1. If you are having trouble with your own build, post up, someone here will help.
2. If you are having problems with a built rifle or upper call the company or person that built it or the last person or company to work on it.
could be caused by low powered ammo such as Remington when talking about the 6.8, in the 6.8 it is also common to see short stroking with light bullets 80-90 gr and fast powders. With the 6.8 and the fast rifle powders it burns it is best not to use a H2, H3 or hyd. buffer or aftermarket springs.
Do not cut off buffer springs as the first move, check the alignment of the gas block over the gas port first and keep everything lubed up well for the first 200 rounds but, DO NOT put a heavy coat of oil in the chamber or barrel, always run a dry patch through to remove excess oil. Is anything binding when you pull the carrier back and forth by hand? I found a carrier key installed with one side up on the rail of the carrier from the factory.
If you are able remove the gas block and look at the soot ring(soot left on the barrel by the large hole of the gas block)
It should be completely around the gas port in the barrel. if it is not make witness marks with a sharpie to the rear of the port and align the gas block with it. While you have the gas block off check to see if the gas port in the barrel is clear, sometimes when they are drilled a small flap of metal is left hanging and when the first bullet passes it shoves it back into the port. On Mid and rifle length systems the port should be 3/32" in diameter. You can slide a 3/32" drill bit in to check you could also open your port to that size just be careful not to ding the barrel on the far side of the bore. Some new light bullet loads are even slower than before and some 20" barrels with rilfe length gas systems need a .106 or 7/64 port for those to function. IMO you should not drill ports or do anything else that is not reversable as a first attempt as finding a cure.
If you are using handloads be sure your size die is squeezing the base down to .417-.420. Most chambers are .422 in dia at the base, some dies size to .422 and a semi-auto needs at least .002 clearance to chamber and needs .004 to be reliable when hot and dirty.
If the rifle in question is older the gas tube could be clogged, remove the gas block and tube assembly from the rifle and try spraying brake cleaner through the tube to remove powder residue.
Make sure the gap in the gas rings on the bolt do not line up.
As long as the upper receiver has M4 feed ramps feeding issues are usually problems related to the mags or a lower receiver with a generous mag well size the allows the mag to move back and forth. Try pushing the bottom of the mag forward while pulling the trigger to see if it feeds, if it feeds while pushing forward but not while hanging free then you know the problem. I see many people grasping the mag with their left hand, pulling back on the mag itself can tilt the feed lips down pointing the bullet low into the feed ramps. Hold the mag well not the mag. They make mag lip tweekers but, I have seen a lot of messed up mags when people don't know what they are doing
See this post for more mag information
Jam does not explain if it's a FTF or FTE, feeding issues(FTF) are usually mag related unless the weapon is hot and dirty and the buffer spring may not be strong enough with the extra resistance. Thats why most that run their weapons hard use a SSS spring or other and heavier buffer and tune the gas system to work with those parts. Fail to eject(FTE) can also be because the weapon was tuned to run with soft springs and a light buffer while cold so when it does get hot and dirty there is not enough gas to cycle the action. If the spent case hangs in the bolt while the bolt strips a new round from the mag then it's an extractor/ejector issue.
CALL THE COMPANY THAT MADE THE UPPER, CALL THE COMPANY THAT MADE THE UPPER
ETA- Sticking cases
seem to be a growing issue, I have posted this everywhere hope it helps.
There have been several post about cases sticking in the chamber. The number 1 cause is not prepping the brass before it is loaded. Check the case mouth to see if a lip is present, trim , deburr or chamfer it until the lip is gone. The necks of a loaded round must be .306 or less to chamber reliably so check the diameter of the neck after it is loaded at the mouth and the bottom of the neck to make sure it is .306 or less before loading 100.
A problem with fired brass is OAL of the case, it should be 1.690 MAX. Trimming to 1.670 will not hurt anything.
When setting up a resize die check the first few cases in the chamber before you size 100. Make sure they work first, it may save you a lot of aggravation.
Seat the primers fully, a primer sticking out the bottom will tighten the headspace just as much as sizing the case wrong.
Burrs on the bottom of the case from swipes will also tighten headspace.
Every once in a while clean the chamber but do not stick a wire chamber brush in a drill and run it in the chamber, it could hit the lands in the throat and hurt the accuracy.
Clean the case lube off of the cases, I had a case stick on me a few weeks ago, turned out to be a powder granule stuck to the side of the case from loading.
Clean out the mags, sand and grit from the mags sometimes stick to the cases, and will scratch the chamber, it is not covered under warranty.
ETA- Pressure signs
? careful crimping your reloads, I found cases that have been crimped at least .050" down from the mouth. They were crimped so hard the neck of the case was swelled to .310!! .004 over size, the scuff marks on the necks confirmed they were large before they were chambered. A crimp holding the bullet is one thing but the chamber squeezing the brass down on the bullet is like a vise and increases pressure.
When you first receive a new rifle, upper etc inspect the bore for obstructions and run a patch through to remove any grit that may have entered the bore, chamber during shipping or handling. All it takes is 1 grain of sand to mess up the head space, one piece of foam peanut to blow up a rifle, grit in the bore to scratch a barrel.
If you drop ammo or a mag in the sand at the range at least knock the sand out, if the cases are covered with case lube and the sand is stuck to them then every case should be removed from the mag and wiped off and the mag should be cleaned out.
If you decide to do a sand or mud test and destroy YOUR rifle in the process, it is not covered under warranty.