My grandfather has a Werner 24' aluminum 200-lb type III extension ladder. I want to know if, in its fully collapsed state, it will safely support ~300 lbs. By my reasoning, if it will safely support 200 lbs fully extended, there is no reason it shouldn't support ~400 lbs while fully collapsed, provided, of course, that the weight is evenly distributed across both rungs.
Your logic is that in fully-collapsed mode, both channels are doubled up, and ought to be able to bear twice the weight.
The problem with that is the second channel is not resting on the ground. It is still transferring all its borne weight to the first channel at some point.
What's more, it is virtually impossible to distribute your weight evenly on the rungs 100% of the time. You will be climbing or descending on toe or heel. You might be super careful the first few times, but complacency will set in.
They do make heavy-capacity ladders... but I am concerned about the general wisdom of taking a great deal of mass up a ladder. Agility is a big factor in ladder work, and I can't imagine being able to be safe managing 120 pounds of gear up a ladder, even if it was securely attached to me, even if I had more muscle than I do.
the rating on a ladder is only one third its maximum strength. although not recommended it will support the weigh but over a period of this type of use it will cause more than desirable wear on the ladder. I am a former roofer who watched a three hundred pound guy carry 160 lb(2 bundles) of shingles up a 200 lb rated ladder daily. its not recommended but it will hold,