A post-war German M&G Diana 25, in either calibre, is usually just below 5.5 ft-lbs, which is the German 7.5 joule power limit. Sometimes a bit less (4.5-5) depending on the pellet.

I guess the Milbro ones should be similar, as it’s the same basic design, built on M&G’s pre-war machinery after it was transplanted to Scotland.

But it’s possible (I don’t know) that Milbro, not needing to stay under the “Freimark” 7.5J limit either put a stronger spring in, or increased the stroke a bit. Or something.

Or that the pre-war 25 was a bit more powerful than the post-war, and that M&G either changed the design a bit or otherwise slightly de-tuned it when the post-war 7.5J rule was introduced.

Either way, I’d be surprised if you could get much over 6 and a bit out of either, unless they were deiselling.

Any chance you can chrono it?