Relay - alternative to mosfet
Mosfet relays are widely used as an airsoft gun upgrade since they prevent the switch  contacts from wearing out. Aside from mosfets car horn relays can also be used as an alternative.

Here's the actual relay and diagram I'm using recommended by Sir Elaz. I think this is the smallest relay you can find on any Car SURPLUS shops price range 70-150Php (roughly 1-3 USD). Just ask for a horn relay with a 3 contact pins and make sure you open it up before buying it coz sometimes either they are busted, rusty or heavely worned out already. Choose the cleanly coiled ones.

With this relay connected to your AEG you'll have a burn mark free trigger contacts all year round especially if you have a highly upgraded gun and loves firing at semi. It will save your trigger contacts.

Note - really follow the diagram or the relay won't work at all. You'll know if ain't working if you still see some arcing/burning on the contacts when you press fire. Actual relay connected after the re-wiring...

kart m14

Jg m16

JG dragunov

Pros and Cons of using a car horn relay...

pros...

- way cheaper and easier to find than mosfets IF you buy the 2nd class types of relay at any surplus store.

- doesn't over heat

- can handle very humid or rainy weather (since car parts are subjected to harsh environments)

cons...

- a bit bulky and very hard to hide on smaller and shorter guns