I have a full auto AR-15, it fired fine till i got a new upper for it. I then switched back to the original upper and it still wouldnt fire auto anymore. It fires semi just fine then i switch it to auto it fires the first round then love taps the primer of the second round. Is it the hammer spring or the sear release? Any advise would help.

When you did the function check on full auto, did the hammer fall or just ride the bolt carrier up?

AR-15 Rep's question is a good one. May be time to replace the hammer spring. A weak one can cause the problem you're seeing.

I have had this same problem for a week now. Today I changed the buffer spring and it is working great! The spring I replaced the old one with was from the stock of an A1 and was a little longer than the one that was in my weapon. Hope this helps with yours.

This post has been edited by gpd279: November 28th 2009 2:10 AM

Works fine now, i replaced the hammer spring, buffer, bolt & firing pin. Thank you all for the info.

You need a heavier buffer. Look online for heavy buffers (like a H1, H2, or H3) or maybe even a Hydrolic (AAC maybe). I run a H3 and haven't had any Bolt Bounce issues since. Remember, the heavier the buffer, the slower your ROF (rate of fire).