Hello All: I own two FNC's with the 4 position sear from S&H
One of them was having ejection issues.The other was not working in 3 round burst
I thought i would get to the heart on the problem if i swapped uppers
and bolt carrier assemblies back and forth
The solution was so obvious I feel stupid when i discovered the
Grenade Launcher sight was in the up postion!!!
2nd problem still exists: One gun in the 3 round burst position only shoots two rounds.
Any ideas?? Thanks
FN factory 3 round burst kit or aftermarket? What ammo are you using, and what is the barrel length? Have you ever gotten 3 rounds, or has it always shot exactly 2 rounds in 3RB? It is possible your one of your 3RB kit prongs is damaged or was incorrectly machined, leading to only 2 prongs engaging rear sear (hence, 2 round burst). If it is inconsistently shooting 1,2, or 3 rounds, it could be you ammo or gas system. FNCs like full power 5.56 NATO ammo, especially in FA and with short barrels. That is what they are setup for from the factory. Using weaker (like UMC) or inconstant pressure ammo (Wolf, Tula, etc) can lead to 3RB and FA cycling issues. One thing to try (regardless of ammo) is to put the gas setting in "adverse mode" and see if it cycle better. If it does, you likely have a gas problem, whether ammo related or gas system adjustment needed. Ignore how far it flings your brass. If your FNC is not flinging your brass far and away, you have a pressure issue. Believe me, I've tried on multiple rifles to adjust the gas to make nice piles of brass. They always end up unreliable at those adjustments.
BTW - you should never swap the BCG on FNC uppers unless you are also swapping the bolt between the carriers (same bolt stays with the same upper/barrel) without specifically checking the headspace with a set of 5.56 gauges. The headspace is specific between the bolt and barrel. They don't have any type of "automatic headspacing" off of a barrel extension, and bolt faces vary in depth. Swapping bolts can lead to improper headspace, which can be very bad... This is why GM aftermarket barrels don't have the gas port drilled. Every barrel replacement has to be headspaced to the receiver/bolt of that rifle before the gas port is drilled.
