Help me understand bolt carriers AR-15

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ohbythebay

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I am looking at Bolt carriers for my S&W M&P 15

I ran across this
Centurion Arms C4 Bolt Carrier Group was created with MPI tested bolts and marked to give you the ultimate confidence in your weapon system. This full auto carrier is heat treated and precision machined. The gas key is staked on both sides of both bolts

When they say full auto do they mean spec'ed for it or do they really mean full auto ?

Also, as I understand it - some are M16 spec'ed to be a bit more robust, yes ?

Thanks in advance...
 
Do a google image search and you will find comparisons that will be easy to understand.

The auto carrier has a circular back end that is designed to trip the auto sear in a full auto Lower.

The semi auto is more like a half circle and won't trip the auto seat in a lower.

Most people prefer auto seat in semi auto because the extra weight in theory slows the cyclic rate a tiny bit by making the chamber unlock a tiny bit later and the extra weight you forward makes it a tiny bit more reliable overcoming a tiny bit more gunk induced friction.

In theory
 
And if your gun breaks and the hammer follows the bolt home a full auto carrier won't let it hit the firing pin.
 
Got it...

So besides the shape, it will work and not convert my rifle to auto (which I didn't really think) but wanted to be sure. I don't really NEED a new BCG...at the most maybe keep a spare bolt..but might be a fun upgrade...but other than durability, I am not really going to notice a difference am I ? :rolleyes:
 
No, you will not notice a difference, if your old one works as designed.

carrier is heat treated and precision machined. The gas key is staked on both sides of both bolts
Click to expand...
Thats just windy add copy.
All bolt carriers are heat treated and precision machined.
And all gas key bolts should come staked in place.

In other words?
If your bolt carrier ain't broke, don't fix it!

rc
 
standard.gif
This image might explain it best
Top carrier is a semi auto type lightened carrier.
The rear lug of the carrier is cut back and the firing pin port shroud is cut away.
This prevents use in a full auto converted weapon as the cut away lug makes it impossible to add an auto sear so just modifying the disconnector will cause the carrier to trap the hammer in the clearanced firing pin port during full auto fire and effectively jam the weapon.
At most, the weapon will only get off two or three shots before jamming in epic fashion.

Middle carrier is a full auto type, notice the full length rear lug and shrouded firing pin port.

Bottom is a standard Match type semi auto carrier.
It features a shrouded firing pin port that allows the use of Match type two stage narrow hammers as it will not trap the hammer in the clearanced firing pin port and the rear heavy lug has been cut back to prevent the carrier from contacting an auto sear and allowing full auto firing.

None of these carriers are considered machinegun parts by and of themselves and as stand alone parts may be added to any semi auto weapon without repercussion HTH
 
all correct, however the top and bottom carriers would never even fire 2-3 rounds in auto. The hammer would drop, fire the first round, the hammer would catch on the auto sear and then nothing would trip the sear so the gun would effectively stop functioning. You'd then drop it to semi auto and it would release the hammer off the auto sear and onto the disconnector, allowing it to function again. Nothing fantastic or spectacular.
 
MIke...

I like the BCM also...

https://www.rainierarms.com/?page=shop/detail&product_id=3653

Sort of came to the same conclusion. The one on my S&W is fine...but I might (haven't decided) get the BCM, use that and keep the original factory as a spare. I think the factory carrier is great..not sure I am as impressed with the bolt itself. We'll see ! Lots of other toys I need to buy first. I keep going back and forth/scope/no scope...
 
Of one were building an AR for plinking/coyotes out to about 300 yards, which type would be preferred?
Click to expand...

coyotes won't notice a difference.

it's part of a system. the barrel length, gas port location, gas port size, buffer weight, action spring, etc all work together to determine how the action cycles.

generally speaking, if you don't know what you're doing and have a really good reason for doing it, you shouldn't mess with it. that said, there's about a 99.9% chance you won't be able to notice a difference when swapping full auto and semi-auto carriers back and forth.
 
ohbythebay said:
I like the BCM also...

https://www.rainierarms.com/?page=shop/detail&product_id=3653

Sort of came to the same conclusion. The one on my S&W is fine...but I might (haven't decided) get the BCM, use that and keep the original factory as a spare. I think the factory carrier is great..not sure I am as impressed with the bolt itself. We'll see ! Lots of other toys I need to buy first. I keep going back and forth/scope/no scope...
Click to expand...
Thanks for the Rainier link. I'd rather order from them as they are such a great company. I love their barrels and their customer service is good. when they screw something up they make it right.

Mike
 
I took an AR15 armorers course last weekend. In that class over half of the class had improperly staked bolt carriers. You could remove the carrier key screws with a hex head driver with not much effort. Many showed signs of gas leakage under the key.

OP... look up what a proper staking looks like and make sure yours is. Being a S&W it probably is.
 
The one on my S&W is fine.
Click to expand...

if you have a reliably running ar with a properly staked key, why would you consider swapping carriers? what do you hope to gain over the reliability you've already experienced?
 
At the minimum, staking should look like this,
standard.gif
Enough material displaced against the sides of the cap screws to grip and the screws should be torqued to at least 45 INCH/Pounds which is just shy of three FOOT/Pounds.
There is no need to hammer peen the screws in place nor is there any need for any kind of glue, i.e. LocTite, if this is done correctly.
 
Buy that BCM BCG if you have the extra money. BCM is a very good company and highly respected in the AR world.

Nothing is wrong with your current S&W BCG, but it never hurts to have a spare esp if its a BCM...
 
A bit of historical background -- the first semi Colt AR-15's (that came out circa 1964) had bolt carriers like the bottom one in Onmilo's picture, above. By the late 1960's the idea sprung up that you could convert these to full automatic simply by removing the disconnector (in reality, you'd just have "hammer follow-down" and have uncontrolled doubling with soft primers). Anyway, ATF put pressure on Colt to stop selling these, and, indeed, around 1970 Colt pulled the AR-15's off the market for a time, pending a redesign of this feature. The solution Colt came up with was to "unshroud" (bevel) the bolt carrier (top one in Onmilo's picture) and use a "notched" hammer. Therefore, if the disconnector was removed, the notch in the hammer would catch the flange of the firing pin and the gun would hopelessly jam. (And BTW, an "unshrouded" bolt carrier has to be used with a small-diameter firing pin or the gun will jam regardless.)

Later, this was found in practice to not be much of an issue and Colt returned to "shrouded" bolt carriers. Later still, Colt went even further and got ATF approval to use full-auto type bolt carriers (middle one in Onmilo's picture) in semi rifles, and that's the situation today.

(As an aside, during the period in the early 1970's when the Colt was off the market, rival Armalite introduced the AR-180, which found its niche because it was the only "black rifle" available to civilian shooters.)
 
If your S&W runs, I'd leave it alone. In theory, an FA carrier is better for the reasons discussed above. In practice, if the rifle is reliable, don't look for a reason to spend more money on something you don't need.
 
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