Posting for a friend. He has a DD lower / DD 10.3 upper that is having alot of FTE's issues (would try to load next round but the spent round would get stuck/not eject) with the Govnah 3 position gas block. Rifle started life as a DD m4v7 (or v5) and has had a couple 1000 rounds through it with no issues. mk 18 Upper was built with DD rail/and DD 10.3 barrel.

Thought it was a ejector issue but after swapping 3 working bolts/bcg's, same issue. (the DD bcg works in other rifles)
Swapped out(working) buffers (carbine, H1, H2) and springs (reg and springco "heavy"), same issue. *H2 and springco white spring*
Swapped out pmag, lancer and gi, mags. same issue.
double checked the alignment of the gas block and it was right (kinda hard to screw it up), but no leaks. 10.3" barrels factory gas port is 0.083.
on the suppressor setting, there is no FTE, because there is not enough gas to cycle the action
on the next setting, its a mix of FTE's and when it does eject, it almost just falls out and not fly out
on the "factory" setting, its again a mix of FTE's and when it does eject, they go around the 3 o'clock position but not kicked out as far as they should.

Took about 200 rounds of ammo and a few times he fired a few strings with no issues. Ill say most he went was maybe 10 rounds before a FTE mostly PMC factory ammo, but had other misc kinds (hotter) as well.

I saw no mention of what muzzle device is on the Mk18 upper.  

I have a DD Mk18 upper.  It ran fine as it came from the factory.  I decided to install a Barking Spider to make the muzzle blast easier on the shooters to the side of me.  I began to have failures to extract the fired case from the chamber with the rim of the case ripped off.  With the BCG held back, the previously unextracted empty would easily fall out when touched by a cleaning rod.  New extractor, extractor spring and rubber ring, different ammo, clean the heck out of the chamber...nothing worked.  Ah Ha, removed Spider and reinstalled DD A2 type flash hider and functioning was instantly back to 100%.  The Spider was increasing the back pressure/dwell time enough that the BCG was moving back before the chamber pressure was reduced enough to allow the fired case to be extracted.  So, is the muzzle device increasing the back pressure on your friend's upper?

Epilog:  Read descriptions of other linear comps and contacted some makers to find a linear comp with minimal back pressure/dwell time increase.  Midwest Industries said their Blast Diverter has minimal increase of back pressure and was aware of DD Mk18 uppers having the problem I experienced when some other muzzle devices/suppressors were installed.  Andy of MI said their Blast Diverter should work fine, and it did.  Yesterday I fired 160rds with different powders and charges and no malfunctions; the Spider would have had several case rims ripped off in 160rds.  Note:  Not putting down the Spider...it works superbly on other uppers, short  bbl and 16" bbl and in .223 and 300 Blackout.

My first guess is the adjustable gas block. Open it up to the setting that supplies the most gas to the action and work down from there. Having too much gas is way better than not enough.

PMC ammo is hot enough straight from the factory. I suspect the "other" ammo, the "hot" ammo, to be a contributor to his problem. I am assuming those are reloads, and whether they are too hot I can't tell from hearsay. What I can say is most reloaded ammo problems are a direct result of insufficient resizing, i.e. he isn't pushing the case shoulder back enough. This will cause all sorts of problems during the ejection process and otherwise binds the ammo in the chamber when the bolt closes.

People who reload need good gages. I recommend Mo DeFina's excellent service rifle gages, RCBS's Precision Micrometer or Hornady's Lock-N-Load headspace gage. The Hornady is the most economical of the three because it can used with almost all common cartridges. Semi-auto rifles need the shoulder pushed back at least .003" from it's fired measurement. I use .004" for a little room to spare.

Simply screwing the die down until it touches the shell plate tells the reloader nothing about the final dimensions of the resized case. Many people mix and match brands of shell holders and dies which compounds the problem. A case headspace gages that gives you a measurement in .001" increments is a necessity IMO.

Warning: I use the reloading data published by the bullet manufacturer.......period. If I'm using Sierra's, I'll reference their manual. Hornady's the same. Hodgdon's online data is hotter than any of the bullet makers data, this should give everyone pause when reading it. Do not exceed the bullet makers maximum load.

well it looked like the plate he got (ordered the mk18 one) was either driller wrong or just got the wrong plate.  basically the holes were to small all the way around.  unfortunately for him, somewhere between him checking the holes sizes at work and going home, he lost the plate.  he wants to order a new mk18 plate, but he cant bring himself to spend $10 on shipping for something the size of a quarter.