It depends on how in depth you want to get, are you wanting to work on spring guns, multi-pump, and or PCP and is it going to be something you do for yourself or are you going to try to make a business out of it (and profit from it). If it's something you want to try at home and tinker with your own guns here's what you can do. Search the internet and all the forums. There is a mountain of information out there. Read up and copy files and print things off for reference. Then (what I would suggest) is buy some cheap or second hand guns (don't learn on expensive guns)and tear em apart and try rebuilding them. Try getting more in depth with your rebuilding and tuning, learning as you go and from your mistakes (you'll make 'em, everybody does) and get better each time. Now trying to crack into the tuning business will require that not only are you good enough (tuning and repair wise) but getting your foot in the door as they say. Tuning some guns for paying customers so you can get some word of mouth reviews and recommendations. A decent website would really be a help too. Most airgunners nowdays have and use the internet. There's not a lot of tuners doing it as a business and there's reasons for it. It's a niche market and it can be a tough place to make it if you know what I mean.
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Nathan