[ShowMeQC]

11.5" is better than 10.5"

and 12.5" beats them both by a good margin

nothing wrong with a 10.3-10.5"

you do that if you absolutely want the smallest 5.56 while still getting decent fps, 11.5" goes up quite a bit and same with 12.5"

 

[PizzaSteve]

My goal is to set it up similar to the 14.5 just shorter so they would be interchangeable when training.

Can we get more clarification on “training”?
The tool needs to fit the job, and in this case we don’t know what the job is. All the comments are correct, but nobody really knows what the mission specs are.
Backpack gun? Get a 7” with side folder
CQB? Get the 10”-11”
CQB then prone 300 yards? Get the 11”-12”

 

[thedownzero]

A few considerations here: 11.5 versus 10.5 has 40% more dwell time on a carbine length gas system.

This translates to an increase in reliability and less issues as a suppressor host.

Lower wear on parts is an added bonus.

All that said with proper maintenance and care, short of a very high firing schedule, the likelihood of encountering wear issues is pretty low between a 10.5 and 11.5

The more dwell time the higher the gas flow/pressure. The less dwell time the softer/less pressure. So it’s actually the opposite of what you said in terms of parts wear.

As far reliability, the MK18 with .070” gas port is the most tested/proven short barrel length of modern time. From Crane’s research and development to being fielded in various climates and environments in GWOT, there have been no known widespread failures or reliability issues.

All things being equal, a properly ported 10.3/10.5 will be less gassy with a can, smoother/softer shooting, and have less parts wear. The only foreseeable issue is you want to shoot weak .223/steel cased crap, then yes…go 11.5.

The 40% more dwell time trope is often repeated and because it has a number and a percent , is often assumed to be better. But clearly not everyone has an accurate understanding of what that means or its implications.

 

[Buccaneer12]

none of the above. 11.75 on a mid length gas system.

 

[oldnrusty38]

The more dwell time the higher the gas flow/pressure. The less dwell time the softer/less pressure. So it’s actually the opposite of what you said in terms of parts wear.

As far reliability, the MK18 with .070” gas port is the most tested/proven short barrel length of modern time. From Crane’s research and development to being fielded in various climates and environments in GWOT, there have been no known widespread failures or reliability issues.

All things being equal, a properly ported 10.3/10.5 will be less gassy with a can, smoother/softer shooting, and have less parts wear. The only foreseeable issue is you want to shoot weak .223/steel cased crap, then yes…go 11.5.

The 40% more dwell time trope is often repeated and because it has a number and a percent , is often assumed to be better. But clearly not everyone has an accurate understanding of what that means or its implications.

Thank you professor. If you'd read the entire post, "with proper maintenance and care, short of a very high firing schedule, the likelihood of encountering wear issues is pretty low between a 10.5 and 11.5" if you're gonna nitpick and parse, make sure you read the post thoroughly.

As I said, between 10.5" and 11.5" there is little difference in reliability. But you do achieve a noteworthy (albeit small) gain in ballistic performance as I'm sure you know.

Mk18 did have some issues, bolt lugs shearing early and some other premature wear issues.

 

[oldnrusty38]

none of the above. 11.75 on a mid length gas system.

12.5", when quarter inches count.

 

[Buccaneer12]

12.5", when quarter inches count.

1/4 inches always count. 😂

I think 11.75 is the cut off for having mid gas run right but I cant remember off the top of my head.

 

[oldnrusty38]

1/4 inches always count. 😂

I think 11.75 is the cut off for having mid gas run right but I cant remember off the top of my head.

I’m sure professor zero will be in shortly to school us.

 

[thedownzero]

I’m sure professor zero will be in shortly to school us.

Don’t get mad at me because you were echoing dwell time percentage when in reality it only equates to a 4% difference in chamber pressure. Even though chamber pressure is (marginally higher), the 10.3 pressure drops sooner as there is less dwell time.

 

[oldnrusty38]

Don’t get mad at me because you were parroting some marketing claim about longer dwell time percentage when in reality equates to a 4% difference in chamber pressure. Even though chamber pressure is (marginally higher), the 10.3 pressure drops sooner as there is less dwell time.

Dropping knowledge is all good and well. The “aherm well acktually” is another story.

You read the brochure on 10.3, good job.

 

[thedownzero]

Dropping knowledge is all good and well. The “aherm well acktually” is another story.

No, it just means your thin skinned and/or take offense too easily when I pointed something out.

You read the brochure on 10.3, good job.

Yep. I sure did. Here is the NSWC Crane report/findings for M8551A1 and the MK18 so you anyone can read it as well:

https://static1.squarespace.com/static/597679b68419c2bc1f2f8ef1/t/598390fdb8a79b354102d3fb/1501794563797/CRANE+Test+Data+Mk18.pdf

Interesting to note that even with M855A1 (10% higher pressure) avg. bolt life was almost 10k rounds…compared to 12k rounds in M4A1 with lower pressure M855.

 

[RuggerAg]

I put together a 12.5 with a rainier match barrel and an a5 buffer, and it's smooth (oh, and accurate). I had a hard time finding a stainless match barrel in 11.5 or 12.5, but that one is doing nicely.

 

[Wake27]

I put together a 12.5 with a rainier match barrel and an a5 buffer, and it's smooth (oh, and accurate). I had a hard time finding a stainless match barrel in 11.5 or 12.5, but that one is doing nicely.

FCD has a newer offering for that.

 

[catatonic]

The more dwell time the higher the gas flow/pressure. The less dwell time the softer/less pressure. So it’s actually the opposite of what you said in terms of parts wear.

As far reliability, the MK18 with .070” gas port is the most tested/proven short barrel length of modern time. From Crane’s research and development to being fielded in various climates and environments in GWOT, there have been no known widespread failures or reliability issues.

All things being equal, a properly ported 10.3/10.5 will be less gassy with a can, smoother/softer shooting, and have less parts wear. The only foreseeable issue is you want to shoot weak .223/steel cased crap, then yes…go 11.5.

The 40% more dwell time trope is often repeated and because it has a number and a percent , is often assumed to be better. But clearly not everyone has an accurate understanding of what that means or its implications.

Interesting. Here's a post on another site by BCM:

I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

I'm not doubting the Mk18s reliability. However, it does have trade offs. Mk18s typically have shown higher wear / parts breakages on bolts as compared to the standard M4 (14.5) platform. Of course we're talking north of 10k rounds, but it matters if you're running high round count and intend on keeping it and replacing parts during its lifecycle.

Also, on the suppressor front, since it abuses cans more due to the short barrel, Surefire came out with an improved design for the shorter barrels and made the blast baffle thicker due to the abuse, also increased bore size for less backpressure:

SOCOM556-SB2

no-reply@surefire.com

www.surefire.com

 

[oldnrusty38]

Interesting. Here's a post on another site by BCM:


I'm not doubting the Mk18s reliability. However, it does have trade offs. Mk18s typically have shown higher wear / parts breakages on bolts as compared to the standard M4 (14.5) platform. Of course we're talking north of 10k rounds, but it matters if you're running high round count and intend on keeping it and replacing parts during its lifecycle.

Also, on the suppressor front, since it abuses cans more due to the short barrel, Surefire came out with an improved design for the shorter barrels and made the blast baffle thicker due to the abuse, also increased bore size for less backpressure:

SOCOM556-SB2

no-reply@surefire.com

www.surefire.com

URGI lineup is showing better longevity (for now) over M4/Mk18 but we out here using 15 year old data points.

The likelihood of a civilian running a rifle to its fail point is pretty low.

OP subjectively 11.5 is usually the better shooter if tuned correctly. With improved ballistic performance. If you are a taller shooter with longer arms that bit of extra hand guard length is better from a functional standpoint as well.

 

[thedownzero]

Interesting. Here's a post on another site by BCM:

BCM pioneered and pushed the 11.5, so to be fair they’re hardly an objective source. They literally came out with the “40% dwell time” (not to mention it’s really only like 33%) nor pointing out its only a 4% reduction in chamber pressure.

I'm not doubting the Mk18s reliability. However, it does have trade offs. Mk18s typically have shown higher wear / parts breakages on bolts as compared to the standard M4 (14.5) platform. Of course we're talking north of 10k rounds, but it matters if you're running high round count and intend on keeping it and replacing parts during its lifecycle.

It’s not that much of a higher rate though.
here it is with M855A1

compared to m4/14.5 with M855 (this is about 55k psi compared to the A1 in mk18 tests that are 61k psi.

Also, on the suppressor front, since it abuses cans more due to the short barrel, Surefire came out with an improved design for the shorter barrels and made the blast baffle thicker due to the abuse, also increased bore size for less backpressure:

SOCOM556-SB2

no-reply@surefire.com

www.surefire.com

Maybe but, was that due to the Mk18 (which afaik is still using RC2) or because they was it a cash grab? Regardless, I’m not saying a 10.3” is infallible, I’m saying there is virtually no real difference between it and the 11.5” based on dwell time (contrary to the “MuH 4o%” claims), and the argument can be made that the longer exposure to pressure before drop off in 11.5 could be detrimental.

 

[thedownzero]

URGI lineup is showing better longevity (for now) over M4/Mk18 but we out here using 15 year old data points.

that’s an apples to orange comparison. You’re talking 14.5” mid-length to a 10.3” carbine. If you have/find newer empirical data by all means drop in here to discuss. (Oh and crane data is 9yrs old btw)

 

[catatonic]

that’s an apples to orange comparison. You’re talking 14.5” mid-length to a 10.3” carbine. If you have/find newer empirical data by all means drop in here to discuss.

USASOC went to a URG-I 11.5

 

[thedownzero]

USASOC went to a URG-I 11.5

I actually just learned/confirmed this a few weeks ago thanks to Bucanner12 and it seems they’re using both still.

From what has been made public that was due to similar ballistics/penetration from the 11.5 compared to the 14.5 and mobility was preferred. I’m saying it’s apples to oranges because the only data available is on 14.5urgi in terms of failure rates, and nothing is out on the 11.5. So its hard to compare 11.5 urgi to mk18 with out m855a1 data.

 

[catatonic]

BCM pioneered and pushed the 11.5, so to be fair they’re hardly an objective source. They literally came out with the “40% dwell time” (not to mention it’s really only like 33%) nor pointing out its only a 4% reduction in chamber pressure.


It’s not that much of a higher rate though.
here it is with M855A1
View attachment 1531514

compared to m4/14.5 with M855 (this is about 55k psi compared to the A1 in mk18 tests that are 61k psi.

View attachment 1531515




Maybe but, was that due to the Mk18 (which afaik is still using RC2) or because they was it a cash grab? Regardless, I’m not saying a 10.3” is infallible, I’m saying there is virtually no real difference between it and the 11.5” based on dwell time (contrary to the “MuH 4o%” claims), and the argument can be made that the longer exposure to pressure before drop off in 11.5 could be detrimental.

It's a consideration is all I'm saying. Plus with those numbers its hardly an apples to apples discussion. Most M4s in circulation in the US Army during the GWOT were M4 Carbines (3 round burst) Mk18s were built on M4A1 lowers (full auto), the M4A1 program to replace Carbine models with A1s didn't come into play until towards the end of GWOT.

Also consider role and placement. Mk18s would've been issued out to more of the specialized or direct action types of units, whereas the M4 has been the US Army standard issue, though as above historically in the M4 Carbine version. There would be more data available in real world use on the M4 but it would certainly be a different use case. Parts wear and breakages aside, with the AR platform its really mission dependent. The common thinking with a vast majority of people who shoot these rifles a lot is the trade off for the Mk18 isn't worth it and the trend has been more toward a longer barrel as its more forgiving.

The US Military is still using the RC2 AFAIK, yes. However, I also wouldn't look to them to procure a variant just to throw on one select rifle that wasn't seeing widespread use. I'm sure in some of the Special Operations units they may have procured some SB2s as well, but its hard to really know. However, my point with it is, they put in the work to create a modified model for usage on rifles that were in the 10" or under range which says something. It's also just not an over bored can to be a faux flow through, the blast baffle at a minimum has been beefed up. Speaking on that front, even if the US military was popping RC2s running them hard on Mk18s, it wouldn't matter because they don't have to pay tax or wait