Originally Posted by David Hineline
In the other section put also to be used with stocks from X" to X" and barrel lengths from Y" to Y"
The ATF won't let you do that any more. As you stated, you need to choose ONE configuration that will be the "permanent" configuration. Then you can mix and match on a "temporary basis" to your heart's content. However you *should* always keep the parts so you can put the gun back to it's "original" configuration.
Should you change your "permanent" configuaration, you would still need to write the ATF a letter. However, it will take a LONG time before they send you a confirmation letter about your configuration change. (Like more than a YEAR.)