I built this with some left over parts I had. As you can see, the upper and lower do not seem to fit well. The charging handle is a little bit harder to pull back then the ones on my AR's, but not bad. Maybe the Cerakote?

Is that a plastic lower?

What lower is that?

80%. TM if I remember right

No, it's a 80% that has been cerakoted.

I don't see why not. If the bolt cycles without any issues it should be fine. Check inside the buffer tube for any wear against the top of the tube. The explosive part is happening in the chamber. The buffer area just has to make sure that the carrier can travel clear.

As long as the headspace is within spec, the gun won't blow-up when fired.  I used to be obsessed with measuring headspace on all my builds, but eventually, after discovering NONE to ever be out of tolerance despite freely mixing and matching bolts, I decided to trust the "modularity" of the AR construction when it comes to factory installed barrel extensions.  Now, if I were building a barrel from scratch, yes of course headspacing would be part of the process.

The only other consideration is free movement of reciprocating parts - the BCG as it were.  As long as the upper and lower align sufficiently to allow the BCG to retract into the buffer tube without binding, the gun is GTG.

With everybody and his sister machining out their own uppers and lowers, it's not uncommon to encounter large gaps between the two at the area of curvature because this is where most deviation from spec occurs, but it only matters cosmetically because the recoil energy is carried directly to the buffer, the upper is merely a "housing" to connect the ends.

Agreed with everything but the headspacing comment: it is a simple and inexpensive process to buy/borrow gauges and check head spacing. There's a 60k psi explosion happening within a few inches of my face. I want to make sure that everything is contained. Checking headspace is the most important aspect of that. The AR may be incredibly modular, but one machinist arguing with his wife or coming in to work a little hungover shouldn't cost me my eyesight. Or deprive my wife the opportunity to look at my pretty face. Or even ruin my build. If the USMC armorer's manual though enough of it to include the check, I'll spend the three minutes. Back to our regularly scheduled program...[steps down from soapbox]

as long as the moving parts move freely, you SHOULD be GTG. Here's what I would do:

* Use some dykem or similar marker on the BCG to see if there's unusual rubbing or scraping as the bolt moves.  Blue or red sharpie can work well if you have a silver BCG. Mark up the tail end of your BCG to see where the BCG impacts the buffer. If its too close to the edge of the buffer head, take off the extension and remove the buffer retainer. Pull the BCG back through the opening to see if the BCG is aligned with the opening in the lower.

* I imagine it very unlikely that given what appears to be very slight misalignment that feeding/extraction would be affected, but it may not be a bad idea to see if you can cycle a bunch of duds or snap-caps.

Good luck

Check your gas tube location in the upper.  If it isn’t properly centered, you will see drag cycling the bolt, and in short order with firing you’ll damage the gas tube.

Pictures please. Let us decide.

OP....I Have a friend who had some various AR15 parts cerakoted and the moving parts were harder to move afterwards than they were previously.

Was getting ready to go to the range and when was messing with it some more. The bcg was getting stuck when charged. I pulled it apart and you can see where it is rubbing. If I undid the rear take down pin, it functioned fine. I've never used an accu-wedge, but do you think it would work to hold the upper receiver up a tiny but more so it will function properly?