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From what you're describing, I assume that you're able to get the gun open, but the cases are still stuck in the chambers? Hmm. The .500/.450 has a base diameter of about .573" and a shoulder diameter of about .530" - a lot of body taper. When the gun opens, primary extraction occurs as the extractors lift the cases roughly 3/8" out of the chambers. At that stage, due to the body taper, it's impossible for the cases not to fall out of the gun via gravity alone - unless the chambers are severely out of spec. A chamber ring in the neck area comes to mind. Which way is the stretching? How do the dimensions of the fire-formed case compare to max cartridge? Time for chamber casts.

Like Dan said, take case measurements before and after firing.

The Hawk is a very soft bullet and obturates readily. Consequently, it is "sticky" and very often requires less powder to achieve the same velocity than other bullets of the same weight. In one of my rifles, I have to reduce the charge of my standard RL 15/Woodleigh load by 3 or 4 grains for Hawks of the same weight. Use Woodleigh for initial load development. Once you have a load developed for the Woodleigh, reduce the charge 5 grains for the Hawk and work back up.

Why did you start with a +max charge of 85 grains RL 15? For a 70 Cordite gun, rough max would be 83.3 grains (70 Cordite X 119%), and that assumes a Woodleigh, not a Hawk. You should have started at 79 or so with Woodleighs, 75 or so with Hawks.
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"Serious rifles have two barrels, everything else just burns gunpowder."

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