Any tricks to make this background true white?

And yet another way ..

A - your original JPG

B - the original into ACR. The results of sampling various areas of the BG with the white balance tool.

C - in CS6, duplicate of B; set a copy of A over-top, applied a hide-all mask and added back some colour to the rounds with a soft white brush; merged down; duplicated the layer; used the quick selection tool to get an alpha channel of just the sidearms; applied Shadows/Highlights to enhance the detail universally and then the alpha channel and a mask to restrict the effect just to the sidearms; sharpened selectively.

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Cheers.
Gary
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