Douglas,
Re: the extraction problemЕ..if it turns out to be too high a pressure; "I have read" (cannot locate the source right now) that Trail Boss is not a good choice in the large Nitro Express or Black Powder Express cartridges.
With the suggested method of determining loads that works on pistol or smaller rifle cases (base of the bullet) the pressures are way overboard in the large capacity cases. Possible that this is the issue? Was the primer flattened more than expected? Chronograph?

The only other possibility I can figure is a poorly reamed/polished chamber that is maybe jug shaped and will not let the fired cartridge slide out.
Have you ever done a chamber cast?

Here's hoping you had a range rod to get the case outЕЕ
- Mike

ADDED: A ha, found it.
Trail Boss load difficulties are outlined In Graeme Wright's book Shooting the British Double Rifle, 3rd Edition, Chapter Six, pages 130-131
paragraph 4
"Е..What Sherman (Bell) found was that the recommended starting load of 70% of the case volume gave approximately equal pressure to the normal full load of black powderЕ.." and "Е..all higher loads were above the normal operating pressureЕ."
paragraph 5
"Е..Trail Boss is not a suitable powder for use in black powder express rifles where loads are duplicating original ballistics. It may be suitable for reduced loads, however, these then may not regulate in a double rifle. If you do wish to use this powder in a BPE Express rifle, then use the 70% figure as a maximum loadЕ.."

So:
I infer that one can extrapolate that even though your .577 2 3/4" is not really a "black powder express" so much as a "Light Cordite", the result may be essentially the same.