Airsoft MODS/Homemaderss

Anyone into Airsoft knows that it costs 100s of $'s for a top quality Airsoft gun, ie Tokyo Marui. The only thing that makes their stuff so good is they actually make/design their products so they DONT break and they're good quality compared to the cheap ass 20-30$ knockoffs. So what really sets this HUGE price gap of 20-30$ and 2000-300$? I really dont care! And that is what this groups about. Designing top quality, DURABLE airsoft, all metal gearing (if electric), Better alternatives and something better than 150fps for a fully automatic. So send in your mods of existing airsoft guns, homemade guns, even a technical tutorial regarding spring variable to get the best power is welcome."