One of the hardest parts of the lower to assemble is the forward push pin.  I've built two AR and while this doesn't make me an "expert" I have learned a trick to help get this part in without having a fit over it.   One thing I've noticed is that the spring is always too long to push the assemble down far enough to get the pin in place.  To fix this I take off 3 to 4 turns, that gives enough space for the pin to slip in without trouble but it is still under enough pressure to be secure.  The next thing you'll need to do is Make the Tool.  Don't try to save time by skipping this step.   If you make the tool needed as listed in the U.S.M.C. manual you'll be way ahead of the game, you must have this tool to do the job right.   One finial tip that'll work for any part that you might shoot across the room is to put a white sheet over you in a tent fashion.  That way if the part does go flying it'll hit the "tent" and not shoot across the room and be lost in the carpet.    I hope this will help someone out, building your own AR is really the only way to go, you'll save money and get to know your rifle to boot.  Talk at ya'll later.  TN.Frank

while i know people often have to shorten the spring i did not on the two lowers i have built so i'd try it first and then shorten if necessary.

There is a 10 cent part you can get at the HW store that works as well as the tool and for the life of me i can't remember what you call it.  It is a pin of the same diameter as the push pin and about 1.5" long with a number of holes through it down its length that are of the same diameter as the detent pin and spring.  Works great.  I'll probably remember the name as soon as i hit the submit button.

I fought with it for about 30 min. before i started trimming the spring, one turn at a time until i got a good fit. The problem i was having was that the pin was sticking out too far when fully compressed. It was just far enough to keep me from getting the push pin in place.

Clevis pin

Yup...and it's not .10 in my area.  It's more like .30 (bwaahahaha).  You stick it in the opposide side the push pin will go in.  Put in the clevis, line up the holes.  Put in the spring and detent, push it down w/ a tooth pic or paper clip untill it is in the hole.  Then turn the clevis so the detent is secured and then slide the push into it's hole (and it pushes the clevis out the way).  If done right, the detent will snap into place and you are done.

The other trick is to use a clear plastic bag so the detent/spring doesn't go flying...and if it does, "It's in the bag" [:D]  (Pun intended.)

What's the big deal about installing the front pivot pin????

All you have to do is take a punch that is roughly the same diameter and use it as a slave pin. Slide it in from the left side and then as you push against it with the pivot pin you can slide it out. Not a big deal.

Or just use a pivot pin installation tool.

All this is entirely too complicated..

Just push the detent into its hole with the pushpin, then rotate the pushpin into its hole. You'll be done installing in about 15 seconds.

For some, it's simple.  For others, it's a challenge.