MK18 Vs. Geissele Super Duty 10.5

[okie]

Just went and checked DD's site to see what their current port size is. Current port size is 0.075, so either they didn't change it, they changed back, or it was something truly ridiculous that they scaled back from.

Doesn't sound like much, but that's about a 10% increase in diameter, which translates into approximately a 20% increase in area, since area is the square of the radius. Simply put, it's a significantly bigger hole, especially when you figure how much flow increases with relatively small increases in diameter. I think it's like a 100% increase in diameter equals a 1500% increase in flow or something.

The Mk18 at 0.07 is already pretty gassy. They have no problem running 5.56 unsuppressed, in all weather, right out of the box. With a can on it, it can run 223 steel on an H3 buffer and still lock back when shot vertically on a cold day, which is worst case scenario. And that's fresh off the line, not accounting for erosion. Mk18 ports erode very quickly. For the most part, you're going to get the bolt outrunning the mags long before the barrels are shot out, so you have to find a medium ground for port size, because three thousand rounds into it, the erosion is already going to be affecting things.

So why would DD do that? I don't know, but I do know that spec Mk18s, unsuppressed, sometimes won't lock back on 223 even on a hot day. I've found it to be mag specific. On a warm day I can get one to lock back on polymer mags, but not USGI mags. What I also know is that when commercial manufacturers properly size their gas ports, they inevitably get a lot of angry phone calls from customers wondering why their gun won't work right, and they don't like being told that their PMC isn't the right ammo (gosh darnit I didn't pay two thousand dollars for a gun that can't shoot everything!).

It's like I've said throughout this thread, when you buy a gun designed for and marketed towards the commercial market, you're buying a gun made for 99% of consumers, who will shoot it once out of the box, and if we're lucky maybe once a year after that. And as long as it goes bang every time they will never complain. That's just how it is.

I'm not trying to be a dick here, but I feel like the person coming to this site for advice isn't the average troglodyte looking for a safe queen, and I feel like they deserve the unvarnished truth. And hopefully, armed with that knowledge, they will go out and demand better and all of us will benefit, including the manufacturers. I'm positive DD would love nothing more than to have a more educated consumer who could appreciate a properly sized port if it was given to them, so I'm not trying to throw anyone under the bus here.

[HKGuns]

About the only way to make this thread better would be to mention the FN SCAR 16/17 and HKGuns would turn it up to 11 and invite the usual suspects.

Dang, you're still butt hurt over the SCAR aren't you? Easy Killa.

[dan1612]

The real answer is both.
Input provided, buy and shoot whatever you want. IME, 0.070 is in fact best.
However, regarding “correct” and “original” or “historical:” Nobody really cares. Nerds. Lol.

[cosmo223]

Interesting about gas port sizes. I shoot lefty, so if anything I should be more sensitive to gas blowback. Never really had a problem with my DD MK18 - i.e. didn't really notice it being any better or worse than other AR's I own. And to the extent that it's overpassed ... that's a pretty easy fix isn't it? Just swap in an H2 or H3 buffer. Irony is that notwithstanding how gassy DD is supposed to be, I just swapped out my H2 buffer because last time I shot the upper I was not getting lock back on the last round. And that's with XM193. Not sure what the problem was. Maybe the mag I was using, since I don't recall having this problem before?

In any event, to me failure to lock back is at least a little problematic, particularly in a gun I could potentially use for HD. Really don't want to pull the trigger and hear click instead of bang. Same thing with poor cold weather performance. I live in CO and shoot year round. That means that I will and have shot when temps are in the low teens or single digits. A gun that only functions when it's sunny and 70 degrees doesn't really work for me.

And if you don't like overpassed guns, try shooting a HK416. Recoil is noticeably different than my other AR's.

[Wake27]

Just went and checked DD's site to see what their current port size is. Current port size is 0.075, so either they didn't change it, they changed back, or it was something truly ridiculous that they scaled back from.

Doesn't sound like much, but that's about a 10% increase in diameter, which translates into approximately a 20% increase in area, since area is the square of the radius. Simply put, it's a significantly bigger hole, especially when you figure how much flow increases with relatively small increases in diameter. I think it's like a 100% increase in diameter equals a 1500% increase in flow or something.

The Mk18 at 0.07 is already pretty gassy. They have no problem running 5.56 unsuppressed, in all weather, right out of the box. With a can on it, it can run 223 steel on an H3 buffer and still lock back when shot vertically on a cold day, which is worst case scenario. And that's fresh off the line, not accounting for erosion. Mk18 ports erode very quickly. For the most part, you're going to get the bolt outrunning the mags long before the barrels are shot out, so you have to find a medium ground for port size, because three thousand rounds into it, the erosion is already going to be affecting things.

So why would DD do that? I don't know, but I do know that spec Mk18s, unsuppressed, sometimes won't lock back on 223 even on a hot day. I've found it to be mag specific. On a warm day I can get one to lock back on polymer mags, but not USGI mags. What I also know is that when commercial manufacturers properly size their gas ports, they inevitably get a lot of angry phone calls from customers wondering why their gun won't work right, and they don't like being told that their PMC isn't the right ammo (gosh darnit I didn't pay two thousand dollars for a gun that can't shoot everything!).

It's like I've said throughout this thread, when you buy a gun designed for and marketed towards the commercial market, you're buying a gun made for 99% of consumers, who will shoot it once out of the box, and if we're lucky maybe once a year after that. And as long as it goes bang every time they will never complain. That's just how it is.

I'm not trying to be a dick here, but I feel like the person coming to this site for advice isn't the average troglodyte looking for a safe queen, and I feel like they deserve the unvarnished truth. And hopefully, armed with that knowledge, they will go out and demand better and all of us will benefit, including the manufacturers. I'm positive DD would love nothing more than to have a more educated consumer who could appreciate a properly sized port if it was given to them, so I'm not trying to throw anyone under the bus here.

Um, the only thing I see on their website is gas block journal diameter which is .75 on the MK18 barrel... I assume you understand that those are two very different things so where did you find their spec for gas port size?

[Wake27]

Interesting about gas port sizes. I shoot lefty, so if anything I should be more sensitive to gas blowback. Never really had a problem with my DD MK18 - i.e. didn't really notice it being any better or worse than other AR's I own. And to the extent that it's overpassed ... that's a pretty easy fix isn't it? Just swap in an H2 or H3 buffer. Irony is that notwithstanding how gassy DD is supposed to be, I just swapped out my H2 buffer because last time I shot the upper I was not getting lock back on the last round. And that's with XM193. Not sure what the problem was. Maybe the mag I was using, since I don't recall having this problem before?

In any event, to me failure to lock back is at least a little problematic, particularly in a gun I could potentially use for HD. Really don't want to pull the trigger and hear click instead of bang. Same thing with poor cold weather performance. I live in CO and shoot year round. That means that I will and have shot when temps are in the low teens or single digits. A gun that only functions when it's sunny and 70 degrees doesn't really work for me.

And if you don't like overpassed guns, try shooting a HK416. Recoil is noticeably different than my other AR's.

When did you buy it? Mine is paired with an LMT E-BCG and VLTOR A5 system so not apples to apples to a stock gun but its very soft and I've never noticed blowback with my KAC QDC on it. Also never had a function issue.

[Five_Point_Five_Six]

Why on earth do people think DD is somehow synonymous with Mk18s? They never supplied a single Mk18 as far as I know.

I think you're literally the only guy in the thread that cares about that.

Um, the only thing I see on their website is gas block journal diameter which is .75 on the MK18 barrel... I assume you understand that those are two very different things so where did you find their spec for gas port size?

Lmao the only thing that can make this thread any better will be if he doesn't understand the difference between the two. They list the "Diameter at Gas Port: .750" which is not the same thing as the "Diameter of Gas Port". I don't think they've ever had their gas port sizes on their website.

okie, the gas port on DD 10.3" barrels is .070" and has been since sometime in 2017. I don't recall a time when they were ever .075, prior to them using the Crane spec gas port they were like .080 or even larger I think.

[Diz]

I disagree that he is the only one who gives a shit about what the term "MK-18" represents. I for one find the original Crane/NSW MK-18 story very fascinating, and it is at a bbl length I have always liked. There is also a huge segment of the gun-buying population who are extremely interested in this as well. Lots of folks follow exactly what CAG or DevGru or whoever, are shooting, not to mention carrying, and wearing. To dismiss Okie as the only one who cares about all this, is extreme hubris. Even if he doesn't know his GB journal from his gas port size. (Rimshot)

Realizing of course that there are guys here that do get issued guns such a these, there are also many more who build their own. To point a finger at him and say "dude nobody gives a shit", leaves three fingers pointing back at your MK-18 type rifle. That you don't care where it came from or who shoots it, right?

To be honest with you, I admire the MK-18 system cuz it definitely has some cool factor. So sure, I imitate the big dogs, to one degree or another in my choices of weapons and equipment. But at least I admit it. I'm having a hard time believing that nobody else here on this site doesn't do the same, to one degree or another.

And no, not confusing this place with the clone-correct threads at TOS, but c'mon, even if you don't get to that level of crazy, most guys do give a shit about what goes into a MK-18-type rifle because many of us have built them, and lots of us like to get as close to bone as possible. Maybe that is considered derp here?

[okie]

Um, the only thing I see on their website is gas block journal diameter which is .75 on the MK18 barrel... I assume you understand that those are two very different things so where did you find their spec for gas port size?

I don't know, I tried doing the same search again and I'm not seeing it. Now I'm seeing that the new port size is .072. Maybe .075 was the old size???

Bottom line, .07 is already plenty generous. Larger is a step backwards.

[okie]

The real answer is both.
Input provided, buy and shoot whatever you want. IME, 0.070 is in fact best.
However, regarding “correct” and “original” or “historical:” Nobody really cares. Nerds. Lol.

Again, not a clone builder here. I simply take what works. Sometimes that just so happens to be "clone correct." Sometimes it also saves money going with a "historical" feature. Case in point, the sawed off carry handle. Lots of times the 6920s come with carry handles from the factory. Ten minutes with a hacksaw and a file and you got yourself a CQBR rear sight, instead of paying a hundred or more for something else that's no better.

Interesting about gas port sizes. I shoot lefty, so if anything I should be more sensitive to gas blowback. Never really had a problem with my DD MK18 - i.e. didn't really notice it being any better or worse than other AR's I own. And to the extent that it's overpassed ... that's a pretty easy fix isn't it? Just swap in an H2 or H3 buffer. Irony is that notwithstanding how gassy DD is supposed to be, I just swapped out my H2 buffer because last time I shot the upper I was not getting lock back on the last round. And that's with XM193. Not sure what the problem was. Maybe the mag I was using, since I don't recall having this problem before?

In any event, to me failure to lock back is at least a little problematic, particularly in a gun I could potentially use for HD. Really don't want to pull the trigger and hear click instead of bang. Same thing with poor cold weather performance. I live in CO and shoot year round. That means that I will and have shot when temps are in the low teens or single digits. A gun that only functions when it's sunny and 70 degrees doesn't really work for me.

And if you don't like overpassed guns, try shooting a HK416. Recoil is noticeably different than my other AR's.

Well a .07 will run an H3 buffer suppressed, and lock back even with 223 steel. So no, swapping buffers probably isn't going to help, much less fix the problem, if your gas port is even bigger than that.