16" barrel with rifle-length gas systems are generally a no-no in my experience and best just not done EVER. There is just not enough dwell time and while some out there will disagree based on experiences to the contrary, for the utmost in reliability and functioning across a wide range of different ammo and shooting conditions...it is not a good combination to run at all.
You can do any number of different things to attempt to get it to run, including: a) opening up the gas port even more (not sure that is advisable depending on how large it has been made at this point and whether the block's port will take on too much more without having to be opened up as well); b) lighten your operating system (carrier, and/or buffer, and/or spring weight or move to a carbine instead of a rifle stock setup and a lighter buffer...rifle's are ~5oz and a standard CAR buffer is 3.0oz or so. If you like your current stock setup and dont' want to change it, you can either get a carbine buffer spacer, buffer and spring and run it in your existing A1 tube or you can buy a dedicated low mass buffer like those from JP which are ~3.2oz); or c) have your 16" barrel's rifle-length gas port welded up and relocated to either the intermediate (Noveske) or mid-length position. As to option "c)" I would check with the guys at ADCO on their offering of this service. It can usually be done for ~$100 if you have to have your barrel re-contoured for the journal or only $40 or less if the barrel is already the proper contour for the new journal location.
