[cblair]

bought an AR-15 lower for an old barrel that I had laying around and the attaching hole in the front is smaller than the one in the front of my upper receiver , is there a bushing for that or what?

 

[woodstock]

Is that a colt upper by chance? If so there is a bushing that will make that work.
http://www.midwayusa.com/product/59...apter-bushing-315-to-250-diameter-ar-15-matte
You can also have the lower drilled out to take the large pin for a more permanent fix.