AR specs and general info

[cmshoot]

Cycle of operation:
Feeding
Chambering
Locking
Firing
Unlocking
Extracting
Ejecting
Cocking


Trigger pin/FCG holes are .155” if mil-spec.
Colt large pins are .170”


Mil spec receiver extension (buffer tube) diameter is 1.14”
Commercial is 1.17”


Gas port location 0.295” (shoulder to center of port)


FSB’s:
Original is shorter than models on flattop models. Models made for use on a flattop are marked with an F.
1.98”: flattop
1.94”: original
Colt is no longer marking their FSB’s because they are no longer making rifles that use the original FSB.


Malfunctions:
Double feed: caused by bad mag
Stovepipe: caused by weak ejector
Failure to extract: weak or improper extractor/spring or chamber too tight
Short stroking: failure of gas system. Check gas rings, carrier key and screws, gas tube/roll pin, and gas block. Possibly ammo, as well.
Failure to feed: Mag seated? Did rounds feeds from mag?
Failure to Chamber/Lock: chamber obstruction, operator induced (riding charging handle), weak or damaged action spring.
Hammer pin come out: J spring inside hammer is worn. Hammer must be replaced.
Trigger Pin comes out: Hammer spring is broken or is improperly assembled
Failure to lock after last round: Bad mag follower or the bolt lock/release is damaged. Also short stroking
Trigger reset/Double fire: check disconnector and disconnector spring


Buffers (Standard/Vltor)
Carbine buffer 2.9oz A5 H0 3.8oz
H buffer 3.8oz A5 H1 4.56oz
H2 buffer 4.6-4.7oz A5 H2 5.33oz
H3 buffer 5.4-5.5oz A5 H3 6.08oz
Rifle buffer 5.1-5.3oz A5 H4 6.83oz

Vltor A5 H2 is the “default setting”. Will generally run in a gun from 10.5”-20”

As you shorten the barrel, you must increase the size of the gas port, and increase the weight of the buffer.


Operating systems:
Pistol: 7.5”
Carbine: 9.75”
Mid length: 11.75”
Intermediate (KAC): Just over 13.16”
Intermediate (Noveske): Just over 13.5”
Rifle: 15.125”


Torque values:
Bolt carrier key screws: 58in/lbs
Lower receiver extension carbine: 38-42ft/lbs
Barrel nut USGI: 30-80ft/lbs


Buffer spring measurements:
Carbine: 10 1/16” - 11 1/4”
Rifle: 11 3/4” - 13 1/2”


Standards:
Colt M16A2/A4: 20” + rifle buffer + .093 gas port
Colt M4 Carbine: 16” + (H)H2 buffer + .062 gas port
Crane MK18: 10.3” + H2 buffer + .071 gas port (Crane)
Crane MK12 (SPR): 18” + rifle buffer + .0995 gas port


Gas port sizes:
14.5” w/ carbine gas .062-064"
16” w/ carbine gas .062-064"
14.5” w/ midlength gas .076-.0785"
16” w/ midlength gas .076-.0785"
10.3/10.5” w/ carbine gas .071 (Crane)"
11.5” w/ carbine gas .076"
12.5” w/ carbine gas .078"
18” SPR .0995"
20R” .0935"

 

[Red Dawn]

Outstanding but of info here. Thanks for helping us out with specs and troubleshooting. You’ve prolly forgotten more than some of us have learned.

Seriously though if there’s a question or concern on any platform rifle you can always ask cmshoot and trust his response.

 

[Moondog1]

As you shorten the barrel, you must increase the size of the gas port, and increase the weight of the buffer.

Why do you have to increase the weight of the buffer as you shorten the barrel?

Awesome info! I wish I really understood the intricacies of different gas systems.

 

[cmshoot]

Why do you have to increase the weight of the buffer as you shorten the barrel?

Awesome info! I wish I really understood the intricacies of different gas systems.

In some cases, it’s because the gas port is moved back closer to the chamber. This results in more violent action.

With the same length gas system, but a shorter barrel, dwell time is decreased.

 

[Allsmiles]

Thanks for the info. Saved this one for future reference.

 

[fast306stang]

Failure to lock after last round: Bad mag follower or the bolt lock/release is damaged.

I've seen this twice with out of spec lowers where the magwell is too far forward for the bolt catch to hit the follower. One lower would work with GI mags but not Magpul or any polymer mags and the other one wouldn't work at all. I tried using a 9mm bolt catch and grinding the stem down, but then I ran into issues with the BCG not wanting to ride over the catch.

One was an old Olympic Arms and the other was a Double Star.

 

[BEREZKA]

Just as good

 

[joesaxon]

I have a 7.5” pistol in 5.56 I would like to replace the furniture with stuff from magpul as my other two AR rifles have this. I do not know what size the hand guard should be for this length? I appreciate any help.

 

[cmshoot]

I have a 7.5” pistol in 5.56 I would like to replace the furniture with stuff from magpul as my other two AR rifles have this. I do not know what size the hand guard should be for this length? I appreciate any help.

Magpul doesn’t make handguards short enough to fit a pistol-length gas system.

 

[joesaxon]

Magpul doesn’t make handguards short enough to fit a pistol-length gas system.

Thank you