M4 vs. Midlength vs. Standard

[amac]

I've been on a learn-as-I-go basis with AR's and have been reading posts on this forum and others for a while. I've read reference's to people choosing midlenght over M4's over Standard carbines. Realizing barrel length changes an AR's purpose, I haven't read any discussions explaining other functional differences. Aside from the M4 feedramp's, what are the operational/functional difference's between the referenced AR platforms? Thanks for the insight.

:)

[fdxpilot]

I've been on a learn-as-I-go basis with AR's and have been reading posts on this forum and others for a while. I've read reference's to people choosing midlenght over M4's over Standard carbines. Realizing barrel length changes an AR's purpose, I haven't read any discussions explaining other functional differences. Aside from the M4 feedramp's, what are the operational/functional difference's between the referenced AR platforms? Thanks for the insight.

:)

You are kind of mixing your terms here. An M4 is a 14.5" military carbine, which introduced extended feed ramps. Hence, the term M4 feedramps. However, these feed ramps have been incorporated into almost all current production AR style rifles. In fact, it is getting hard to find quality upper receivers and barrel extensions that don't incorporate them.

As to "midlength versus Standard Carbine." Here, you are discussing gas-system lengths. The carbine length gas system was originally designed for 14.5" or shorter barrels. When used with a civilian length barrel of 16" or more, it has much more dwell (which if I understand correctly, is the time between when the bullet passes the gas port, allowing pressure to the gas tube and gas key on the bolt, and when the bullet exits the barrel, releasing the pressure in the gas system.) This increased dwell causes the bolt control group to cycle faster and with greater force than when used with shorter barrels. This can cause greater parts wear, and might require a heavier buffer (H, H2, etc.) to function properly. Any civilian "M4" style rifle probably has a carbine-length gas system.

Mid-length gas sytems have the gas port placed approximately 2" further down the barrel, reducing dwell on longer barrels. Practical results are a slower, softer cycling weapon, with longer handguards, to cover the extra 2" of barrel between the Receiver and the front-sight-base or gas block. Of the 4 ARs I used to have (sad result of a home breakin,) the last 2 were midlength rifles. One was a 16" military style weapon, while the other was a 18" stainless-steel barreled 6.8SPC hunting rifle. Both also had "M4" feedramps. In the case of my 6.8SPC hunting AR, the feedramps are almost a necessity, as that caliber round feeds poorly without them.

[thopkins22]

Dwell time is actually being increased with longer gas systems, not decreased.

This allows time for the chamber pressure to drop significantly, decreasing the amount of gas(force,) required to extract the case and cycle the action.

So because the case extracts easier, the lower pressures found at the further gas ports, and the shorter length of barrel beyond them will suffice, and with a more gentle(easier on parts) impulse.

Somewhere around here I've seen a good diagram of how dramatically reduced the chamber pressures are at the time of extraction in midlength and rifle length systems compared to carbine length gas systems.

[OutlawDon]

Somewhere around here I've seen a good diagram of how dramatically reduced the chamber pressures are at the time of extraction in midlength and rifle length systems compared to carbine length gas systems.

http://ar15barrels.com/tech/pressure-time.gif

[MistWolf]

Dwell time is actually being increased with longer gas systems, not decreased.

This allows time for the chamber pressure to drop significantly, decreasing the amount of gas(force,) required to extract the case and cycle the action.

So because the case extracts easier, the lower pressures found at the further gas ports, and the shorter length of barrel beyond them will suffice, and with a more gentle(easier on parts) impulse.

Somewhere around here I've seen a good diagram of how dramatically reduced the chamber pressures are at the time of extraction in midlength and rifle length systems compared to carbine length gas systems.

You are correct that the dwell time from the moment the bullet leaves the case until it passes the gas port increases with a longer gas system.

When you increase the length of the barrel from the gas port to the muzzle, it increases the dwell time that the gas is pressurizing the system.

Both are dwell times but of different events

[Whootsinator]

Though generally when people just say 'dwell time', and do not specify, they mean the time the projectile is between the gas port and muzzle.

[markm]

They should all be banned.

[HeavyDuty]

They should all be banned.



What?!?

[fdxpilot]

Though generally when people just say 'dwell time', and do not specify, they mean the time the projectile is between the gas port and muzzle.

That was what I meant in my post.