Hello. I'm a new member to the AR-15.com forum but have been a frequent visitor for some time. I looked to see if there have been similar posts on my topic but could not find any. This is kinda a two-part question.

First, I can already answer part of my question. Ans: If it shoots well - its fine. And it does.

With that said, something has been troubling me for quite some time and I need your take on it.  I have two complete AR-15's that I have put together using different two diffent lowers and uppers, all post ban.  My parts for one rifle include a Mega lower and Bushmaster barrel/Diemaco upper reciever.

When I assemble the Mega lower to the upper receiver I notice that they are mis-aligned on the exterior and also in the magazine well. To put is more simply, the parts don’t line up seamlessly, and the lower is off to the right with an overlap. This is happening to my other rifle as well.

But the combo does shoot well. I have shot this AR-15 several times at the range with no malfunction but cant figure out how the round chambers correctly when these two components are not lined up. Please someone explaine.

Second part, I am told that all these parts are "mil spec" but have found through my experience that mil spec is a load of bull. All manufacturers' forging specs are in fact different but all claim to be "mil spec.' So what the heck really is mil spec when there are varies in measurements.

In fact all parts are not really interchangeable. To prove my point, I recently attempted to put together a Rock River lower to a Diemaco upper receiver and they simply did not fit at all. The rear lug block is too shallow for the Diemaco's rear lug. Try it sometime and see. Both manufactuers claim to be mil spec but obviously do not fit eacother! If everything was truly "mil spec" then everything should line up perfectly, but the reality is they don’t.

So, I am assuming that my first problem is simply cosmetic and does not have any bearing on the faction of the rife. I'd like to know if other people have similar experiences. And please someone explaim this mil spec myth.  

Thanks.
-Spaten

Dry land is not a Myth!!!!!!!!
(It just doesn’t exist)

In regards to the M-16 rifle, the government has specifications that the rifle/parts must be within or the parts/rifle are rejected.  Because of this, rifle/parts produced by government contractors are checked no less than three times to make sure that they are not outside of the tolerable specifications (the +/_ that you see on plans). The rub here is that no Military contractor can sell these parts to anyone else than the US government. (See top post labeled FN)

Now we step into AR-15 semi-auto land.  Each non-military manufacture has their own proprietary specifications that they use to produce their parts.  Since the military does not buy AR-15's (semi's), they do not have any specifications for semi parts. Colt was the first to design the SP-1, and current non-Colt semi spec parts are loosely based off it. This means that there is no such thing as a Mil spec for semi parts, and only semi parts that could be interchangeable with a M-16 could be built to Military specifications.

Now we step into the world of being PC.  As stated, the first semi rifle produced was the SP-1 by Colt.  Colts concept was that the rifle was to be a semi only, and when items like the LL and Dias were designed to turn the rifle into a MG, they started to charge parts on the production rifle to prevent such. The original SP-1 has a screw type front take down pin and the upper receivers were not sear channel relieved from the start, but latter, parts like their FCG pin being oversized, the sear block, and even the open bottom C carrier were further steps to keep there rifles as only semi platform.

In the beginning of non-Colt clones, few producers were concerned with their rifles ability's to be used at LL/Dias platforms, and these rifles grew a gathering since no smithing was required to the rifle/parts for conversion. But, after a while, some of these producers became PC and began to make changes in their designs to prevent full auto conversions (high or low shelf receivers).

In regards to RRA, there focus has been that there Semi auto rifles were meant to be target rifles.  They not only designed their receivers to not accept Drop in conversion parts, but also specified that their receivers be milled to tighter dimensions to take out the slop found in the original Sp-1 design. Because of these changes, even their own upper receivers fit very tightly to there own lowers.

Now the rub, when referring to Ar-15's, you must think of each manufacture as producing their own systems or versions of the first semi rifle.  The parts that could be used as M-16 parts are built to some what of the standard military specifications, and their other parts being built to work around with there own version of those specifications. Since AR’s are produced as semi target rifles, each manufacture produces their parts to find a niche in the market.  Some focus on the ability for their parts to be interchangeable with other producers, while others focus on their rifles producing the tightest groups.

To sum it up, the Sp-1 rifle was the first semi AR, and over the years, and even Colt has made changes to their design in the progression of the rifle.  Since there are no military specifications to hold a constant from one manufacture to the next, each manufactures created there own proprietary specification to produce the parts. Some are just produced closer to what could be considered military specifications (overall theme of the military specifications, but still in a semi auto configuration).

Almost forgot, Welcome to the site!!!!

I've had the same problem with several manufacturers parts, get used to it, and have fun with it.  It is usually not a big deal to figure out how to make things work if you have some mechanical inclination and a few shop tools in the garage.  I too have done the RRA lower thing several times and have grown to love it, the rear lugs on brand X always have to be dremeled "just a hair" on both sides at the top to make a PERFECT fit, I love it!  Conversley, the last complete Bushy I received had so much slop between the upper and lower that I almost returned it.  Yes, different manufacturers use different machine shops to finish the forging into a final product, and you would be suprised how many BIG manufacturers don't even make the receivers at all, it's all contracted out to various machine shops, with who knows what sleepy high school senior pushing the buttons for minimum wage.  There are variations, tools wear, people get sloppy, but still these things are VERY close to original blueprints considering all the variables.