I just built my first AR 15. I bought an accuwedge buffer to ensure a tighter fit between the upper and lower, but it came with no instructions for installing it. I found one site that shows it dropped into the back of the lower, with the "tail" sticking up and against the rear of the lower, and with the round disc down.  I tried this, but the upper and lower won't close completely.

Can anyone help with better instructions, or perhaps a picture of a properly-installed accuwedge? My rifle is pretty tight without it, so maybe I have a solution in search of a problem here. Thanks for the help!

1.  It sounds like you installed it correctly.
2.  It doesn't sound like you need it.

If you still want to use it take a razor knife and trim some off the bottom of the accuwedge until you are able to close the upper & lower.  

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If you can't get it to close after you've installed it you don't need this thing, your AR is tight enough.

Take it out and throw it in a drawer and check it again in a few thousands rounds.

If it aint broke don't fix it.

Suspicions confirmed. Thanks for the good advice. I will take it! In the drawer it goes. Good thing it only cost $2.95!

The fit will depend on what brand of lower receiver you have. A Colt SP1 (old thing) takes the Accuwedge as-is. A Rock River lower has a much higher "floor" where the Accuwedge fits and the base of the wedge would have to be shortened with a razor blade. Depending on whose lower you are using (high floor or older standard type), the wedge will have to be shortened or fit as it is.

Yep, what you said... I have a new Eagle Arms rifle that also has a "raised/high floor" type lower, and will not accept an Accuwedge without shortening.

This new design has been adopted by nearly all current manufacturers (I believe that only Bushmaster and Colt are still "low floor" in this respect) so that a drop in auto sear cannot be installed in their receiver.

When installing an accuwedge there is supposed to be a degree of opposite tension while closing recievers.  It is not supposed to slap right together.  A light squeeze is required.  BUT you don't want to have to use excessive force to close up.

I had to trim off almost 2.5mm from the base of my Accuwedge in order for it to fit my Armalite M-15A2, and then it was still a tighter fit than I would have liked.
However, it only has about 100 rounds throught the rifle from new, so maybe it will loosen a little after more firing.
Skippy