DnPRK - so if I gather what you're saying, there are more issues with a 16" barrel and getting the gas system to function correctly with the bolt carrier?
In a nutshell, yes, 16" barrels can be harder to get running correctly. This is because propellant gas is tapped off closer to the chamber where pressure is higher. Its called "over gassing" which causes the bolt to unlock and attempt to extract the case before the chamber pressure has decayed. Symptoms of over gas is case rims distorted/ripped by the extractor, extractor skipping over the rim (leaving the empty case in the chamber - the symptom seen in Vietnam) or broken extractor hooks. Trying to resolve such problems is a terrible experience for a 1st time builder.
To get a 16" gun to run correctly, all of the following have to be perfect:
1) the propellant has the correct burning rate
2) gas port is the correct size
3) mass of the bolt/carrier buffer is balanced to prevent premature unlocking
4) buffer spring is the correct weight
If any of those 4 are off, you can have a failure to extract and/or failure to feed.
Some folks crutch their 16" builds by adding extra strength extractor springs and o-rings around the extractor (in an attempt to prevent extractor skip over the rim). Others use gas blocks or gas tubes with adjustment screws that allow them to shut off part of the gas flow to the bolt carrier. The LMT enhanced carrier has a different cam pin cut that delays unlocking for a fraction of a second.
None of the above is an issue with a 20" or 24" barrel. The extra 4" of barrel allows the chamber pressure to decay to a level where extraction is easy and not as violent. My recommendation would be start with a 20" build and get it running right. After gaining some experience, you can experiment with a 16" build.
