They are not firearms, they are accesssories.
Keep repeating this phrase to them when discussing your uppers. "How can they be a firearm when I can legally sell them to a 12 year old?"
Get them, no demand that they send you a copy of where the term firearm is defined in their policies. If they don't have it defined then tell them they have to go with the legal definition. This doesn't mean that you get to only count the cost of the lower against your policy maximum though, they will get to count a complete gun.
They are just looking to get out of paying you any more money then they absolutely have to.
I had to fight a similar fight where the insurance company was claiming that anything that was attached to the firearm (scope, scope mount, sling, buttstock magazine pouch) at the time of the theft was part of the firearm and therefore counted against the policy maximum. (so slap a $2000 scope on a $700 rifle and it's now $2700 and over alot of policy limits, when it should only be the $700 rifle)
I beat them down and got my money.