The price is set to zero because I am open to hearing offers. What is for sale or trade is my custom gas blowback m4. I bought it in pieces and built this. I bought high quality parts to put into the m4, internally and externally. The gun shoots great, feels amazing to hold especially with the kick. It's hardly used since I went off to school after building it. Offer me anything and I'll let you know. Feel free to ask questions!