If you are looking for relatively cheap. Penn foster has a good starter program. It will give you a knowledge base to start from. If you can afford it the sonoran desert institute has a more advanced program that includes hands on projects and you can then further your studies with their associates in firearms tech program. You might be able to find an nra armourers course in your area. If possible look for an apprenticeship. Nothing beats a brick and mortar school if you have the $60k. Any of the other correspondance courses are just bs. Agi does have great videos to pick up on the side.
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