Cutting Down a 16" Middy

[e1harris]

Hear me out here.

If a Mk18 has a 10.3” Carbine Length Gassed Barrel with a Gas Port opened to 0.070, that would give it 3.3 inches of dwell time.

I would like to cut my 16” Mid Length to 12.3”. That too would give it 3.3 inches of dwell time.

Now let’s talk Gas Ports / Buffers
A. Open my port to 0.070. Same, same right?
B. I don’t mind holding Buffer tryouts to find the winner.

Someone please explain to me how a 12.3” Mid Length opened to 0.070 would act any different whatsoever than a 10.3” Carbine Length.

[RyanB]

You'll need a much bigger port than that. And since port pressure is lower for longer gas systems, they perform differently.

[Clint]

Try reading this first.

http://www.ar15barrels.com/prod/operation.shtml

The short answer is lower initial pressure at the mid location.

The gas port would probably need to be bigger than .070"

One thing that is not covered in the article is that the total pressure is not simply "dwell" time or dwell time x pressure.

There is also "blowdown" time after the bullet exits the bore.

The pressure does not drop instantly. It is quick, but does add to the total gas drive.

Suppressors extend this blowdown time and are commonly known to increase the gas drive and cyclic rate, even though the dwell time and distance from the gas port to the muzzle remains unchanged.


I've heard at least one person doing 12.5" middy, but it was not very reliable.

[Col_Crocs]

I remember this being discussed a while back so I did a search. Interestingly, I found this. http://m4carbine.net/showthread.php?p=1147094
Not trying to be an ass but my question is why and to what benefit?

[dpaqu]

Not trying to be an ass but my question is why and to what benefit?

Why is a 14.5 middy better than a 14.5 carbine? Or are they better?

[Todd.K]

The gas pressure in the barrel decreases over the length, as the gas has more bore area to expand into.

Variation of pressure in the bore from temperature extremes or the powder burn rate used will be greater the longer the gas system. This is a bad combination with less dwell time.

[e1harris]

Thanks for all the responses.

I guess the why would be
A. I already have a 16" mid length.
B. I want a shorter barrel.
C. Obviously 14.5" mid lengths are getting good results these days.

SO the next evolutionary question was.... If you can run a carbine length gassed rifle with only a 10.3" barrel (3.3" of dwell time)..... why couldn't you do the same thing with a middy.

I understand in advance that buffer selection and gas port enlargement would be critical.....

My hypothesis is....

12.3" Mid Length Gassed Barrel =
a. longer barrel compared to carbine length SBRs (inferred better muzle velocity)
b. decent sight radius with an 11" fore-end
c. gets my support hand out further on an 11" rail
d. More Dwell Time = "Harsher" Cycling (see 16" carbine length gas)
e. Less Dwell Time = "Smoother" Cycling (see 14.5" mid length)
f. Running a suppressor on a 12.3" mid length would be a hell alot more maneuverable than a suppressor on a 14.5"

And last but not least.... If the gas port is done "exactly" right on a 12.3" mid length SBR, then it should run even better suppressed.

Now this is all hypothesis based upon the MK 18 set up.

[RyanB]

Either use the "carbine +1" gas port location or use a can on it 100 percent of the time. 10.5s work because there is a huge amount of pressure at the port. The less that is true the more issues you have in low dwell time systems.

[e1harris]

OK, OK....
So 10.3" and 10.5" carbine lengths are already inherently finicky.

11.5” Carbine Length Gas….. what…. 4.5” of Dwell Time?

How about a 13.5” Mid Length? (forget about the stupid $200 stamp, we’re talking theory here)

Hear me out again…. 13.5” Barrel with Mid Length Gas…. That’s 4.5” of dwell time…… I’m thinking it would run beautifully with a Carbine buffer, or an H Buffer suppressed.

[e1harris]

Just think, a 13.5" Mid is the same as a 11.5" Carbine...

Only with greater Muzzle Velocity.


Am I crazy, or should I just stop trying to be inventive and cut my 16" Mid down to 14.5", suppress it and call it a day?

[Todd.K]

It is not the same.

There is less pressure at that length, and less pressure through the dwell time, and less dwell time for the same amount of barrel length past the port.

You should buy a barrel of the length you need.

[Col_Crocs]

Why is a 14.5 middy better than a 14.5 carbine? Or are they better?

Totally different. A 14.5 middy retains enough dwell time to properly function, hence, the reduction in pressure actually brings a benefit. Middy doesn't equal better in all circumstances as seen and best explained in the above posts by ToddK.
That said, I get it (the OP's interest), seeing as you have a 16 middy on hand. The more ideal route though is to just get a good 12.5CAR.

[markm]

Just think, a 13.5" Mid is the same as a 11.5" Carbine...

Only with greater Muzzle Velocity.


Completely wrong. As ToddK. pointed out, it's more than simple inches. Dwell time isn't measured in inches.