I'd let SIGARMS examine it again. Give them a chance to figure out what’s happening with your P220 Compact.
It's impossible to diagnose problems via the internet, and I certainly wouldn't imply otherwise. I can offer you some educational info for the purpose of polite discussion, though, which may, or may not, be of interest to you ...
First of all, in the current SIGARMS Pistol Armorer Manual, in the Troubleshooting section for the 220 models, there are a number of suggestions for an armorer to consider when it comes to cartridges not being properly fed into the chamber.
They include: Magazine not seated properly (insert properly); magazine dirty or deformed (insert fresh magazine, clean or replace the removed magazine); weak magazine spring (replace spring) & dirty/damaged ammunition (change ammunition).
Then, if the cartridge does not chamber, it lists probable causes of: Cartridge incorrectly fed (check or change magazine); Low quality ammunition (change ammunition) & Shooter error (Keep wrists locked and maintain a firm grip when firing the pistol). Does this last sound familiar?
These are just the basic probable causes and corrections, and there could be other factors present, as well.
Anyway, you might want to consider informing the SIGARMS technicians what ammunition you're using; when the feeding issues arise during the range sessions; whether it’s occurring with only one, or more than one, magazine, and how your pistol is lubricated at the time.
Ammunition is not all created equally.
I ran 300 rounds through it on two occasions after that and still having problems.
Did the feeding issues occur with only one magazine? Did they occur in the beginning of your session? Throughout the entire session? Toward the end of the session? 300 rounds is a fair number of rounds to fire at one time. Some folks might experience their grip strength starting to lessen, or perhaps become erratic. Slide travel and feeding 'timing' can sometimes be affected by grip stability considerations.
How well is your pistol lubricated? SIGARMS repeatedly emphasizes to its LE armorers that Sig Sauer pistols are intended to be 'wet' pistols when it comes to lubrication. The armorer/operator should be able to verify the pistol is lubricated by being able to 'see' and 'feel' the presence of the lubrication (in the appropriate locations, of course). Granted, over-lubrication is still a problem which can affect proper functioning, and lubricant shouldn't be applied where it doesn't belong, or applied in amounts that allow it to 'run' because of gravity (and perhaps end up where it shouldn't be present, attract dirt & debris or contaminate ammunition) but Sig Sauer pistols are not intended to be fired dry, or with less than proper lubrication.
The owners manual can provide directions for lubrication, and a Preventive Maintenance Guide available online (prepared by the SIGARMS Academy staff) may also be helpful.
http://www.sigarms.com/CustomerService/documents/PREVENTMAINTGUIDE.pdf
The Guide lists a couple of references to lubrication ... “Environmental extremes such as coastal salt air, humidity and broad shift in temperatures expose unprotected metal to attack, requiring frequent attention with a lubricant/preservative versus a desert environment where the natural attraction of dust and grit to the lubricant becomes a negative factor.”
And then, “A weapon is not considered properly lubricated unless the lubricant’s presence can be visually and physically verified by the operator.”
Also, accumulated fouling and debris around the extractor can potentially create feeding issues. The Guide states it as: "Foreign material and firing residue around the breech face and extractor can cause extraction related stoppages as well as failures to feed."
If this happened toward the end of a 300 round firing session, and especially if the ammunition was of a 'budget' variety which used less clean burning powders than some of the more expensive 'premium' varieties ... well, accumulated fouling can start to affect functioning when it reaches a certain point.
The other day a fellow brought me a pistol which was exhibiting feeding issues. The make/model is unimportant. What was interesting about it is that after I tentatively discounted the ammunition being used (same ammunition which functions fine in many other weapons) and the shooter’s skills & abilities (another firearms instructor), before I started trying to diagnose what might be ‘wrong’ with the pistol itself, I took a look at the magazine being used. Believe it or not, the magazine spring had been installed backwards. The owner hadn’t disassembled it, either. The follower wasn’t being properly tensioned upward at the front, and this was apparently causing an erratic problem during shooting. I reversed the spring and the ‘problem’ was resolved. I’d like to say that this is the only time I’ve encountered this issue, but as you might’ve guessed, it’s not.
While we’re mentioning magazines, I’ve seen magazines which have been very dirty and filled with accumulated gunk, and which coincidently seemed to be connected to feeding/functioning issues. Magazines should be clean, dry and free of debris. Excessive fouling can sometimes cause apparent ‘feeding’ problems, and as you might imagine, a magazine spring is at its least amount of tension when it’s almost decompressed near the end of a magazine load. Are all of your feeding issues experienced only toward the end of a magazine load? Is the magazine(s) clean and free of debris? If you have more than one magazine, does one exhibit less felt tension than the other? There’s a reason magazine springs are stocked as spare or repair parts, you know. Even a new magazine spring may be unknowingly defective. I’ve heard of more than one manufacturer having to replace relatively new magazine springs in LE magazines when it turned out a batch or production run of them (often vendor supplied) failed to maintain tension, or slipped through without it being discovered that they may have failed to meet the required specifications. Things happen.
Granted, 500 rounds ought to have brought your pistol to ‘broken in’ point. FWIW, we were told in the armorer’s class that if one of the new stainless 220's were exhibiting functioning issues in the first 150 or less rounds, that the barrel may have to be ‘seated’ into the slide in order to correct it. Also, the new stainless extractors which cam into the slide and are tensioned/slipped into position are different than the other models which were held within the breech block, or retained by a spring & plunger. The new stainless models are intended to be installed only once, and armorers are told to replace them if they’re ever removed from the slides for any reason. They’re tensioned a bit extra to permit the initial installation.
So, when trying to diagnose apparent functioning issues it’s not necessarily as ‘easy’ as some folks might think. The things I’ve mentioned are just the tip of the iceberg, so to speak. Let the folks at SIGARMS have another chance at trying to determine if there’s a problem with your pistol. Just remember that there are several things which can influence the normal operation and functioning of a semiauto pistol after it leaves the manufacturer, only one of which is the way the pistol was manufactured ... The manufacturer has no control over the ammunition used, the shooter’s skills & abilities, the maintenance or the environment in which the pistol is used.
Sorry if my babbling isn’t interesting, or doesn’t help reduce the understandable frustration you must be feeling. I just don’t (and can’t) have the ‘answer’ to your problem. I’m certainly nobody’s ‘expert’, nor am I a licensed gunsmith. I’m just a LE armorer, which is kind of at the bottom of the totem pole when it comes to knowledge and experience.
As I learned myself, as a new LE armorer ... and have later tried to help train new armorers to realize ... sometimes ‘fixing’ a firearm problem is the ‘easy part’. It’s properly diagnosing the problem, or problems, that’s often the more difficult task. Fixing things which aren’t problems, or are related to existing problems, can take you further from your original goal ... and sometimes even create new, even more annoying ‘problems’.
I remember being called into the bench once by a couple of other armorers. They were having a problem with a Commander. They were debating a number of parts replacement courses of action they were hoping would correct the problem. After listening to the one guy who had actually been shooting the Commander when the problem occurred, and asking what ammunition had been used, I looked at the pistol AND magazine. A different magazine (different follower design and a fresh magazine spring) resolved the functioning ‘problem’. Of course, it also turned out a couple of other conditions existed which really required attention and correction, and which hadn’t been discovered by the guys about to start ‘fixing things’, but they hadn’t been responsible for the immediate observed ‘problem’.
Calling to talk to the customer service technician who may be working on a pistol may be possible, or it may not, but it’s also very helpful if someone writes a letter which carefully ... and clearly ... lists not only the perceived problem(s) experienced, but also the conditions under which the perceived problems are experienced.
Just my thoughts. I hope it’s resolved to your satisfaction. These things can be frustrating.
During slow fire the last round does not feed and I must manually eject the round somewhat forcefully. This was with "personal defense" rounds of all sorts and Winchsester White Box.