Comparison of a pair of AR-15 lowers

About a week ago a picked up a pair of AR-15 lower receivers.The top one is a Century Arms lower, the bottom is (unsurprisingly) a Smith & Wesson M&P lower. Today I built up the Century Arms one with a DPMS lower parts kit. I chose that one first because the finish wasn't as nice with that one compared to the S&W version.I ran into a problem as I was function checking the assembled lower: the magazine well was pretty tight. The stock metal mags were tough to seat and the PMAGs were a real struggle to get in and out.I checked the S&W lower and the Colt rifle (that was obviously assembled) and in both cases there was no interference between the magazine and the magazine well.After an hour of hand-fitting it I was able to get the metal magazines to drop free and the PMAGs to almost do so as well. Maybe a few more thousandths and we'll be good to go. A bit of work with a bastard-cut file and a mill file opened up the clearances enough. I wish I had a good measuring rig and knew were the datum points were on the lower so I could blueprint it based on the specs.You can see how much material I took off the lower. There was perhaps 3-5 mil of paint (hard paint I might add) and another 10-15 mil of aluminum I wound up taking off of all the mag well rails before it got to "good enough" for me.You can also see the machining is a bit sketchy as well. Here's the S&W for a comparison:All the angles are sharp and all the flats are flat. That is how it should look. That is also how the Colt looks.So, while I've been told that "a lower is a lower is a lower," I can say for certain that it's not really the case.I'll have more on the assembly of the lower on Tuesday for an anti-teardown.  :-)[smugmug url="http://photos.vec.com/hack/feed.mg?Type=gallery&Data=27752306_x64BMx&format=rss200" imagecount="100" start="1" num="100" thumbsize="Th" link="smugmug-lightbox" captions="false" sort="false" window="true" smugmug="true" size="L"]