Anyone else have trouble getting the mag catch installed without having to grind/polish the sides of it?  I had a parts kit I got from CMMG (not sure of the brand) and I installed the catch, but it was too stiff to engage with an empty mag.  I ended up taking a dremel to the flat sides and now it works great.  This was my first build so I am curious if this is normal or not.

I had the same problem with mine. I ordered two kits from Model 1. The first mag catch works great but the second was the same as yours. Both kits are Model 1 and both lowers were CMMG. I have not disassembled and smoothed it yet but I will. Did you have problems with the detent pins? I did on both. I changed to DPMS pins and they worked fine. The others were to long, couldn't get them compressed enough to get the takedown pins in.

Every other part worked just fine out of the bag, I just have a few scatches on the lower from hammering out and in the roll pin multiple times.  No big deal though, it's a tool.

Have built 6 ARs and not had that problem; but then I've never bought anything from CMMG or Model 1.

Brother, you got sent an out of spec piece, which is why I don't use anything but Colt milsurp or Bushmaster parts in my ARs.  Tried both DPMS and RockRiver, early on, and got at least one defective part in each. Learned my lesson early

Did you try the mag catch in a different lower or a different mag catch in that lower.  I was building 2 lowers and swapped the mag catch from one to the other because of this ver y problem.  Now it is not a problem.