1/2x28 is the normal USA threading for 9mm and the easiest to find ..Some like Oyspre make square cans that will be easier to use with stock sights or you can get rasied sights .I would just keep stock and learn to shoot over them
As for can you need to firg if you want a one calbier can ie 9mm only that might have better sound damping or a muilt can that can handle a lot of calibers so might not sound as good but will give you more bang for your buck ..Some like the Librety cans and I think the Omega or the such will handle 30-40 calibers all the way up to rifle fire
Another thing to think about is full auto rating if you are planning on ever getting a full auto gun a can that can be used on it wold be nice a lot of the cheeper cans are not FA rated ..And lastly make sure you can take it apart to clean that is a big one
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