If you try to paint an M4A1 while your inventory is full or nearly full you will loose all the parts that won't fit in your current inventory. I had two single slots (not together) and after painting my M4A1 green i saw that only the handle (which can fit in 1 single slot) was saved, all the other ones disappeared. I waited in the house i was for 10-15 min in case the other parts could appear there after a while, but no luck.