What is the difference between them? I know about the pistol with a forward grip type AOW, but I see short 870s and such that have a pistol grip and short barrel that apparently are AOWs. What makes them AOWs and not SBSs?
Sorry if this is a stupid question or if I missed the easy answer somewhere.

What makes them AOW's,at least in the 870's case,  is they were manufactured without a shoulder stock installed and registered as such with the BATF&E. Making an AOW is a $200 tax. Transfering an AOW is a $5 tax. Advantages to the AOW's are: 1) more states allow AOW's than SBS's. 2) no 5320.20 required for interstate travel. 3) Other than the mfg.'s no additional markings are required.  

Can you slap a stock on an AOW 870 with no paperwork changes?

Quoted: Can you slap a stock on an AOW 870 with no paperwork changes?

no

that has always confused me. I can have a 870 with a 12" barrel and a pistolgrip for a $5 tax and to add the buttstock is $200.00. It seems flawed>>>>

Quoted: Quoted: Can you slap a stock on an AOW 870 with no paperwork changes? no

Ok, that's what I thought.
Doesn't make any sense though.

Quoted: What makes them AOW's,at least in the 870's case,  is they were manufactured without a shoulder stock installed and registered as such with the BATF&E. Making an AOW is a $200 tax. Transfering an AOW is a $5 tax. Advantages to the AOW's are: 1) more states allow AOW's than SBS's. 2) no 5320.20 required for interstate travel. 3) Other than the mfg.'s no additional markings are required.

Not to beat the proverbial dead horse, but why does an SBS/SBR have to be marked, but not an AOW.  After all, the person registering it is the person "making" it.

I am pretty sure that they are still required to be marked by the manufacturer, whether that be a C2 or an individual. The Remington 870 AOW that I have was made in to an AOW by Scattergun Technologies and is engraved with their info (company/city/state).

As for marking requirements: If your making an AOW then yes you would have to mark and pay $200 making tax. If your purchasing/transfering( $5) a Factory Remington 870 AOW then all required markings are already on firearm.
As for installing a buttstock: You can convert a title 2 to a title 1 firearm at will meaning you must install atleast an 18in bbl. with  buttstock installed or re-register as SBS to permanently install buttstock with short bbl installed. BTW, This info comes from NFA FTB Chief Sterling Nixon.

Ok so the stock and markings. Got it.

A "shotgun" is by definition a shoulder-fired weapon.  A short-barrel shotgun is an NFA shotgun, shoulder-fired with a short barrel.  But an AOW made out of a virgin or non-shoulder-stocked shotgun receiver is not a "shotgun" at all, it's a "smooth-bore pistol,"  a firearm NOT designed or redesigned to be fired from the shoulder. That's the primary difference here and it helps to think of it that way when trying to figure out which to do.  

A Remington 870 AOW is an NFA pistol, so of course it cannot have a stock as an AOW nor can it become an AOW if it had a stock as a Title I firearm.

AOWs are cheaper to buy because they transfer on a $5 tax versus a $200 tax for an SBS. As for Form 1ing your own, there is no savings as the tax on a Form 1 is $200 regardless of what you're making.

AOWs can be a loophole in states that don't allow short-barrel shotguns, but this can work in the opposite direction too.  Some states use definitions that do not differentiate between an AOW built on a shotgun receiver and a short-barrel shotgun, they partially define it through the ammo it fires or is designed to fire.

Quoted: As for marking requirements: If your making an AOW then yes you would have to mark and pay $200 making tax. If your purchasing/transfering( $5) a Factory Remington 870 AOW then all required markings are already on firearm. As for installing a buttstock: You can convert a title 2 to a title 1 firearm at will meaning you must install atleast an 18in bbl. with  buttstock installed or re-register as SBS to permanently install buttstock with short bbl installed. BTW, This info comes from NFA FTB Chief Sterling Nixon.

You're right, but to add one thing...

Truly converting to a Title I shotgun means adding the barrel AND having the weapon removed from the NFA Registry with a letter to NFA Branch making that request.  A registered SBS can have an 18" barrel on it, yes, but it is still an NFA firearm as long as that firearm appears in the Registry.  Even with a Title I-length barrel, it still needs to be treated as an NFA weapon, all NFA regs need to be followed.

So can a SBS have a pistol grip installed and still be legal ?

Quoted: So can a SBS have a pistol grip installed and still be legal ?

Sure, an SBS does not NEED to have a stock attached.  If can go out with a pistil grip only.