Ive done it a few times, so heres how i do it.
Regular bolter holding arm.
Cut away from the shoulder section and shave it down so it angles more upwards than before.
Cut at the wrist and angle downwards more.
Will require
GS to fill gaps, but looks good.
This should have the bolter held higher and closer to the face, rather than away from the chest.
For the supporting arm you can use any one, but may need to make a slight cut under the shoulder and twist it a little more outwards and upwards to make it fit 100%
Hope that helps.
thats not my pic, but its the same pose as you will get and shows it alot better than words will.
Should also help you work out the rough angles needed.