Greetings everyone.

I first wrote Zhukov before starting this thread because I wanted to make sure it was allowed.  He gave his permission so I’ll just use the same words I used with him.  

I'm former Army, had my fill of pray and spray shooting, honestly I have no desire to have full auto again (unless someone else is paying for my ammo). I did leave the Army before I really learned what makes things tick. That's part of the fun of all this. Since I'm such a tinker/geek I'd like to know the differences between the full auto parts and how they function differently. It seems like knowing how things work would also prevent me from making a really bad mistake with gun show parts.

Basically and for instance I see references to a FA BCG but I have no idea what the difference is and most importantly WHY. One of the 1911 manufacturers has an animated view of the action (STI?); maybe someone knows of a similar animation, that sort of thing.

I suspect that blueprints are available somewhere, nothing patented is a secret, but knowing how the parts work together is really what I am after.

FWFW, I have an ATF type 20 and am a pyrotechnician. I would assume a flash powder discussion on the pyro forums (what goes in M80's) is about the same as FA is here.  I am very much aware of the long arm of the BATFE and want to stay on the proper side of the line.

Thanks in advance for reading and your consideration.

The difference between the semi auto and FA BCG is weight.

The original design was the FA BCG.  The semi was obtained by removing part of the tail of the carrier.  This made it so that the BCG did not trip an autosear.  

A lot of manufacturers have gone back to the FA BCG because the added weight in the carrier slows the cyclic rate down and helps to increase reliability.  Also, I'm sure its easier to just produce the mil-std BCG.

There's a second difference: semi-auto carriers "expose" the firing pin.  In the event of a mechanical failure that causes the hammer to follow the bolt home, the hammer will jam on the firing pin and preventing possible o-o-b kabooms.  The original ("full auto") bolt carrier shrouds the firing pin so that hammer would harmlessly ride the BCG back into battery.

I don't have an animation handy but here's how it goes.  The trigger, hammer, disconnector, bolt carrier, and autosear interact with each other in a timed way to fire the rifle once the bolt is fully closed and the trigger is still pullet.  The trigger catches the hammer at rest on a notch, pulling the trigger disengages the hammer and allows it to fall.  As the bolt carrier cycles, it cocks the hammer which catches either the disconnector in semi or the autosear in full.  The tail of the bolt carrier hits the paddle on the autosear once the bolt goes into battery and disengages the tail on the top of the hammer.  

In full auto the trigger is out of the way and the job of the disconnector (disengaged by the selector switch) is taken over by the autosear.

ETA Not a bad animation.

This link should answer all of your questions:

http://www.ar15.com/content/legal/AR15-M16Parts/

So the tail end of the FA bolt carrier where it sorta "closes" again trips the sear?

I'm not following then how exposing/not exposing the pin creates a follow fire condition?  Doesn't the hammer have to move through the same area no matter what?

EDIT:  More posts while I was typing. I'll go read that reference.

The notch on the AR15 hammer jams up against the collar on the firing pin creating a horrendous nightmare to fix.

M16 (or "full auto") BCGs do have a purpose, and they're legal in an AR-15 as long as your AR-15 doesn't magically fire full-auto.

All the other stuff in an AR should be of the non-auto variety.

Basically it is real simple.



In F/A the hammer wants to go forward, but the sear holds it back until the B/C slams home. When the B/C slams home, it trips the sear like a lever and releases the hammer.



 

No weight of B/C is not a factor in making to go F/A.



The difference is the F/A B/C can trip the sear, while the S/A cannot.
 

Here's part 1...

http://www.youtube.com/watch?v=C2x8Oot-x88&feature=results_main&playnext=1&list=PLEDF6B93667EB3E44

Right.  The weight is not what makes the full auto firing occur.  The weight (by subtraction of material in the tail of a semi auto bolt) is just a physical difference in the part.

The exposed FP is not to prevent a kaboom, but to prevent semi-auto guns from going F/A if someone removes the disconnector. It is designed to cause the gun to jam up. That is also why the S/A Hammer is NOT rounded, but notched, to make it easier to catch.








No, the hammer does not ride the BCG; it is held back by the sear when  going into battery. It only contacts the B/C coming out of battery, not going into battery.



 

I used to be curious about this as well.  Eventually I stumbled on one of our member's websites: http://www.quarterbore.com/ar15m16/index.html

IM your email and I'll send you a flash animation of a few different configurations in operation.
 

Not all semi auto carriers are unshrouded. The one that came with my Bushmaster circa 2007 has a shrouded firing pin.

I think the unshrouded semiauto carriers are starting to go away, thank God. They seem to bend the firing pin retaining pin, at least in my experience. I have 4 semiauto carriers, non are unshrouded.

The selective quoting obscures the context.

The non-shrouded firing pin prevents a malfunctioning semi-auto from becoming an un-timed full-auto with the potential of an out-of-battery firing; the weapon fails in a "safe" manor.  The shrouded firing pin keeps the weapon running in the event of a malfunction, desirable when a run-away gun may be the safer alternative to no gun (e.g. in the middle of a firefight).

Excellent information, thanks folks.  I'm also pretty happy I was able to ask in a way that didn't get me flamed.

I followed a few of the links and was also amazed at the simplicity of the LL shown. That's American ingenuity right there.  

I also got side-tracked on You-Tube and following different links and found a few people attempting to make FA by having the hammer follow the BC.  Aside from it probably not working too well I was in awe of how stupid some people are.  The ATF could fill a year of Sundays chasing all the morons down on the Internet.

I was an A1 era person - how does the A2 time only three rounds then?  Some sort of ratchet?

Not all semi carriers have an un shrouded firing pin.
Left to right> Colt Auto Carrier, Bushmaster Semi Carrier, Colt Semi (SP1) Carrier.


Can't find my FA vs SA diagram of lower parts.... yet

Yes. It has a "wheel" ratchet that rides on the right side of the trigger. It makes the weapon have 3 different trigger pulls on semi - not the best theing for accuracy.

That's interesting.  I've never pondered how burst fire worked.  Does anyone have a pic illustrating the ratchet?
 

Huh?  Your picture seems confusing, especially since the lightning link was designed to work with an SP1 carrier.  You might be better describing that as a sear block compatible carrier.

That block upon which the FGC is mounted - is that an armorer item?  There's not enough of the block in the pic for me to go find it.  It looks like a similar jig I have for a 1911.