To finally answer the TM M3 + PDI SPAS 12 Spring quesiton
 

It's been considered and asked many times in the past.
Will the PDI upgrade springs for the SPAS 12 work in the M3, which has nearly identical internal parts?
The simple answer: no.
The spring, even when fully compressed is too long to allow the gun to cock far enough back for the piston to lock back and let you fire. It goes almost all the way, and will go no further back.

This is for the PDI 0.8J spring, though I don't know for certain about the others, one can safely assume that the 0.8's bigger brothers will not be shorter when compressed. I will look into the possibility of cutting the spring down, but with an upgrade spring, what's the point of shortening it to make it fit? No doubt the reason for the M3 having 2 springs (one inside the other) rather than one like the SPAS is because of the small space that they have to compress into to cock.

The good news is that the PDI piston head set works beautifully!

Research since the creation of the thread has suggested that the J rated springs will not work in the M3, but the % rated variable pitch line from PDI will...

Many thanks for this, I was trying to find PDI upgrade springs to use in my M3 shorty... I guess it's a good thing none were available. Please let us know if cutting down the spring helps and thanks for the tip on which spring to get. :)

I think I will avoid cutting the spring down, after all. At this point I'd rather sell the spring new to a SPAS 12 user, than just throw my money away, I think. I will hang on to it until I can see if I can replace my tired stock spring.
Will inform.