This has come up here (and other places) so it's about time that we started documenting some of this information, so I took some measurements and a photo.
The information and instructions were inspired by all the HK94 clones that had a .308 rifle spring installed instead of the 9mm spring. It seems that switching over to the 9mm spring greatly helps cycle the weapon better and extractions/ejections improve as well. This can also be used on any of the standard (91,93,94, etc.) rifles/pistols that don't use a pinned hammer/strut pair, as it doesn't require hammer removal from the trigger pack.
Now remember, this can vary a bit. But the numbers are far enough apart that it should still be easy to tell which one it is, especially if you have some calipers.
.308 Rifle Spring:
Strand Diameter, .05"
Strand Diameter, .04"
So now that we can see what the differences are, it might be time to change them...
It's pretty easy and all you need for tools is a philips screw driver and a small "L" shaped allen wrench. Please note, you don't use either of these tools for their original designed purpose, but they just happen to be perfect for the job.
First thing you do is remove your lower from the gun and let the hammer forward to unload the spring as much as possible. You can then use the tip of the screw driver to push down and reward against the hammer strut, then bring it up to allow it to push forward and up. Then you can just extract it right out of there. But before you do all this, make a point to see where the strut shaft sticks through the rear of the metal pack. There's a hole there.
Now you just slide the new spring on and work it down and reward. Now this is where it gets tricky, but not too bad. There is a cross support right in your way so that you can't just push the pointy end of the strut through that hole, and the 9mm spring is a bit longer than the .308 rifle spring so it takes a little bit of coordination. Ok, so now you get to use the end of the allen wrench as a little hook. As you are pushing the "U" end of the strut down and compressing the spring against the metal pack at the rear, use the hook of the allen wrench to pull the strut point up toward the hole, underneath that cross member.
It will take a little playing around with it (took me about 5 minutes on mine the first time) but you will eventually get it through there. Now that I know how to do it, it only takes about a minute.
ETA: The previous step can be made even easier by removing the trigger pack from the lower grip. That would allow you to push something up from the bottom, into the pack, to more easily align the strut point with the hole at the back of the pack. But either way will work fine.
But I re-iterate, please make sure you gotta really do this before going to the trouble. It might be interesting just to try a different spring anway though.