Sounds like there isn't enough momentum to chamber a round.
Mass would inhibit momentum, no? Yes if the bolt moving at the same speed was heavier, it would have more KE to drive the round, however a heavier bolt would not be moving at the same speed.
I would have suggested he needs a heavier buffer spring with more push, or a lighter bolt to generate more speed.
There could also be excessive friction in the system preventing the BCG from achieving sufficient momentum.
Either that, or there is an interference somewhere in the mag housing.
I would be interested to know if the bolt gets hung up when there are no rounds in the mag.
Do you own or have access to any other ARs? I would measure the inside width of the mag well on yours, and compare to others. May explain why mags are tight to fit. Wouldn't explain the F2Feed though.
If you remove the uppers, insert the mag and measure how high the mag feed lips sit above the lower receiver, and compare to others.
I suspect the amount of force needed to seat the magazine properly is another symptom of the problem. I would be interested to know if magazine seating feels the same with the upper removed as it does when assembled.
Could be a bad fit between the upper and lower.
Ideally, the bolt should not be contacting the mag. You want to inspect the mag feed lips and the bolt for signs of metal on metal wear. If you are unsure you can mark them with a sharpie or thin coating of nail polish and see if it gets scratched off. If there is no contact between bolt and mag, than another explanation is that there is too much resistance in the magazine follower spring, making the round difficult to push out of the mag.
Try to get the oldest most worn out mag you can and see if you still have the problem.
Get a different BCG, preferably standard, and see if you still have the problem.
Get a different heavier buffer spring, and see if you still have the problem.
If none of those things make a difference, then you are probably going to have to take a look at the feed ramp where the round goes into the chamber. When a round gets stuck, where is the tip? is the round still parallel to the other rounds in the mag, or has it started to point up and guide into the chamber.
Take a close look at a round that has gotten stuck. Is the tip damaged as though it had been smashed hard into a brick wall? Is the round set back into the case? If so the feed ramp geometry is causing problems.
If the round looks like it is getting fed properly and not getting hung up on the feed ramp, try using the forward assist to drive BCG into battery. Does this work? If it move forward freely with the added push? If so, your BCG mass/Spring push is part of the problem.
I am just thinking out loud and throwing as much as possible out there in case something sticks.
