Quoted:
Did it crack the wall on the magwell? If not then you are ok. Don't let it happen again.

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Having read multiple threads about how you should never dry-fire an AR15 without the upper attached, I decided to find out how many times it would take to fail and if something failed, what part gave out.

Background: I had milled an 80% lower out of 6061 aluminum from 80percentarms. While the lower is functional, it's not going to be a keeper because of a milling mistake affecting the safety area (I had one of the jig pieces upside down!), so I decided to use the lower for this test.

For those of you unfamiliar with 6061, it is generally regarded as more than strong enough for use in an AR-15 lower, but not as strong as 7075 - the mil-spec alloy of aluminum. People frequently make 80% lowers out of them because they're much easier to mill.

After the first 10 dry fires, some paint was removed from the bolt catch and the hammer. After 100 dry fires, there was no evidence of any damage. Same with 200 and 300. After 400 times, there was an every-so-slight bulge in the lower, where the bolt catch was hitting the edge of the lower. To put this in perspective, I had to hold it at just the right angle to notice it and even putting a straight edge on it, it was really hard to notice. Roughly, I'd guess 1/10th of a mm.

After 500 dry firings, I removed the bolt catch and inspected it. Honestly, I would not have noticed anything unless I was specifically looking for it. That said, there was a tiny upraised area that I couldn't see, but which my finger nail would catch on where the hammer was hitting the lower. Other than that, it functioned perfectly.

I then re-installed the bolt catch and continued the test, but this time with a magazine in place. Maybe it's having a magazine in the lower that causes problems when it's dry fired without an upper, since an empty mag will lift up the bolt catch. After dry firing it 100 times that way, there was still no detectable damage beyond the missing paint on the hammer, bolt catch, and the two almost imperceptibly small dings in the lower I described before. To put these dings in perspective, I'm confident I could show the lower to 100 buyers and not one would notice these dings.

Quoted:
Did it crack the wall on the magwell? If not then you are ok. Don't let it happen again.

Quoted:
Hi all! I was cleaning my ar15 and I accidentally drop the hammer on my AR lower receiver without an upper (I did up my thump on it, but not hard enough). I was told that slamming a hammer on an ar15 lower without an upper is a big no-no. This is my first ar15 and I am worry that I might cause damage to my rifle. Could one hammer strike on the lower damage the firearm?
Thanks