Please note, having *a* drink while in the possession or even firing a firearm is vastly different than getting drunk while firing a loaded firearm. My point is that there are many impairments and you should always be aware of your capability under your current conditions (having a beer being a possible one of them). I would agree with you in that being impaired to the point where you become a danger is not just dangerous with this very dangerous tool but a plethora of other items as well. Yet almost daily I see at least one person drooping behind the wheel of a car on the interstate on my way home.
While it is possible that in the case of a diabetic (for example) that a person may be aware of their dip, that is far from a guarantee. Many fights were had with my wife when she was low blood sugar who believed it did not impair her in any way. Recording the situation and convincing her to watch through it changed her tune right away hence why I brought the example up in the first place.
I agree with your latter assertion however and it is my primary issue here. People dismissively assume that consuming even a thimbleful of alcohol impairs you and makes you a danger to others but even using one of the earlier tests for alcohol impairment while driving they showed that small amounts actually increase attention being paid rather than decreasing it in many people and in some people that amount is even larger. Instead, we've created this hysteria around the subject to the point that I won't even have a single beer when out to dinner because it's not worth the bull**** they put you through if you're pulled over and there is still a possibility the cop can or will throw the book (DWI anyway) at you even with a BAC of 0.001%.
Further, how far do you extend it? Do you not allow people with literal "blind spots" to fire a weapon in any manner? My father cannot see until you've passed 30 degrees on the right side of his head with his peripheral vision. Is this not a similar sort of danger also? How about someone with a cough? Long hair? Poor hearing? If you're going to stigmatize something because you might be impaired the tiniest bit where do you draw the line?
Please don't misunderstand me, I'm not trying to imply there isn't significant responsibility and safety risks involved with handling what is arguably one of the most dangerous tools ever created. However, I also am not of the mindset that a single beer (or even two in some people's cases) should impair the average person to the point they are unable to handle the firearm in question safely. I am, however, a heavy advocate that people should be responsible for all their activities, this one included, and should handle such dangers with the proper discretion. Granted opinions vary here and I do not fault people with different ones but that also doesn't mean I'm going to just blindly agree with them either.
Sorry if I rambled a bit. It's early and the baby is sick so I'm half out of it. (IMPAIRED!!
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I have the lower, LPK installed, pistol buffer tube (with buffer and spring), and upper already. Just need to get the BCG, barrel, gas block, gas tube and a few other parts to complete it. I might opt for a heavier buffer (4.6oz or 5.2oz instead of the stock 3oz) to reduce recoil. With a 7.5" barrel, I might need the heavier buffer. 
