I am still fairly new to AR's so I have no idea if this is normal.

I had a Seekins lower on the M16 knock with an upgraded trigger. then I bought a complete Daniel Defense upper. the take down pins are very difficult to push in a remove. We took it too the range and shot about 125+ rounds through it. The only problem we had was when it got too cold between cease fires, the magazine would not feed into the magwell unless you slammed it in.

Questions:
1) is the very tight fit of the take down pins something I need to worry about OR is just something that needs to wear down?
2) has anyone else experienced a magazine not feeding well because of the cold?

Did you have this issue before with the magazines? Did you try different magazines? I've had one issue where an old Colt mag wouldn't feed into my PSA magwell. Also are you positive that you didn't over fill the magazine. Sometimes you can get 31 rounds in the mag and it will bulge a little at the top.

If you have never had this issue before with the same lower and same mag I would make sure that you were not over filling the mag. If you've always had this issue then I'd say the lower is out of spec. With the take down pins some are tight some are not. If they are brutally tight like you have to pound them in with a mallet to get them seated I'd say either the lower or upper are out of spec and aren't matching up well. If they are just stiff that is no biggy. Mine are stiff to push out, I use the tip of my knife or the tip of a bullet to push it out. Pushing them in is easy though.

More likely than not, the pin holes are a bit tight. the steel pins will polish the holes a bit and the problem will eventually work itself out if you work them back and forth enough.

#2 is an interesting question (also -- was this on an open or closed bolt?) and I'm not sure I can answer this without knowing the dimensions. Do you have/have access to a decent set of calipers? If sounds like your magwell could be a little under spec, in which case I'd probably want to talk to seekins for a resolution.

Good question. If it was on a closed bolt a full mag with a stiff spring can be almost impossible to drive home unless you give it a really good smack.

#2 - If you are using a full 30 round mag on a closed bolt then yes, it can be tough to get the mag to lock in. Try only loading 25 rounds in to the mag and then install. You will probably see a difference. The other option is insert the full 30 round mag on an open bolt then release.

Are the takedown pins stiff without the upper on it? Are they stiff all the way through or are they just stiff at the ends when they hit the detents? If it is just at the detents, you can remove the springs and cut a couple of coils off the spring and then replace. Take off a little and test it, then if needed take a little more and test again. Makes the takedown pins a lot easier to move.