I just bought a BCM new M4 upper.  I have a like new LMT SA BCG that has maybe 300 rd on it.  Is there any functional advantage to buy a FA BCG when I have this one?   I am thinking no because my Colt has one of those strange looking cutout carriers and runs like a top.

it add weight to the setup, which helps locktime or dwell time.
It also shrouds the firing pin completely.

All new Colts have the FA BCG

I Would never buy a Semi-Auto carrier, but if I had one, and it was from a company like LMT, I would use it

Is the Bolt MP Marked? You could just buy a FA carrier and save a little coin if you wanted,,

The carrier is the "correct" design.  It is heavier, which slows the cycle down(similar to an H bufer).  I think the weight difference is negligible in feel.  A FA carrier has a shrouded firing pin, which mean that the hammer isn't hitting the collar of the firing pin.  Less stress on the firing pin.

I've never had a problem with either.  Someone else may know more.

It does shroud the firing pin, MP bolt, and is otherwise identical to the FA LMT except for the rear of the carrier.

I would stick with the LMT.  No advantage worth $130.

I agree.Spend the $130.00 on something else,like mags or ammo etc..
The LMT bolt also has the bolt lugs relieved at the root which is a suprior design to most others IMHO.Is my preference in bolts anyway.

If you're not having any problems with all the different ammo you're using, I would suggest not to change anything.

I'm a very long time AR owner and had used the old first semi auto BCG which was on my first AR Colt SP1 in the '70s, also those Colt (1991 to 2005) half moon shape at the rear of the carrier with the entire bottom rear end had been removed, all of them ran without any problems for me for about 34 years straight.

Except back in 2005 when I bought a new LMT Defender 2000 M4 16" carbine with LMT semi auto carrier and standard carbine weight buffer, when I used Wolff extra power magazine springs on my most used USGI aluminum mags, every once in a while I had failure to load, the bolt won't close all the way, either when firing and sometimes when I release the bolt catch to load the chamber.

I tried the LMT full auto carrier with LMT H heavy buffer and no more FTF.

All my 5 AR have full auto carriers, my noted LMT carbine, USGI M16A2 Colt upper on Colt A2 stock lower, Colt 6721 (2004 model) with midlength system barrel, 6721 (2010 model) HBAR carbine, SP6920 Sporter M4 carbine (2011 model), the newer Colt models after 2005 have full auto carriers from the factory.

If you want to improve on your reliability and not spend much money, you could just get a BCM M16 full auto carrier ($69) and use that with your existing LMT bolt assembly, it's up to you, your call.


http://www.bravocompanyusa.com/BCM-Bolt-Carrier-Auto-M16-p/bcm%20carrier%20with%20gas%20key.htm

No reason not to run an FA bolt.

How many rifles do you have?

Each rifle should have a bolt (at least).

I run FA carriers. I like having the extra metal.

Just go to a gunshow and you can find used ones. check for good carrier keys/staking, steel blue it for worn finishes, and G2G.

Locktime is the time from when the trigger breaks (sear releasing) to the time the firing pin hits the primer.  BCG weight has no bearing on locktime.  

As Alpha-Romeo3 said - if it works fine, there's not really much point in changing it.  

I don't buy SA or cut carriers by choice any longer, but the few I have, I still use, and don't necessarily feel like I need to replace them ASAP.  I also tend to use the cut carriers in my 20" rifles, though, since they have the longest action stroke and heaviest buffer and spring weight that should compensate for the somewhat increased sensitivity of carbine systems.  

In all honesty, while cut and unshrouded carriers can hammer the crap out of the FP retaining pin and chew up the buffer, given how much people like to change buffer weights around, I don't see a shrouded SA carrier making much of a difference - for me, it's just "the wrong part."

Plastic magazine releases and other "alternative" parts work - I just prefer not using those parts.  

~Augee

There are a few issues with used carriers to be aware of though, mainly that you could be getting unknown metalurgy on the carrier, and more importantly, the cam slot may not be milled correctly, which may present big problems.

I ran into this once with a used no name carrier from gunshow, wont do it any more.

Agree.  Used carriers should be marked "C."  

~Augee

LMT SA carrier is FINE as is

No.  I don't think there is an actual advantage.  At least not one you can feel in your hands or see with longevity of the part.

There are plenty of semi carriers out there that are very good quality.  I like to think that quality of manufacture is more important than whether or not it is auto or semi.  

I would not change my semi if it worked fine.  

I would change to an auto carrier if I were shopping for one.  Just cause' ya know.

I just wouldn't go out of my way to do it until it needed to be done.  That's my nickel.

You know Augee, I was trying to think of a way to say that without sounding like...well...me

I like shrouded carriers because there is less wear tear on the firing pin retainer. Replacing bent retaining pins constantly is a pain in the ass.

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I only use semi carriers, and have never had a problem after many thousands of rounds.

Thanks for the replies.  From reading the above, it seems to confirm my belief that a quality BCG either the FA or SA will work 100%.  But, like Augee said, I want this build to have the "correct" part, since it is a M4.  I can get a LMT FA carrier only from Quantico for $88.00.   Maybe I'll keep  the SA for a spare or sell it on the EE to make some of my money back.

Well if thats the case then thats what I'd do....have done the same myself in the past.In fact if you need a spare bolt,firing pin,cam pin,FPRP buying a complete LMT "standard" M16 BCG at $129.00 will be the most cost effective route to take as well.Keep the AR15 "enhanced" BCG as a spare.
Of interesting note,LMT uses the correct large spool firing pins in all their BCG's..

Sell your sa carrier.......keep the lmt bolt, and buy a fa carrier for about 70/80 bucks......done

I am still running two of the Colt half-moon un-shrouded carriers without any problems at all.
With the round-nose hammers and full-auto, full-skirt firing pins, I don't even get the damage to the retaining pins for some reason.
I agree that full-auto carriers would be the better option simply "because", and I do prefer them, I don't see them as being necessary.
Since it requires extra machining to make a semi or un-shrouded carrier from a full-auto carrier, it's no wonder that all the manufacture's are offering the full-auto as standard.....it's actually cheaper.