It sounds to me that even though you have the gas on setting four,you're still not getting enough to properly cycle the rifle.The only thing I can think of right off is that your gas block may be positioned incorrectly.In other words the holes in the block and barrel aren't lining up completely.I'm not that familiar with the old gas system,but I'm almost certain that it indexes the same way as the new gas systems.
Since it is the old gas system,did you buy the rifle new or used? Since I would think used,did you talk with the original owner(if possible) about how the rifle performed before you purchased it etc? If you can talk to the original owner,ask him if the gas block was ever removed.It might not have been replaced properly.I'm just trying to determine if these problems are new or have always happened with this particular rifle.The gas block could have became loose as well allowing it to move some.Either way I believe it's a gas pressure issue.From what I gather your using descent ammo,so I don't think that's it.It also seems to be giving problems with different magazine too.The XCR usually isn't really finicky about magazines anyway and Brownells are better than most an U.S,G,I. magazine that I've seen.
If you look at the gas block check to make sure it is flush against the shoulder on the barrel.If not use an Allen wrench and loosen it,then slide it all the way down against that shoulder and re-tighten.Lastly take the gas adjustment knob out and see if the inside of the block is extremely dirty,that might cause this issue as well.If it is soak is in some bore cleaner(Hoppes) and also use some action cleaner inside the gas block.A toothpick is also handy as well as q-tips to get all the powder residue.Try these and get back.
Ok on the FTF.By that you mean that the new round fully chambers and when you pull the trigger you get a click and no boom? If so,have you tried re-chambering and firing them again,and if so did it fire.If so have you ever had a trigger job,and by whom.You may just need a new hammer spring,which is a really easy fix.