I have an extra barrel (take off) that I purchased on Gunbroker.com. If it were illegal to purchase a barrel of any size without an approved Form 1, you would not be able to buy it without the approved Form 1, from anybody. This would require the remittance of $200 to the ATF even before you bought it.
If you are going to buy a pre-manufactured barrel that is already cut crowned and has the flash hider, it would be wise, but not necessary, to obtain a Form 1 first. You may decide that its just too much of a hassel to make your PS90 into a SBR. Just do not keep the parts next to your operational PS90 (even though there is an awful lot of work that has to be done to replace the standard barrel with a short barrel, as well as specialized tools, time, effort, ability, knowledge, Blah Blah Blah!!!).
If you are going to buy a take off barrel (one that has been taken off of a PS90 that was cut at the flash hider) then there will probably be no problems having it without the Form 1 (unless you have a milling machine) because it requires additional machining before it can be made operational as a short barreled rifle.
Either way, it is not illegal to have a short barrel as the regulations are now written but others will say that their interpretation is different. This is the gray area of INTENT and ABILITY and CONSTRUCTIVE POSESSION and PARTS and TOOLS and KNOWLEDGE and TIME and Blah Blah Blah Blah!!!
Its a barrel and not a firearm, therefore it is not subject to regulation UNTIL you marry it to a receiver.
