Here is a pic of some of my springs. Working form *Right* to Left...The last 3 on the right are German post war MG3 springs with the last one being a new MG3 spring...you can see how much set they take over time which I mentioned in my other post (note how much longer the new unused MG3 spring is).
The 2 before them are used Yugo uncut springs. You can tell the difference if you look at them close as the Yugo springs have a gold/brass tint and are brazed on the ends, where the MG3 German springs are more of a silver color with no brazing....thought that was worth noting for those interested as an easy well to tell them apart.
Next to them are 3 BRP single wire semi auto springs.
The 4 next to those are a cut down mix of MG3 and Yugo springs cut to varying lengths...I use these in my tuning kit, much like the varied sizes of gas nozzles to fine to each gun dependent on tightness of the gun, ammo, etc.
That said, assuming you are talking semi modified springs...rather than going by length or number of coils as you can see will vary depending on the spring, again as I mentioned earlier in another post I would start with say 22-23 coils which will be way to much for a semi...but then cut a 1/2 coil off at a time till you get the *maximum spring your gun can run* with that spring..
I do this by placing the uncut end of the spring into the B/C notch, give it a circular twist to 'seat it', install your buffer, then pull the charging handle back, watching carefully as the spring compresses. You want the buffer to carry the load of the full recoil...you do not want the spring to bottom onto itself, thus defeating the buffer.
Having to weak a spring or not enough will lead to rounds not stripping fully or the action absorbing most of the energy leaving little left to put the bolt in battery, reset the hammer, etc..but a 'one size fits all' # of coils in this type of gun with so many variables is not a good rule of thumb IMO. Sort of like 2 muscle cars running the same engine may need different tuning, carb jetting timing, etc.
These semi's in this platform are running right on the edge of functioning due to the shortened R/C spring, added drag of the hammer, which again is why you need all the spring you can run along with getting the gas right. All this too is assuming the gun is straight, built to proper specs, not binding, has a good recuperator and is running proper spec ammo ...all combined leaves much to go wrong with a used kit which is why so many have trouble building these.
This is why many of us plead with new builders to first study the stickies, read the build threads and buy the German manuals in orders to understand this is not an AR build.
BTW,this last advice is not directed at you Hackentt, but rather some of the newer members, As I'm sure you are aware of of this.