Sloppy fire selector
 

I changed out my grip on my WE M4 to a magpul MOE grip, and since I've changed it my fire selector is really sloppy. When its in semi it feels very loose and move around, any tweaks to fix this? Or do I need a new fire selector?

The fire selector detent spring is probably missing from the swap......you need to replace it (rule of thumb, put back things they way they came apart)

Watch this video, skip to the 36sec mark, this is for a real ar-15. Just like the real ar-15, the WE m4 uses a detent spring for the fire selector. Find it, or buy a new one and follow the install instructions from the 36sec mark in that video

I kept the same spring and pin that was in there previously.

there could be a difference in the depth of the spring pocket giving different spring tensions. Try measuring the depths and offset the difference with something.

Change it back. Does the problem go away?

While you're changing it back take pictures of how the pin and spring are oriented.

I had to drill out the new grip hole a small amount for the spring to comfortably slide in, I wonder if it just ended up being deeper then the other one. What could I try shimming it with?

Compare the depth of the old vs. new with a feeler and then use something not compressible like a steel ball bearing.