Below are some pics of the lower my FACTORY Daniel Defense Mk18 SBR. Bought the rifle as a factory new SBR…
I had some initial issues getting the carbine to run reliably as new. It was sent off to DD because of numerous malfunctions and it is running better now (I am positive it is overgassed). Then, I noticed this today while cleaning. Appears the gas key has been or had been lightly hitting the receiver. The carbine has been primarily shot with a Gemtech Trek T suppressor.
I verified the length of the buffer tube (appears in spec) and at full stop to the rearward position of the BGC, there is a about a 1/16" gap between the gas key and the receiver. More than anything, it appears the anodizing was chipped off and I can barely make out a light rub mark in the shape of the key. Using a razor straight edge, it does not appear the alloy itself has been deformed.
I can guess what’s behind and I’ll contact DD tomorrow but I’d like some of your input in the meantime. I am assuming this is not normal for a suppressed SBR.
Not the best pics but the best I could do. Thanks for the input.
What buffer assembly are you using? Does the bumper look like it has been taking a beating? The other part that you need to inspect is the disconnecter. Check the location indicated below to see if the hood on the hammer has been making significant contact. It’s sounding like an H3 or X Buffer may be necessary.
Currently an H2 with a white Springco spring. Bumper looks normal. The disconnector appears to have some slight contact marks but my other ARs appear to have similar marks. Thanks.
Should be closer to 3/16. It’s improbable on a properly built AR for the carrier key to come in contact with the receiver. In any event, a carbine with a 10 inch barrel needs an H3 buffer without the suppressor. The suppressor certainly changes things. Very often on seriously over gassed shorty carbines, the hood on the hammer will come into contact with the spine of the disconnector to the point of deforming it.
If the receiver extension is manufactured and installed correctly (should be 7 inches deep) and the buffer assembly is the correct length and the bumper is manufactured with the correct material, something else is going on. Is there any chance it was shot without the buffer assembly and action spring installed?
I’d also consider a Tubb flat wire 36 coil buffer spring or at least a Sprinco blue. You could also trade the DD GB for a Syrac. That would smooth it out a lot!
Actually, it is possible for the gas key to make contact with the lower even if everything is in spec (aside from the gas port). If requires being grossly overgassed.
The Army research paper in the calibrated over gassing thread notes that this was seen with the rifle length gas system when you open the gas port up enough.
This is a gas issue and needs to be remedied by an adjustable gas block or a new barrel with a smaller gas port.
I just got back from the range to do some troubleshooting. There is something out of whack with the lower. I put my BCM 11.5 (non-supressed) on the lower and put a piece of tape between the upper and the lower at the back end. Even the 11.5 took a small bite out of the tape.
Upon further inspection at the range, it appears to me the receiver extension should be turned one more time around… It has the extended lip on the bottom but the lip barely sits over top of the buffer catch. The additional twist looks to me like it will give me the +/- 3/16"…
I was able to try the suppressed upper with an H3 and blue Springco as well as with an H2 and white Springco. Same results.
There is no chance the gun was shot without the buffer and spring. I am the only owner of the carbine and again, it was new from DD at my LGS when I took possession after my stamp arrived. I did not shoot it sans buffer and spring. I still think it’s overgassed. Especially when compared to how my 11.5 BCM shoots. The H3 and blue spring seemed to help but the thing still ejects brass about 9 feet and 1 o’clock even with low power .223 ammo. At least I haven’t had any malfunctions since I got it back from Daniel Defense. Brand new the thing was frustrating - I had a bunch of malfunctions of every type. I don’t know, experts who know more than me say not to read into the ejection pattern so long as it’s reliable and it is has been upon fixing by DD. The BCM creates a nice pile of brass about 6 feet at 4 o’clock and it is so smooth.
Excellent troubleshooting. I think everyone will agree that DD barrels have big gas ports to begin with and the suppressor exacerbates the problem but compressing the bumper on the buffer assembly to the point of impact is unlikely. One rotation of the receiver extension will shorten the travel of the bolt group by 1/16 of an inch. Since you have likely isolated the problem as the lower receiver and you are unable to get another turn out of the receiver extension, you may need to look at the location of the buffer retainer.
If you know your key clearance is that small and continue to troubleshoot by test firing, then do yourself a favor and pull a quarter or two out of your pocket and drop them down the RE before the spring. There are many possible causes to your issue, including possible stacking of issues. The quarters will give you a margin of safety while you get your issue(s) resolved.
Yes, it is the original receiver extension as delivered from DD. The castle nut is staked albeit barely. And just think, this was a gun I had purchased pre panic. Not impressed with the quality.
Just FYI DD Mk18’s come with a super overgassed .081" gas port to allow use of wolf and tula ammunition. Proper port size is .071", so even in spec there could be striking due to how overgassed they make the Mk18.
Damn that BRT insert worked wonders! I could get an 18" with rifle length gas system to run on a .081 gas port. That’s absurdly huge for an SBR (which DD should KNOW has a very high likelihood of being suppressed).
Our COTS MK18 Uppers, Rifles, and Barrels do have a gas port diameter of .081". Yes, it is larger than the norm but we did this for good reasons. I think that we’d all agree that not all ammo will perform the same. Taking into consideration the short dwell times on a 10.3" barrel w/ carbine length gas system and which ammo is available on the commercial market, we had to optimize our gas port diameter to work with the widest range of ammunition which is available to end users and law enforcement agencies. The gas port diameter we’ve selected has proven to be very reliable with both 5.56 and most .223 ammunition. As a result of the reliability of our platform, the MK18 has been selected by many local, state, and federal agencies for duty purposes.
We welcome any and all feedback regarding our products. Both the good and the bad. It helps us become a better company and makes sure we stay on the cutting edge of manufacturing.
