Ok, checking MegaMekLab, I either can't find the Modular Weapon Mount for Battle Armor, or I did find it and it's not working as it should. I'm trying to build the IS Standard in MegaMekLab, and I'm putting the item "BA Manipulator Adaptation [Modular Equipment Adaptor]" in the right arm. I can add the Small Laser, Flamer, and Machine Gun to the Right Arm just fine (though I have to select "Add to Right Arm"), but the Light Recoilless Rifle doesn't fit.
Based on the Sarna.net description, I thought it was supposed to work the way you coded the Anti Personnel Weapon Mount, but it's not working that way.