According to Print volume limitations, and how to use the full build volume 256x256x256mm³ | Bambu Lab Wiki “The full print volume is only accessible when the AMS system is NOT used.”
You can at least use the full 256x256 mm area if you remove the exclusion area and stow away the collapsible stopper.
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Print this small blocking model https://wiki.bambulab.com/knowledge-sharing/print-volume-limitations/bambu_lab_printable_blocker.stl
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Collapse the lever stopper and fix it in place using the blocking model that will keep it tucked away during printing. (For a detailed instruction for this process, see the above mentioned reference.
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In “Printer settings/Basic Information/Printable Space”: Clear the “Excluded bed area” (delete 0x0, 18x0, 18x28, 0x28)
If you are using the AMS, the filament will not be cut at the end of the print, which will result in an error. To avoid this, you have to pause the gcode before the cutting command:
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In “Printer settings/Machine gcode/Machine end gcode”: Modify “M400 ; wait for buffer to clear” to “M400 U1; wait for buffer to clear”
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When the printer pauses, remove the blocking model or manually actuate the filament cutter and continue with the print.
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If you removed the blocking model don’t forget to reactivate the exclusion zone by resetting the printer settings to default.
Obviously, this method will not work for multi color prints that require spool changes.