#1-  I am under the immpression that you can use British SA-80 mags in your AR.  Is this true?

#2-  I am building my first AR and a guy I know wants to sell me his upper.  It's a DPMS 20" Bull barrel rig with a Tri-Mount upper and comes with a scope (Not sure what kind, I think he said 6x18).  He wants $700 and says that he'll throw in a carry-bag.  He shoots comp with it, and says that it'll shoot the tips off of popsicle sticks at 100yds.  Does this sound like a good deal?

1) Yes.

2) DPMS makes good stuff, but why is he selling it?  What make of scope is it?  Answer this and we can give you a better answer.

Jabber;

There is a lot more to a scope than the magnification. Find out everything, and ask to go with him to the range and watch him shoot some 10 shot 100 yard groups. Then come back here and ask if you should buy it after you know what scope is and tell us about the groups.

Good luck.
Him

Scope is a Bushnell Banner 6-18x50 Dusk&Dawn model.

He says that he is selling it to buy an ACOG for his 16", as he wants to get into tactical competion.

The bag is a Lambskin Assault Case.