What is the benefit of a full auto bolt?
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entropy
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If you mean a carrier, it's mostly ease of manufacture. For a while, semi auto carriers were made for fear of people modding their AR's for full auto, but most carriers made today are full auto profile.
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shoobe01
Member
Not me:
The M16 bolt carrier serves two functions (over the "semi auto" bolt carrier). The first is that the firing pin is fully shrouded so that the hammer is cocked by the carrier and not the firing pin itself. The second is that the M16 bolt carrier is heavier and therefore increases "lock time" (or the amount of time that the empty case remains in the chamber after the primer is struck by the firing pin) which aids in extraction. The heavier carrier also reduces the felt recoil impulse which in turn reduces wear and tear on the other internal parts of the carbine.
edwardware
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That's unlocking delay. Correct explanation of the effect, wrong term.
Lock time is the delay between the sear unblocking the hammer/striker/cocking piece, and the primer igniting.
And to the OP's question: added bolt mass increases unlocking delay, and reduces BC velocity.
as others have mentioned, mass reduces unlocking speed, much more effectively than a heavier spring. One effect of this is that brass does not fly as far, and in a smaller area. Another advantage, if you use an A1/2 Carry handle, it distributes some counter barrel weight a little back. If you have overgassing, it can help, some say it can help undergassed rifles as well, because it allows more pressure into the piston chamber before unlock, though I don't know personally. You can also get these advantages by adding an H1/2/3 buffer cheaper than replacing a carrier. Extra weight also makes things run better when dirty. If you believe "bolt bounce" is a meaningful issue in a semi auto, you can help that with a heavier carrier. Bolt bounce can be an issue in a FA, but then you would already have the 16 carrier to trip the autosear.
I don't know what the AR manufacturers are putting out today, but back when I built my M4 in 2010 there were some significant differences in bolt carrier groups between machine gun carriers (full auto) and those that weren't of the same spec. The most significant of these differences were the steel used, HPT and MPI. The following info is taken from Bravo Company Manufacturing website regarding their full auto carrier group.
"Bolt carrier group (auto version) for your AR15, M16, or M4. Parkerized exterior and chrome lined inside carrier. Machined to USGI specifications. This M16/M4 carrier includes the proper Mil-Spec gas key. Gas key is chrome lined and heat treated per GI specifications. Gas key is secured to carrier via USGI domestic mfg Grade 8 fasteners and properly staked per Mil-Specs. These properly hardened parts ensure a strong staking and proper gas system functioning.
The bolt assembly is machined from the correct Mil-Spec Carpenter No. 158® steel, shot peened for increased strength, includes tool steel machined extractor and ejector, BCM Extractor Spring, and is HPT (High Pressure Tested) and MPI (Magnetic Particle Inspected)."
"Bolt carrier group (auto version) for your AR15, M16, or M4. Parkerized exterior and chrome lined inside carrier. Machined to USGI specifications. This M16/M4 carrier includes the proper Mil-Spec gas key. Gas key is chrome lined and heat treated per GI specifications. Gas key is secured to carrier via USGI domestic mfg Grade 8 fasteners and properly staked per Mil-Specs. These properly hardened parts ensure a strong staking and proper gas system functioning.
The bolt assembly is machined from the correct Mil-Spec Carpenter No. 158® steel, shot peened for increased strength, includes tool steel machined extractor and ejector, BCM Extractor Spring, and is HPT (High Pressure Tested) and MPI (Magnetic Particle Inspected)."
Gordon
Member
I have two of the post first ban/ pre 2nd ban Colts that have those hardened pins in the trigger portion to keep from inserting milspec triggers (or full auto BS stuff) they also had ones restricting what kind of bolt. Finally took the completed and registered Colt receivers to a guy who had a CNC program to make 70% lowers and bingo , they milled out the crap Colt put in so I could put a good bolt and trigger in them . The BCG supplied in those Colts were both Crap andin heavy training would constantly fail to eject until I milled the receivers to accept good ones. In the mean time I expanded my AR inventory in the 5 year duration before milling the Colts. I left the HBAR A3 and the Milspec A2 stock except for the milspec triggers and bolts now in them, they work great now .
shoobe01
Member
Agree. I'd call it dwell time*, but didn't bother to edit that bit.
*But feel free to tell me why that term is wrong also.
Demi-human
maybe likes firearms a little bit…
Because dwell time consists of the time a bullet passes the gas port to when it is free of the muzzle, and no longer pressurizing the gas system.
(It could also be considered the total time a bullet is under pressure in the barrel.)
(It could also be considered the total time a bullet is under pressure in the barrel.
)Owen
Moderator Emeritus
Those aren't different steels, they are different testing methods. HPT is High Pressure Test, MPI is Magnetic Particle Inspection.
HPT means that it fired a proof load without a visible failure, and it still passes headspace
MPI means that is was exposed to a magnetic particle that glows under black light. The part is basically dunked in a fluid under a strong magnetic field, then removed, rinsed off, and examined under a black light. If there is a crack, the crack will glow under the black light.
AlexanderA
Member
The "unshrouded" (beveled) semi bolt carriers were a transitional thing. They went along with the notched hammers, so that the hammer would catch on the firing pin collar and jam the gun, if you attempted uncontrolled automatic fire by removing the disconnector. This was an issue in the early 1970's. Before and after that transitional period, semiautomatic bolt carriers were fully shrouded. The FA bolt carriers aren't as open in the rear portion, so that they can trip the FA sear. Today, any of these (including FA bolt carriers) are acceptable in semiautomatic guns.
MachIVshooter
Member
If you know who did say that, inform him/her that it is not correct. The portion of the carrier trimmed back or removed completely in old style semi auto carriers is on the back end, has no bearing on hammer cocking whatsoever.
That's all well and good, but the bolt is not the "full auto" part; the carrier is, and they're made from alloy 8620.
AlexanderA
Member
That was talking about the special "unshrouded" bolt carrier (which dates from the early 1970's), not the ordinary semiautomatic carrier. There are several styles of bolt carriers.
243winxb
Member
The m16 bolt carrier is more weight and trips the auto sear.
Can always search here.https://discover.dtic.mil/search-results/?q= M16 bolt carrier
Can always search here.https://discover.dtic.mil/search-results/?q= M16 bolt carrier
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Dwell time is the time an engine piston is at a stop at top dead center where it is neither going up or down.
The gas system is still pressurized for a brief time after the bullet has uncorked the muzzle. It's this residual pressure that runs the rifle.
Demi-human
maybe likes firearms a little bit…
I am absolutely certain we are talking about engine pistons...
Yeah, I'll buy that...
As well as all the gas that has been pumped in while the bullet was traversing the distance from gas port to the muzzle. Correct?
I like light wieght bolt carriers and their entourage, myself.
Yeah, I'll buy that...
As well as all the gas that has been pumped in while the bullet was traversing the distance from gas port to the muzzle. Correct?
I like light wieght bolt carriers and their entourage, myself.
Don't tell the Colt fan boys that. Lol
Thanks for the good information.
I don't want to be part of the whole I have a truck. No it's a pickup. No it's actually a F150. Type of argument.
I got what he meant in the first explanation.
Remember we are TheHighRoad and don't pick the low hanging fruit.
I don't want to be part of the whole I have a truck. No it's a pickup. No it's actually a F150. Type of argument.
I got what he meant in the first explanation.
Remember we are TheHighRoad and don't pick the low hanging fruit.
Most of the gas needed to operate the action enters the system after the bullet exits the muzzle.As well as all the gas that has been pumped in while the bullet was traversing the distance from gas port to the muzzle. Correct?
Colt "FanBois" will be the first to tell you
AlexanderA
Member
What is labeled in the picture as the "SP1" bolt carrier is actually the "post-moratorium" beveled, or unshrouded, bolt carrier. I have two SP1's, that I bought new in the "pre-moratorium" period (circa 1968), that both came with shrouded carriers. (The "moratorium" was circa 1970, when Colt temporarily pulled the AR-15 off the market to address the disconnector-removal issue.) The unshrouded carrier is nothing but trouble. In particular, you have to be careful to use a small-collar firing pin, or else the gun may jam even with the disconnector in place.The m16 bolt carrier is more weight and trips the auto sear.
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