I've been having some success with JM's new Global Tarantula spring in these.It's the best spring I've tried in the Model 27 by far.So far I've done two, a .177 that drove Silver Bears at 718FPS for 8.1FP and an oldish .22 that drives SB's at 575 FPS for 9.3FP.
Just a FYI.
I bought one of these springs, since his website said the fit the IZH38. Well, it fit, and the gun cocked, but it was WAY too much spring for the gun, slamming the piston head into the end of the chamber. I took 2 shots before I became worried about the sweat joint at the front of the gun, and removed the spring. After only 2 shots, it had taken on a decided curvature. I was able to remove some of it, simply by hand-bending.
Is this spring ruined, or could I use it in my 27, given the fact that it has a guide, and the IZH38 has none, (hence the curvature.)
I've also heard that the Canadian Princess Auto surplus machine-gun springs can be used in 27s, althought they need to be cut to length. Specs are .105 wire, .504 ID, 63 coils.
Its leather, of course, and fits tightly. I reconditioned it with silicone oil several months ago, and probably put 6-800 shoths through the gun with the factory spring before switching springs. The seal has a good tight fit, and some drag when fitting by hand. The gun shoots fine with the factory spring reinstalled. My 38 has the designation 38C, and has no internal spring guide whatsoever. I forgot to measure the factory spring when I had it out, but I do recall that there was A LOT of preload when installing the GT spring. I was worried that the gun wouldn't even cock, but it did. I guess there's big a difference between what fits and what works.