British flare guns and appropriate pressures?

[Corporal]

I have experience with break top revolvers and scaling down the charges within the safe pressure ranges but I have not found much information on flare guns and how much they can take. I know there are 26.5mm sleeves to convert the chamber down to a 12ga, but the implication here is likely that the flare gun be used with 12ga flares.
I do however have 12ga adapters that will take a 45acp cartridge. So I was wondering if this 'sleeve within a sleeve' combination + the strength of the flare gun's break action would be able to stand up to an underpowered 45acp lead round at about 12,500 CUP.

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[Peter Laidler]

It all depends on the flare gun that you're talking about. If it's the little 1" fabricated 'Berridge' black fabricated wartime signal pistol, then the strap and catch are insufficient to retain the pistol in the closed position with anything other than the standard signal cartridge

[Corporal]

Thank you for the reply. Is there any information on the avg pressure generated by such a cartridge?
The lever on this MkIII flare pistol is a bit encouraging. That being said I dont know what the locking lugs on the barrel actually look like.
WW1 Schermulys Trench Flare Pistol