Just having a rifle stock in the same place as a pistol AR is considered constructive possession and you can be charged for it.
If you have a rifle AR and a pistol AR, then having the stock around isn't necessarily constructive possession, as you can claim the stock is for the rifle.
But, so long as it is of legal length, 16" barrel and 26" overall length, then you are good to go.
It doesn't matter what the action is. If it is a rifle (bolt, semi, etc.) then it has to be 26" in overall length and 16" barrel. They measure the barrel length using a dowl that is inserted down the barrel with the action closed. It wouldn't hurt to leave a little extra, just to be safe. That's why most shotguns are 18.5".
Honestly, I don't think it's advantageous for a 16" bolt rifle. A bolt rifle is a single shot accuracy machine, so why give up velocity and increase muzzle buffeting for the sake of 4" of barrel?
One other thing I've seen is people thinking they can add a fore-grip to a pistol length AR. That also is a no-no. You can do it on a pistol grip shotgun, but the shotgun must then have a 18" barrel and an overall length of 26". The Remington 870 with the dual pistol grips fits that bill.