I have an SP1 lower receiver that I sent to M60Joe to modify the big hole pivot pin so that it will take a Mil spec pin.  Not cheap but then who else does this?  I bought a 601 pivot pin from NDS and I also ordered a dimpled takedown pin from someone on this site.  Now here's my question: I have the proper A1 upper that has the recessed pivot pin hole to accept the 601 pin.  However, I was thinking I'd like to build something not so Retro (I already have built an A1 and A2) so I was wondering if anyone knows whether there is an A3 flattop upper that will take the 601 pin.  If not, I was thinking of maybe using my dremel or drill press to try and recess an A3 upper to fill the bill.  Anyone know of any bits that might work for this? Ideally, I was thinking what would be perfect is a big that had a smooth pole coming out of the middle that was the size of the milspec pivot pin hole but then had a small sleave on it what would ream out the recessed area.  I doubt if anyone makes such a tool but just thought I'd mention it.  On the other hand, I might just go ahead and use my recessed A1 upper but build a carbine instead of a rifle using a Colt 6920 barrel.  Any thoughts would be appreciated.  And yes, I expect most if not all of you will tell me to stick with the A1 recessed upper.  If anyone has any pics of a carbine build using a recessed A1 upper I'd appreciate it if you'd post them here.  I know there's some on another site but I'd like to see some real closeups of the build.