carrier travels home it hits it hits the trip on the auto sear, which cants the lower portio nof the sear rearward, disengaging auto sear bob on hammer, hammer travels home and FA fire is achieved.

2. Disconnector hook that catches hammer at it's bottom most travel when bolt carrier presses hammer downward in its rearward travel. When nothing is touching the disconnector its spring loaded tension retains the hammer until the bolt carrier is home and the trigger is released allowing the hammer to fall into trigger sear notch. When selector is in FA position the disconnector is depressed against the trigger allowing the hammer to follow the bolt carrier slightly untill the hammer is caught by the slightly higher FA sear. In SA the bolt carrier retuns home, pushing the FA sear out of the path of the hammer leaving a clear path of travel for SA opperation.

3. Ant-slam fire notch intended to bind the hammer and bolt carrier when disconnector is not present, fails, or when other M16 parts are used. Prevents dangerous slam fires.

4. ? I have no idea. The only thing I can think, now that I give it more thought, is to prevent the burst ratchet from being installed; but I'm not sure if it actualy rests that far forward.

Anyone care to take a stab at number 4? Thanks so far for the help on number 3. I'm half way to having a clear mind that can properly understand all of the FCG's function!

[saki302]

I think area #4 rests on the receiver whent he hammer is full forward, and would help reduce the effects of the hammer striking the aluminum there.

I used to have an old pre-1990 AR15 hammer (cheap deal at a gun show, what can I say!). when my disconnector failed, the rifle barfed out half a mag in less than a second!
I now run a RRA NM trigger which has a prominent anti-slam fire notch (though the RRA disconnector hasn't let me down yet).

-Dave

[superhondaz50]

I have a feeling you may be right about the burst ratchet idea, but im not sure.

[superhondaz50]

ya the elongated part on the right side of the AR hammer is to prevent the ratchet from being installed.... The fat disconnector hook on the M16 hammer is to catch the second disconnector on the 3 round burst models. Early M16 hammers do not have the fat disconnector hook, but rather a skinny hook like the new AR hammers.