as far as the suppressor goes the easiest way to get one is to find a class 3 dealer near you that has what you interested in for sale and buy it. after it's purchased your dealer will give you 2 copies of a form 4, 2 copies of fingerprint cards, and a citizen ship from. the form 4's will each need a self picture attached and signed by your CLEO (chief law enforcement officer). that's usually your county sherrif. of course you'll have to be fingerprinted on both fingerprint cards. the citizen ship form is basicly you signing a form stating your country of citizenship. assumbly the USA. once all that's filled out and signed etc. etc., you'll drop it all in the mail with a check or MO for $200 to the address listed on the form 4. then a long wait begins. usually 3 to 4 months later you'll get a call from the dealer you bought the can from saying the forms are back and you can come pick up your can.
as for putting a short barrel on your rifle (I'll assume we're talking about an AR) the easiest way to legally do it is to buy a lower reciever or rifle, get 2 copies of a form 1, 2 fingerprint cards, and 1 citizenship form.
fill out the form 1's (a form 1 is an application to make and register a NFA weapon) there are spaces on the form 1 for info on the reciever you'll be registering as a SBR along with caliber and barrel length. because it's an AR and the caliber and barrel length are easily changed the best bet is to list all the different calibers and barrels you might use.
attach your pics to the forms, get them signed by your CLEO, get fingerprinted, then send off the form1's, the fingerprint cards, the citizen ship form, and $200 to the address listed on the form 1. then a long wait begins. 3 to 4 months later you'll get an approved form 1 in the mail.
after you have the form back approved you can then put a short barrelled upper on your AR. your lower will ahve to be engraved with your name, city, and state because you are the manufacturer that made the short barrelled rifle
HTH,
Luv