Hans Von Lohman, on 25 April 2014 - 10:22 PM, said:
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Weapons are sorted by how "old" the weapon is. I.E. if you bought a PPC months ago and have it laying around in your private stash, and then buy a brand new medium laser, the old PPC will go into the 1st slot, and the medium laser goes into the 2nd slot.
Steps to take to fix things.
1. Remove your weapons that won't stick properly.
2. "Hide" any excess weapons on another mech for the time being, or sell them off.
3. Install your first weapon, which can be one you already own.
4. Save your build. Change to another mech, change back.
5. Buy a new weapon to live in the 2nd slot.
6. Save your build, Change to another mech, change back.
It has worked for me to get a Griffin to sort it's LRM-10 to be on top of the shoulder, and an SRM-6 to be in the chest plate.
Definitely the best explanation I've read of the workaround needed to do this.
Now PGI just needs to eliminate the need for this workaround. Please prioritize this guys!