Quoted:
Oddly, there seem to be almost as many posters complaining about(too) tight fitting uppers/lowers.

As long as the two parts stay together, they're good to go.

Quoted:
A "TRUE" mil-spec lower you will not own without a tax stamp and 20K.  So now that we have that established a civi lower one brand will fit slightly different than another.  Take for instance an LMT lower vs a Aero Precision lower they will have slight variations not huge but still qualify as a mil-standard like lower.

But most lowers are made close to the mil standard but some manufactures make them with tighter tolerances to allow for tighter fit and no rattle like RRA.
Rattle is not anything to be worried about it can be fixed by an O ring or accuwedge if it bothers you.

Quoted:


As an interesting aside and perhaps as a testament to the adaptability of the AR beyond what most people feel is prudent to do, there have been the occasional reports of people complaining that their AR is functioning, but doing so unreliably, particularly with regards to feeding problems.  

Further examination then reveals that the user has in fact mated an SP1-style "big hole" .370 pivot pin upper with a standard .250 pivot pin lower without any shims or bushings, but simply assumed it was "plug and play."  

~Augee

Quoted:


As an interesting aside and perhaps as a testament to the adaptability of the AR beyond what most people feel is prudent to do, there have been the occasional reports of people complaining that their AR is functioning, but doing so unreliably, particularly with regards to feeding problems.  

Further examination then reveals that the user has in fact mated an SP1-style "big hole" .370 pivot pin upper with a standard .250 pivot pin lower without any shims or bushings, but simply assumed it was "plug and play."  

~Augee

Quoted:
Ive been doing some research and am wondering about Mil-spec lowers. I found a couple of internet discussions about Mil-spec lowers being a little loose fit. Most of these discussions were a couple years old. Is this still the case now?
Do the Mil-spec lowers fit better with same brand uppers? I realize some people on this forum will criticize that the way the lower fits is not a factor to concern yourself with. I understand this point. I have never owned nor shot a Mil-spec rifle.
I am preparing to buy a Mil-spec rifle and am curious about what to expect. Again im not saying that this is an important factor in deciding,I would just like some input from experienced AR enthusiasts. Thank you in advance.

1. With the upper receiver attached to the lower receiver, and the pivot pin and takedown pins in place,
perform the following test:
(a) Apply hand pressure to push the upper receiver as far to one side as possible.
(b) Attempt to insert a 0.020 inch thickness gage between the pivot pin lugs or the upper and lower
receivers.
(c) If the thickness gage penetrates to the pivot pin at all accessible locations, repair by replacement of
the upper receiver (see (b) below) or replacement of rifle is required.
2. If the rifle fails the above test, remove the upper receiver and install a "NEW" upper receiver and perform
the test again.
3. If the rifle now passes the above test, it shall be considered serviceable and continue in use.
4. If the rifle fails the test with a new upper receiver, this failure shall be considered a shortcoming. This
shortcoming requires action to obtain a replacement rifle. Once a replacement has been received,
evacuate the original rifle to depot for overhaul.
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