Can a 6 position AR stock be replace with a fixed butt stock

To go with the stock, and stock screw.

The buffer tube, buffer, and buffer spring will have to be changed to rifle length to match the fixed stock.

stock need to change other parts. Magpul makes a CARBINE fixed stock which fits without changing anything else. It is shorter than rifle length so you may need an extended butt pad to compensate.

But it looks like a Magpul stock. I've seen other brands too. I haven't seen one that looks like a regular A2 style.

both the stock and the stock kit which includeds the spring etc and a series of shims.