Stupid Question - Why wouldn't a 12.5" Middy Work?

AR/M4 AR Technical Discussion
1

I understand that current thought is that a 14.5" barrel is the shortest that works with a mid-length gas system but I’m curious why a 12.5" barrel with a 9" gas system (3.5" of dwell time) would be that much different than a 10.5" barrel with a 7" gas system (3.5" of dwell time) if the gas port was properly sized/opened.

I understand that it would be significantly more reliable with a can on the end but would it work without and still be reasonably reliable?

PS. Grant this is all your fault for letting me shoot your SBR at the group meet.

2

well then here’s a little eye-candy for you.

psych. just a concept photo… this weapon does not yet exist.

i’ve sort of been entertaining the idea in my mind off and on. i’m sure you could get one to run. but the 10s are finicky precisely because they have so little dwell… so you’re kind of taking a step backward.

i’m just trying to figure out how i can justify such an endeavor- what gain could possibly be made? i happen to have a mid barrel that’s not being used…

i’d like to do one just because

3

Shooting Grant’s 10.5" with a can was a wet dream. I’d like a bit more velocity/range/accuracy that the 12.5" provides with that shoot-ability.

4

The closest thing to something like this is an Armstech that uses a gas trap at the end of the barrel. IIRC they mfg a 9.5 or 10.0" SBR version and a 14.5" carbine version.

5

I think it would work, but in order to make it reliable, you’d have to open up the gas port to such a degree that the benefits of the midlength system would be diminished.

Dwell time is obviously a (the) key issue here, but with the AR DI system, system is the key word, it can’t be boiled down to dwell alone.

Consider what dwell time really is – yes, it’s the time that the bullet is in the barrel past the gas port – but what’s really happening is that it is the time window that the system is allowing the expanding gas to fill the gas tube, and apply a enough force for enough time to operate the action.

The distance from the gas port to muzzle is probably better represented as the “dwell length” than the “dwell time”. The actual time the bullet spends in a fixed “dwell length” will change as a function of barrel length. Think about it - if the bullet is exiting the 10.5" barrel at 2363 FPS or a 16" barrel at 2669 FPS*, that’s about a 300 FPS difference. That’s a 0.001 sec extra time to travel that same 3.5" of dwell length. That’s not much, but if you’re operating with a system that the timing is already pushed to the brink of having the enough “dwell time” to function reliably.

So, for the same dwell length, the longer OAL barrel has less dwell time. Further conspiring against us, is the fact that the longer gas system, needs more volume of gas to expand into the gas tube, and operate the system – the gas tube just has a larger volume to fill and pressurize before the action can cycle.

And then another factor working against us, is that the further down the barrel we put the gas port, the lower the port pressure. Which, normally, we think of being a good thing about the mid- or rifle-length systems. But now with our short dwell length, tiny dwell time, and larger volume to fill…we’ve got less pressure to work with.

So what do we do? Well, we either increase that “dwell length” (and therefore dwell time) or, since in this exercise we’re trying to hold that length fixed, then we enlarge the gas port, to remove a restriction and pressurize that gas system faster to make up for the decreased dwell. The problem now is that you’re slamming a bunch more gas back into the action at high velocity and pressure, undoing big part of the whole purpose of going to a longer gas system in the first place - the less violent operating system. And now, you’ve created a system thats much, much more sensitive to weak loads, temperature variations, a dirty/restricted/poorly lubed system, etc. because it is all so very carefully tuned and timed.

Summarizing - as you increase the barrel length, and move the gas port out to keep the dwell length the same, you’re actually shrinking the dwell time - so to compensate, you have to take drastic measures that really undo the advantages of the longer gas system.

At least that’s my theoretical take on why we’re not seeing a lot of 12.5" middys pop up in a world where both 12.5 and midlength seem to be “the new hotness”, but you never see them combined.

*FPS data pulled from some MK262 chrono data from MSTN found on TOS… a 300 FPS difference from 10.5 to 16 was a pretty common figure I found though.

6

Thanks LocknLoad.
I now have a better understanding than I thought possible.
I appreciate your time and consideration in replying with
what is obviously real life experience and logic.

That helped me a lot.

7

Locknload- great post.

I don’t see much point for a 12.5" midlength given that the carbine version works so well (so far, anyway). The recoil in my Noveske and DD 12.5" barrels is negligible.

Use good mags, springs, ammo, etc and you should be good to go.

8

You’d still have the significantly lower port pressure at 9 inches. I don’t think it’d be hard to make run at all. I bet a port of .080-.085 would run fine.

And then another factor working against us, is that the further down the barrel we put the gas port, the lower the port pressure. Which, normally, we think of being a good thing about the mid- or rifle-length systems. But now with our short dwell length, tiny dwell time, and larger volume to fill…we’ve got less pressure to work with.

Man… I don’t know… I’ve seen a Colt rifle barrel cut down to 16" that ran great with a port openned to .090". It’s more gas at dramatically reduced port pressure.

9

Great post Loknlod. Thanks also to markm.

I’m not sure that I understand how a the pressure would not be sufficient to cycle the bolt reliably but still be strong enough to make recoil more significant (and the advantages less) than say a 11.5" with a carbine system.

10

My 12.7" LWRC is a middy and it works just fine, FWIW.

11

Mark, you may very well be right. My experience with a cut-down rifle barrel was the opposite; it wouldn’t run unless clean, freshly lubed, and with hot ammo.

Two thoughts on that - first, I believe that both the pressure and the duration are in play there, so it’s not a matter of sufficient pressure alone; and second, I’m not saying the recoil would be more significant than a 11.5"/carbine setup. Maybe it wouldn’t even recoil quite as much. What I did intend to convey is that the recoil wouldn’t be appreciably less than the equivalent barrel length with a carbine gas system, in which case you would have sacrificed a measure of reliability without gaining an worthwhile trade-off in recoil characteristics.

Is it a piston-operated system? If so, it’s interesting that they chose the middy position for the gas port, but I’m not sure it’s apples-apples to compare it with a DI system. In a short-stroke gas piston system it’s got a much smaller volume of gas needed to pressurize the piston chamber and force the op-rod rearward.

12

My understanding is that you have to deliver the same volume of gas to the carrier no matter what. You can do that by running high pressure/low volume (short gas/small port) or low pressure/high volume (long gas/big port). The complicated part for me is seeing how dwell time (barrel length after the port) affects things. With a short dwell time you have to deliver the proper amount of gas faster, so a bigger gas port makes sense. A long dwell should need a smaller port. The thing I don’t get is how these shorter or longer impulses affect the cycling of the gun. Do you just slow the bolt down with a heavier buffer? Can a 12.5 with a middy gas system beat up the gun like a carbine gas system? It seems like delivering a super-fast burst of lower-pressure gas would be about the same as the slower, but high-pressure impulse of the 16" carbine gas system.

…or am I on completely the wrong track… :confused:

13

truthseeker,
It’s not so much the same volume as it is the same amount of force. The gas has to apply enough force to accelerate the reciprocal mass (bolt ass’y, buffer) sufficiently to cycle the action, overcoming inertia, friction, and the recoil spring force.

The timing issue is also critical in relation to the unlocking of the bolt and extraction of the casing from the chamber. The bolt/extractor have to pull the empty out, and if the system tries to do this before the pressure has subsided, then the casing is locked in the chamber by the outward expansion of the gases inside, and we get some issues. This is why a really heavy buffer and higher extractor tension can help with certain operating systems, like 16" barrels with carbine gas. With the longer barrel and closer gas port, the forces imparted on the BCG are much earlier, relative to the total cycle time, than if the port was farther out with a longer gas system, so it’s trying to tear that casing out of the chamber while it’s still smashed against the walls by the still expanding gases. We can increase the inertia of the reciprocal mass by using an auto carrier (slightly heavier than a semi version), and heavier buffer, to retard the operation (same force, more mass = less acceleration). The stronger extractor is so we have a better grip on the case rim as we yank it out of the chamber.

Assuming “fast” = “early” and “slower” = “later”, then sticking with the same terms, the 16" with carbine system is a early burst at high pressure for a long duration. Moving the port to midlength position on the same barrel length would give a later burst, at lower pressure, for a shorter duration. Because the 16/CAR setup is overgassed, this is a positive improvement, moving towards being “optimized”. If you keep the 16" barrel, and move the gas port to rifle position, you have a much later burst, at much lower pressure, for a much shorter duration. We’re at a point then where the 3 don’t necessarily add up to enough “oomph” to properly cycle the action (leading to possible functioning issues). Obviously there are about eleventy-seven combinations to run through.

In terms of “moving parts” the DI system is pretty simple. But if you think about what a complex dynamic system the AR becomes when you pull the trigger, it really gives you an appreciation for the genius of Stoner and other pioneering gun designers.

14

Wow, awesome explanation, thanks!

Just to make sure I’m following… A 10.5" barrel with carbine gas system should be less of a pain in the butt than a 12.5" barrel with a middy gas system even though the length after the port is about the same. This would be because of the higher starting pressure of the carbine gas versus the middy gas. We can slow down the over-gassed action easier than we can speed up the under-gassed action (theoretically).

This is what blows my mind. Is there a chart somewhere that tells you what port size, barrel length, buffer, carrier, etc. will work? Is it just trial and error? Voodoo? Power of prayer?

In any case, the title of this thread needs changing. I don’t think this is a stupid question at all.

15

other than trying to save some money for a tube that is on hand for budget project, where is the advantage of a middy gas??

i’m still wrapping my head around the dwell explanations but isn’t a 12.5 in a carbine gas system superior in every way??

16

Yeah, I think that’s kind of the deal.

This is what blows my mind. Is there a chart somewhere that tells you what port size, barrel length, buffer, carrier, etc. will work? Is it just trial and error? Voodoo? Power of prayer?

That’d be a heck of a chart. Thankfully most of the “worth the trouble” configurations are out there in one form or the other already.

I keep waiting for someone who knows more than I do to come in here and point out how horribly wrong I am :wink: If anybody’s really interested in this I’d encourage them to use what I’ve said as a starting point and then go research on their on to get their own understanding of the AR system… I’m certainly no authority on the matter.

17

I’m not really looking for a 10.5". 11.5" is my minimum and 12.5" seems to offer most of what I’m looking for as I intend on using the 77gr which seems like kind of a waste out of a 10.5.

I’m primarily interested in getting better performance with the shootability of the 10.5 and still be relatively reliable.

I didn’t really think that there was going to be significantly more cost/effort to make it so.

I had thought it was a pretty straightforward proposition. If it’s not really feasible than I’ll probably go with an 11.5" but that’s why this board is a resource.

18

well im gonna do one. its about No. 5 on the priority list right now, and i still need to chop the barrel and buy a stripped upper, but shouldnt be too long till i can post some findings.

19

Yes, very much so.

When you build this, I must shoot it.

20

I can tell you I’m tickled with my 12.5 build. I do however run suppressed mostly…i’ve tried all kinds of ammo but mostly feed it 75gr reloads. I took a page from Grant’s small port research and cut down a carbine gas / 16" dd and running the lmt bcg / bolt per his recommendation. The gun is 100% reliable sans can but bolt does not lock back on an empty mag. With the can, it does. Not 1 failure but I only have a little over 1k rounds down range.

While this set up is ideal for me, the whole standardized gas port size and location has had my head spinning for custom sbr builds. If the dwell time on a 14.5" middy is considered optimal why not adjust the gas port ‘location’ for all the other sbr length’s??