Building an "other" question

I've been doing research on building an Other, i see people saying if your under 26" you just need to pay the $200.00 to register the gun as an AOW, does that mean i can be completely below 26" ? for example go with like a 8"-10" BARREL ? and build it to come up to around 18-22" ?

not too familiar with this, just asking for help and advice.