UPDATE 17 July 2014 - Appears Gas Key Striking Lower- Input Please

[arptsprt]

SEE POST #51 FOR FINAL UPDATE - If you care...

SEE POST #35 FOR UPDATE

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.

[MarkG]

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.

Disconnector.jpg

[arptsprt]

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.

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.

Disconnector.jpg

[MarkG]

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.

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?

[Sparky5019]

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!

[BufordTJustice]

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.

Sent from my SPH-L720T using Tapatalk

[MarkG]

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.

If I were driving on the road behind you, I'd call 911 and report you as DUI...

[arptsprt]

Thank gents for the replies.

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.

[MarkG]

Thank gents for the replies.

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.

[TF82]

Is that the original receiver extension? Its hard to tell from the picture, but the castle nut doesn't appear to be staked.