The MK18 Thread

[HCM]

@TGS and @psalms144.1 have experience with Mk18s. My job issues 11.5” guns so I’ve had no experience with the 10.3”/10.5” guns. They sure do look cool though.

Very similar other than 11.5” have a more forgiving reliability envelope and slightly longer service life on parts like bolts/bolt lugs. That little bit of barrel gives a large increase in dwell time. Hence SOCOMs recent switch from the 10.x guns to 11.5”

[Wake27]

I can confirm these feature the newer barrels with crane spec gas ports.

Yup, that’s what I got a week or two ago. Only downside is the plastic ejection port cover.

Very similar other than 11.5” have a more forgiving reliability envelope and slightly longer service life on parts like bolts/bolt lugs. That little bit of barrel gives a large increase in dwell time. Hence SOCOMs recent switch from the 10.x guns to 11.5”

What switch? I’ve only seen the 14.5s here but last I heard the URGI switch still has a 10.3 barrel and haven’t heard anything else for the rest of SOCOM.


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[TGS]

How would an 11.5" give greater service life?

The gas port is in the same location, and many 11.5"s are running gas port sizes comparable to the Mk18.

I guess if we're going to re-do the other thread, I'll post mine here too:

[rd62]

Here's my 10.5" LMT with DD Omega Rail and rear sight.

Looking for a new weaponlight setup and considering upgrading my rail to something MLOK for a lower profile.

Its been very reliable though.

[TGS]

How would an 11.5" give greater service life?

The gas port is in the same location, and many 11.5"s are running gas port sizes comparable to the Mk18.

Just to clarify:

Colt HBAR "FBI" 11.5s have a .069" port.

Colt 6933 have a .074" port.

BCM 11.5s have a .076" port.

Crane-spec Mk18s (including DD commercial post 6/2017) have .070".

The 11.5" has 40% more dwell time, along with gas port sizes either the same or significantly larger.

There's no way that would mean they would have longer service lives. They'd need to either have smaller gas ports or have the gas port placed further down the barrel for that to make sense (ala middy vs carbine). As currently made, they're hitting the BCG with just as hard pressure spikes (or harder) as a Mk18, maintained over a 40% longer time period.

ETA: This idea might be a hand-me-down from MAC-V-SOG's 10" Colt 607s during Vietnam, and the switch to the 6933. In that case it makes sense the 11.5" 6933 would have a longer service life than the 607, given the 607 has gas port sizes reported from .093" to slightly over .1xx" (yes, that's not a typo).

[Casual Friday]

Just to clarify:

Colt HBAR "FBI" 11.5s have a .069" port.

Colt 6933 have a .074" port.

BCM 11.5s have a .076" port.

Crane-spec Mk18s (including DD commercial post 6/2018) have .070".

The 11.5" has 40% more dwell time, along with gas port sizes either the same or significantly larger.

There's no way that would mean they would have longer service lives. They'd need to either have smaller gas ports or have the gas port placed further down the barrel for that to make sense (ala middy vs carbine). As currently made, they're hitting the BCG with just as hard pressure spikes (or harder) as a Mk18, maintained over a 40% longer time period.

The OG MK18's were .082" and were hell on gas rings and extractors. DD was replacing barrels in 2017. Post 2018 MK18s are pure sex.

LMT uses a .073" gas port for their 10.5", and SOLGW 10.5" is .074".

I have two BCM 11.5" barrels, the ELW is .076" while the govt profile CHF barrel is .073".

Will Larson (RIP) said that .071-.076" is the optimal range for 11.5" barrels with full spec nato ammo, and should run suppressed or not with nothing more than a buffer change if necessary.

Older Colt 10.3" barrels had like a .092" and even larger gas ports according to some, I can only imagine how pleasant those were.

ETA: Just saw you added the part about older Colt barrels.

[TGS]

ETA: Just saw you added the part about older Colt barrels.

Such is life, no worries.

[TGS]

Here's another photo. One of our legacy Mod 0s.

[HCM]

How would an 11.5" give greater service life?

The gas port is in the same location, and many 11.5"s are running gas port sizes comparable to the Mk18.

I guess if we're going to re-do the other thread, I'll post mine here too:

The gas system is 7 inch so the Gasport is in the same place on a 10.3, at 11.5 and the 14.5 carbine length. However, the more barrel you have in front of the Gasport the more dwelltime you have. The more dwelltime you have the softer or easier the system operates However, the more barrel you have in front of the Gasport the more dwelltime you have. The more dwelltime you have the softer or easier the system runs. An 11.5 will run more reliably and have longer service intervals than a 10.3, just as a 14.5” with carbine gas will give more dwell time and correspondingly run softer /be easier on parts than a 14.5”.

Though there is obviously a bigger difference between a 14.5 verse 11.5 then 11.5 verse 10.3, There is a reason why colt and some other makers will not make 10.5 or 10.3 inch guns. There is also a reason why most non-DOD federal agencies went with 11.5 inch guns versus 10.3.

All of that is before you get into the size of the Gasport. If you know you’re only running certain types of ammo 0.70 Gasport on a 10.3 will run fine, even though it will reduce component life. When you start running all kinds of random s**t ammo you need a bigger Gasport to ensure reliability. That’s why the older DD commercial MK18 used to have a larger Gasport about 0.76. The price for that increase spectrum of reliability is a heart of shooting gun that wears components faster. There is no free lunch.

The block III you URGI supplied to SOCOM has been updated to 11.5” barrels since last year per Geissele who is supplying them.

[TGS]

The gas system is 7 inch so the Gasport is in the same place on a 10.3, at 11.5 and the 14.5 carbine length. However, the more barrel you have in front of the Gasport the more dwelltime you have. The more dwelltime you have the softer or easier the system operates However, the more barrel you have in front of the Gasport the more dwelltime you have. The more dwelltime you have the softer or easier the system runs. An 11.5 will run more reliably and have longer service intervals than a 10.3, just as a 14.5” with carbine gas will give more dwell time and correspondingly run softer /be easier on parts than a 14.5”.

Though there is obviously a bigger difference between a 14.5 verse 11.5 then 11.5 verse 10.3, There is a reason why colt and some other makers will not make 10.5 or 10.3 inch guns. There is also a reason why most non-DOD federal agencies went with 11.5 inch guns versus 10.3.

All of that is before you get into the size of the Gasport. If you know you’re only running certain types of ammo 0.70 Gasport on a 10.3 will run fine, even though it will reduce component life. When you start running all kinds of random s**t ammo you need a bigger Gasport to ensure reliability. That’s why the older DD commercial MK18 used to have a larger Gasport about 0.76. The price for that increase spectrum of reliability is a heart of shooting gun that wears components faster. There is no free lunch.

The block III you URGI supplied to SOCOM has been updated to 11.5” barrels since last year.

Please see my follow up post.