I've talked about these polymer lowers and uppers before, but I thought it'd be a good idea to share my experience with them.
I bought an American Tactical (ATI) lower back in March and put an Anderson slab-side upper and barrel on it.
I also put a 4-12x40 scope on it too. I sighted it in and did get an 1-1/2 five shot group with my reloads.
I put another AR together with both upper and lower being a polymer and both the same brand - ATI
And in just putting them together right out of the box -I found that the pins do not line up.
The upper is too far forward:
Here's another picture:
And right here seems to be the problem:
The upper is pretty "square" on the rear lug where the pin-hole is and contacts the rear of the lower.
In the next picture, you can see where I removed some material.
It is more material than I'd like to remove with a file or something similar.
I used my air powered die-grinder and removed a little material at a time.
It didn't take very long at all and the pins lined up just fine.
In the 1st pictures, I showed there was a good sized gap between the upper and lower.
While the gap was smaller - there is still a gap there - but it doesn't seem to matter.
It doesn't bother me anyway.
Here is where I removed the material:
You can see it right under the threads where I rounded the back end of it.
I removed material from the lower here because this is where the polymer is cast around
the aluminum, and I figure it's a little stronger there.
The rear lug in the upper is all polymer - but I'm sure it could be rounded off a little.
I wish I woulda took a picture of it.
Another thing with these polymer parts - the front pin-hole lug on the upper is aluminum.
It seemed to have a few edges and small burrs - So i used a flat file and got rid of the edges smoothed things up,
And on both the upper and lower, I felt along every edge and smoothed anything I thought needed it.
And in shooting an AR made with a polymer upper, I noticed it smelled a little different that an aluminum upper.
Why? I'm not sure...
And with the ATI polymer upper, a regular "clam-shell" type upper clamp won't work.
I used the Magpul BEV Block and it worked great.
I wouldn't be afraid to use the polymer upper and lower combination.
I would like to spend some more time using them - but what the hell - they work!!
You can have a working AR-15 for a nice price.
And I'm figuring on keeping the the AR I have with the polymer lower and Anderson upper for a while longer.
I just like it!!
Plus - this election could make it worth a LOT of money before too long...