Cutting springs has always been discouraged by most people in the know, but I came into a 1911 that already had one in it, so here are my questions.
The recoil spring wire diameter is noticeably heavier than the stock 16# spring, but it is also shorter. In effect, the spring seems lighter when the cycle begins, peaks at full recoil and then returns to rest. FWIW, it also had a FLGR. It has run flawlessly, other than an infrequent premature slidelock. And yes, it did not have it, but I did put in a buffer. There is no doubt this Norinco has seen many, many rounds downrange. It continues to run very well. Subjectively, it seems to recoil less than other 1911s I've shot, but I chalk that up to the heavier weight of the FLGR and the buffer. But maybe not.
Does the heavier, but shorter, spring move the pressure curve towards full recoil, thus saving the frame from battering (relatively) and easing up faster (because it's shorter) as the slide goes into battery, thus again saving the frame (relatively)?
Does it affect felt recoil since the slide impact against the frame is decreased by the higher spring pressure at that moment?
Is this (heavier, shorter) an old armorer trick from the past that's gotten dusty with the easy access to a wide assortment of springs in various weights and buffers?
Trimming the length of recoil springs used to be about the only way we had to fine-tune'em when the slides outran the magazines. That, and sliding the spring over a suitably-sized rod, and letting it roll against a belt sander to reduce the wire diameter. (UGH!) With the widespread availability of Wolff magazine springs, those times have thankfully come to an end. I still occasionally run into identical NIB Colts with recoil springs of different free lengths and numbers of turns... so it's a good liklihood that somebody is fine-tuning springs somewhere along the line.
Basic rule of thumb for this practice is that if you have to remove more than two full coils, it's time to step down to the next lower rate and start over. Best to just invest in a three-pack of Wolff mag springs and be done with it though. Those things are money well-spent, and are a lot like that rabbit that keeps on goin' and goin' and goin'...
Another rule of thumb when dealing with coil springs is that 32 coils of smaller wire is better than 28 coils of heavier wire. The springs drive their component for a given length, and the longer you can keep driving it, the better... even if it means reducing the total load at full compression.
Also... reducing the slide velocity and frame impact with a heavy spring is often counterproductive, because the magazine timing becomes more critical as the slide's speed on the return to battery goes up. There's another very good way to accomplish that, and that's with a firing pin stop with altered geometry on the bottom. It accomplishes that by utilizing more of the mainspring's loading to delay the slide, thus dissipating some of the slide's momentum at the very beginning of its travel.
What you said all fits the apparent history of this Norinco. Its slide's been hardchromed, and hotrodded a bit. I'll post a pic of a clever way the previous armorer adapted a beavertail. But it's worked well with a variety of magazines, so I'm not touching anything on it for now. Well, I DID add a buffer [ducking].
I should add that when I first took it apart, the original FLGR had a flange that had been beaten down to half the original thickness.