Found a good deal on a used 220ST last week and finally got it to the range to give it the shakedown. I had a few problems that I did not anticipate. First, the recoil spring felt very heavy, requiring more force than I am used to in order to cycle the slide (even compared to other P220s I've had in the past). When I cleaned the gun prior to going to the range, I noticed that it was very difficult to remove the spring from the guide rod. I had to use quite a bit of force to separate the 2 parts, which didn't seem normal to me. The gun came with 4 Novak 8-round mags and a 10-round Pro-Mag (junk). When I loaded the Novaks to capacity and inserted a mag into the gun with the slide in battery, the slide would bind as I pulled the slide rearward, and I was unable to cycle a round into the chamber. If I locked the slide back, then inserted a mag and hit the slide catch, the slide would move forward and chamber a round. The gun would then cycle as you would expect. If I downloaded the mag to only 7 rounds, then I could draw the slide back and it would strip a round off normally when the slide came forward. However, the slide would not lock back after the last round for all 4 Novak mags. Needless to say I was pretty irritated at this point. I noticed that the follower on the Novaks would not push the slide catch upward when inserting an empty mag. But when inserting the empty Pro-Mag, I noticed that the follower would push the slide catch upward (strangely, the slide would fail to lock back with the Pro-Mag despite this).

What do you guys think is going on here? Do I have a magazine problem, a spring problem, both, or something else altogether? It seems as if the gun is oversprung, given how hard it is to cycle the slide. I am concerned that I was sold a lemon. Normally I only use Sig factory mags, but this gun only came with Novaks. So I don't even have a factory mag to try. Your thoughts on what I can do to fix this are appreciated. Thanks.

If it was me, I'd head over to Top Gun Supply's website and order a factory recoil spring (if not a complete spring kit) and at least one factory mag. I don't have any experience with Novak mags, but promags are junk...like not even for range use junk. You need to get your gun back to a factory baseline to even know where to begin with troubleshooting any issues.

check the feed lips on the Novak's mags...there were some issues with them (heat treating or brittle IIRC)...

As to the spring, it should not be difficult to remove (though it can be snug) the recoil spring from the guide rod...many people replace the guide rod with after market ones so I'd make sure yours (both the rod and the spring) are OEM...

Good luck,

Bill

Yeah, it’s junk. Just another example of the crappy P220 platform. Send it to me and I’ll dispose of it for you....

Seriously, though,...One of my P220ST pistols had all kinds of weird issues when I got it home.
Lock back on a whim, or not at all...
Weird feed issues...
Didn’t matter what mags I used, factory, Mec-gar, Act-mag, dimple, no dimple. I had never experienced such behavior out of any of the several P220 pistols I owned. Most of them would feed, shoot and shuck from a 5 gallon bucket if you got it close enough.
I finally broke down and put a complete spring kit in it from SIG. It solved the problem. I really think, in my case, it was the spring associated with the slide catch. Plus it was pretty bad gunked up.
Like was said above, you’re probably gonna have to get it back to factory baseline before you can even start to diagnose it. Sounds like someone may have been experimenting with “extra power” springs for whatever reason.
New to you pistol, no idea of its history...factory spring kit FIRST.

So it sounds like I need to get a factory recoil spring at minimum. That won't be so bad, but if I have to replace all the mags...then my "deal" is no longer such a good deal. I am going to have to call the guy and see if he will tell me what the heck he has done to the gun. I can understand that Novak mags may have had some problems...but all 4 of them?

IIRC ACT made the mags for Novak. ACTs work great in my 220 carry SAO, but like you I can't insert a full mag on a closed slide and charge the gun using those magazines. BUT the same thing happens with factory Sig 220-1 magazines. I guess I just learned to live with it.
ACT mags have been known to crack at the rear near the feed lips, but given that I can get them for less than half the cost of Sig factory mags I'm happy to roll those dice. The only feed issues I've ever had with my 220 was with the factory Sig mag, so there's that.
I also have a hell of a time removing my factory recoil spring from the guide rod... so I don't. I just hose it down with brake cleaner, lube and re-assemble. Again, I just decided to live with it and Stop over-analyzing things.
FWIW!

The P220ST has a very heavy slide, thus a very heavy recoil spring to be able to move the slide.
Not the gun if you have to operate under bad situations were one handed operation might be required.
I have a SS solid recoil spring rod and never have a problem removing the rod. The spring needs to be turned against its winding to come off the rod.

Its possible that the previous owner chose to shoot very heavy loads and installed a heavier than stock recoil spring.

I agree, replace the mag springs (+5# or +10#),and recoil spring and guide rod. You should have a spiral wound wire spring on the recoil spring rod.

This should be done when ever you buy a un- known, used semi-auto.

I spoke to the previous owner and he claimed he did not make any modifications to the gun. What he did not tell me was that he bought the gun used. Given the Novak mags, jeweled barrel (at breech) and G10 grips, I think there is a possibility the recoil spring has been replaced. So after talking to a friend that is knowledgeable about Sigs, he has recommended that I source a factory recoil spring and mag to see if that fixes the problem.