Hello- real quick, I have a P320 that I converted from the full-size to a carry configuration for duty carry. The issue I'm having is the following:
1. Insert full (17 rounds) mag into pistol
2. Rack the slide to put one in the pipe; very difficult to rack the slide with a full mag
3. Drop mag and insert one more round to top off mag
4. Fire, slide does not come back to eject or feed a new round. Have to eject the spent round from the chamber. Sometimes it stove pipes.
5. Experimented with all 7 (17 round) mags; each one does the same as above.
6. Insert a full mag; rack the slide and let it operate with only 16 rounds in the mag- works fine.
7. Both 21 round mags function as should, no issues when fully loaded and topped off after putting one in the pipe.

I've used all sorts of different ammo- factory 115, 124, 147; FMJs, JHPs, & GDHPs with the above described failure. Reloads of the same weight created the same issue.

Approximately 400+ rounds have gone through the pistol. Could it be the recoil spring still needs breaking in? I've stored the mags with rounds in it for weeks hoping to break in the spring set of the mags.

Pistol has already been to the Sig factory for a once over replacing the extractor and extractor spring. The factory declared it G2G using 50 factory 124 gr. FMJs. Pistol never did act this way when I had it in the full-size configuration. I like the more compact carry version for duty carry, that's why I'm not converting it back to the full-size.

Hoping to hear from some that have a P320 with similar or no issues at all. Email or IM me if you need more info.  Thanks, CJ

did you try just putting in the 17 rounds? without adding the extra to top it off?

With all due respect, I'm not clear on your question. I've inserted a full mag, racked it and it ran better using only 16 rounds in the mag, if that's what you mean.  I'm thinking it's a mag issue pushing too hard against the bottom of the slide. Even with a full mag, trying to rack the slide is very difficult. As for my Sig pistol experience, I've been carrying Sig P220s & Sig P226s for over twenty years with the agency I retired from. It's what they issued us. I've never experienced this sort of issue ever with a Sig pistol.  Also, I've been a Sig Classic Pistol armorer in the past with my former agency...

Sorry, I'll clarify.

You loaded the mag to capacity. You inserted and chambered a round (noticing added drag), then dropped the mag and topped off with "extra" round.  

My question was, are you noticing extraction/feed issues if you only load the 17 rounds(after the first round drag issue), and don't drop and top off.  (removing some tension)>>>> I misread #6, yes reading is fundamental.

How hard is it to seat a fully loaded mag with the slide closed?

Either way, sounds like something is slightly out of spec with the chassis/mag release, and as you stated, you are getting added pressure on the bottom of the slide with magazine at full capacity.

It's not that difficult to seat a fully loaded magazine, but then again I was taught to 'slam' the mag home; a habit carried over into my retired life. However, I did hear from Mr. Bruce Gray of Grayguns fame on the Sig forum.  He mentioned having the same problem with a few of his guns. He recommended loading the mag with 17 dummy rounds and pushing the top round against his workbench to 'break-in' the magazine.  It's a good day when I learn something new...

Is this a gen 1 or gen 2 gun?  The latest version has a revised slide.  The part (on the bottom of the slide that runs from the breech towards the back) that puts pressure on the loaded mag is now not as pronounced. This might be the reason for the mod.  You can YouTube (sig p320 evolves by hammer strike) to see the difference in the slides.

Too much pressure on the mag stack.  Have you tried hotter ammo like +P?  Recoil spring too heavy impeding slide movement.  Too weak ammo to overcome recoil spring and ammo stack.  21 round mag might have more room for the spring to compress in the mag body.  That's my guess.

DOE- I think you just won the gold star...

Thanks for recommending that YouTube video.  That made all the difference in the world and now explains why my P320 in full-size configuration worked without any issues.  I compared the compact slide against the full-size slide.  And what do you know, there is a dramatic difference in the loading ramp of the two.  I guess I ended up with a Gen 1 slide for the Compact.
Now, the bigger question is, how do I get the compact slide replaced or cut to match the slope of the full-size.  Funny how the Sig factory didn't say anything about that hump in the slide?

Thank you again!  I just knew the hive mind would come up with an explanation...

No problem.  Glad to help.  That's what these forums are all about.....mostly.    

Amazing that you sent to Sig and it passed.  Maybe their ammo is hotter.  It won't hurt to ask for a new slide after you discovered this.  And ask why they revised it.  This would really test their CS.  Forged slide and probably tempered.  Would be hard to mill and grind down and then maintain the temper.

These non disclosed revisions are product improvements to known issues or consolidating manufacturing processes.  Same thing with Glock in the late 80's with the "nickel colored parts."  They called it an "upgrade" instead of "recall."

Another option......mag springs are cheap.  You could cut one coil off and test for reliability.  Mag spring is too long and over compressed, adding to the drag that the slide must overcome with more velocity.  Shorten the spring to have less compression and more room off the stack.  This should allow the slide to move freely.

DOE- the ammo that the Sig factory used to test the pistol was off the shelf Speer Lawman 9mm 125 grain TMJ, 50 rounds.  The stuff I've already run through the pistol before.  

Good points about the heat treating the slide, so not a good idea to machine it.

I've already sent that picture to the Sig rep I had been dealing with before sending the pistol in; I'm waiting to hear back from him.  Another Sig rep suggested I try the hotter NATO ammo to see if that'll help break it in.  I don't know how long this pistol is going to take before being 'broken in'.  My experience with Sig pistols (and I've handled hundreds of them as an armorer) has always been 'to hell and back reliability'...pull a new one out of the box, clean it up, load it up, and shoot it until you hand grows tired; that's a Sig pistol...

Speer lawman 124 gr tmj is hotter than usual ammo.  Try looking for either +P or +P+ to run thru a box.  Run it in the same full cap that you described.  Sig should not take 500 + round to break in.  The higher velocity ammo should accelerate wear, overcome the new recoil spring and overcome the mag stacking.  

FWIW, I went to several lgs and looked at the compact slides.  They all have the longer part.  I need to find a carry version to compare next week.

Once your mag springs loosen up the slide won't be as hard to rack.  I've been issued my P320 Carry since Oct 2015.  We have transitioned over 300 people at work and a few at the beginning had issues with the slide cycling with some old frangible ammo we were using up.  Last year we bought some Federal FMJ and the second round cycling issues went away.  The new 17 round mags are just tight getting that 17th round in. Once your mags are easier to load the slide will be easier to rack.  David

My P 320 has over 3000 rounds.  I use 9 different 17 rd magazines and the 17th round is still a bitch to get in.  I use the UPLULA on a bench and it takes some force to depress the spring to get the 17th in.  Magazine goes in with out undue force.

When I sent it in to SIG to get the extractor replaced they used Aguila 124 gr for testing, sent the box flap back with the gun.  They may use a range of manufacturers ammo for testing to ensure reliability across brands.

Main issue is that you didn't buy a glock

I've carried a Glock 40 for 15 years.  Even had two KBs; one with an issued weapon and another with a personally owned one with factory ammo.  Glock replaced both.  Nothing like having two guns with three different serial numbers (barrel, slide, and frame).  I still have Glocks, but they are not "perfection" as you fan boys claim.  Broken pins, recoil springs, E-series, extractors, plungers, disconnectors, trigger bars.  If they are that perfect, why do I see most with aftermarket triggers, etc.  They have been around since 1984, so there's lots of aftermarket support for it.  They are on gen 4 going to gen 5/M.        

They are just tools in a tool box.

I wonder if my constantly loaded mags makes a difference.  I have 3 issued mags that have been loaded unless shooting since Oct 2015.  I finished a Firearms Instructor refresher course last month and went over 4200 rounds through my pistol using the same 3 duty mags. I had my first malfunction in the class, actually 2.  We were shooting and moving with the support hand only shoot at steel with 98 grain frangible Federal ammo.  I had 2 different failures to eject. I blame that on the ammo though and my support hand grip.

The only only other pistol I've owned where the mags were as tight was the original XD9.  My Sig 17 round mags load easily now.  I do know of the 1500 or so mags at work there are two different versions of springs.  The latest version are longer than mine.  David

David,
Are these the same size springs compared to the 21 round mags?

Which one of these is the latest and greatest slide?  (top or bottom)

Compact ontop, fullsize on bottom.  Looks like all the compact version I've seen are that same profile now in gen 2 frames.  The slide milling is different than the videos.  This is after the military contract pistol testing.

I'm the OP.  Neither slide is the latest & greatest.  The slide on top is my Compact slide as rec'd from the vendor.  The bottom slide is my full-sized slide as rec'd from another vendor.  

My belief is that there are newer, maybe better engineered Compact slides out there.

It was pointed out to me by DOE that quite possibly the newer Compact slides have a more flattened stripper rail akin to the full-sized slide.  I base this on a YouTube video DOE pointed me to where there was a side-by-side comparison of an 'older' Compact slide and a 'newer' Compact slide.  The newer version definitely had a more flattened stripper rail.

MAC has a new video of a P320c firing 1k in 13 minutes.  It shows the new slide variation.

MAC?  Link please or expound?

You tube Military Arms Channel.  Latest video of Tim shooting a P320 relatively fast with 25 magazines preloaded and then reloaded.  Gun got really hot, but no problems.  Mean rounds between failure were zero.  Should of ran another thousand to get up to army standards.  Surprised that there wasn't any cookoffs.

Eta.......damn auto spell.