Glad that the problem is fixed, but for future reference, another way to do that is :
1) SELECT both weapons, in this order:
first the existing, working weapon and then your new one
(keep ctrl pressed for multiple selections)
2) make sure the "View mode" is set to "Combo", so that you see the two things combined
(at the bottom of OpenBrf windows),
3) now "Selected"--> "roto-translate-rescale...".
This open a dialog.
4) check "apply to Last selected object only" (at the bottom of the dialog window)
so that your rotation-translation will be applied only to the new weapon
and the other one is kept where it is, for reference.
5) fiddle with the angles etc until the HANDLES of the two weapon match and the two weapons match in orientation
6) accept what you have done with OK