Just did a aero precision build and when i laid the gun upside down to make sure the upper and rail was nice and straight there is a gap about 2-3mm , i am wondering why there is a gap? No gap at the rear of the upper then gradually is a gap then when you get to the front of the rail it is nice and flat again. Is this a problem? I noticed this when I had the rifle upside down on a nice flat surface

Either I'm dumb or we need pictures. I'm not getting what you're saying
Edit: Are you saying where the upper and lower meet together it is tight on either end, but has a gap in the middle (like below the bolt catch)?

A picture would be very helpful, but if I understand you correctly it sounds like there are 3 possibilities:

    1. The upper is warped/out of spec
    2. The rail is warped/out of spec
    3. The rail isn't mounted quite right

Again, a picture would be very helpful.

Dont recall how the Aero handguard mounts but the holes on the barrel nut for the handguard could be out of spec causing the gap. However, if the Handguard is a clamp style then something else is wrong. Take it back apart and reinstall etc. Send photos if possible.

As long as the receiver top rail itself is flat, if you aren't planning to mount lasers or other aiming devices on the handguard then forget about it. Check if the handguard is pressing into the bottom of the gas block, since it'd be rare for one to be bent like that, but common to be mounted a bit off. But, if it's not making contact with the gas block, and is securely mounted... Handguards are for hands and lights in most cases, I really don't care if it's perfectly aligned.

Please post a pic.

If I'm reading this right he's saying that when it's set upside down, the receiver and start of the top rail are flush with his table, but the top rail at the front of the handguard has a couple mm gap.

If im right, are you sure your table is actually that level op.