You need an application capable of ignoring the margin (Standard is 4mm)
Acrobat/Reader will do so, try saving the file to pdf
Then you can see the standard reduction (Scale 96% and the gray bit around the edges)
Set scaling to none and the image will not be reduced and go to borderless
But keep in mind, this is a toner based output device, you are not going to get a borderless print, that isn't possible, you are still going to have a border around the page, it will just delete any of the image that was going to be in that space.
I have a Xerox Phaser 7500 printer.
I have set up a custom paper size (SRA3 - 450mm x 320mm) on the printer itself, as well as on the Windows Preview paper option.
Document size in Photoshop also set up as SRA3 - 450mm x 320mm
When I print an image the actual size that prints is a tiny bit smaller, not a lot, but enough.
For example, a 95mm block will print as 92mm and a 280mm block as 270mm.
I have tried the standard settings, as well as "fitting the image to frame" option in Windows Preview while setting the margin to none.
On both occassions the image will print the same size.
Any ideas?