I find the idea of bolt locking in a barrel extention where receiver strength isnt critical, like the AR where the barrel with extention is screwed into an alumnium receiver... it seems the AR-10 is like this as well except they fire 308's... so maybe someone could make a bolt action by making a barrel without gas port, add extention and thread them into a tube with ejection port milled in like remington 710, weld a bolt handle to the bolt carrier and have a straight pull bolt action.... Imagine a rifle with simplicity of a 710 but can still be rebarreled if someone wants match grade barrel.

Remington, Winchester, Armalite, are you listening?

On a side note, has anyone ever turned a AR upper into a rifle without a lower by making them a straight pull bolt action (no gas system etc)?

As to making a straight pull bolt action by removing the gas system from an AR, there are many of them custom made for places like England.

Eugene Stoner would PUKE!

The Steyr Scout locks up like an AR.

The TUBBS 2000 is another similar rifle to your idea Rahimiv.