Just purchased a 12 round factory magazine for my P365.

This magazine only likes 11 rounds.  I can barely jam a twelfth round in, but then the magazine absolutely will not seat in the handgun.

Is this a known issue?  Am I missing something?

Thanks!

known issue. they are tight for 12. Sometimes leaving them packed in there will help things settle out. I have this issue that's getting better the more I shoot it.

the other option is Magguts spring replacement, and you get extra capacity there

Thx for the info.  
Very disappointed that a company like Sig would put an item on the market that doesn't work.  Then again, recalling all the troubles the P365 had when first released, I guess I'm not that surprised...

It's full. No surprise it's tight.

Lock slide back.  Insert mag. Release slide.  Works fine for me.

No harm disassembling the mag to make sure the spring isn't in backwards.  The front of the follower gets the highest, thinnest portion of the spring.  

I have a few of the 12 rounders and while they were tight, they could be fully loaded.

After leaving them loaded and shooting a few mags worth mine loosened up more. I had to use a loader to get them to capacity the first few times.

They do work, you just need a Maglula to load them.  I’m up to 11 mags for my 365 now.  Had issues with my first 2 and now I load and unload them a couple of times before shooting the first time.  This has proven to eliminate the problem.

My first 2 mags would misfeed the first round out of the mag when starting with13 in the gun.  Did it the first 2 mag fulls with both mags and then stopped.

Yea I wish they were easier from the beginning, but they do get much better after just 2-3 loadings.  All 11 are 100% now.

I have bought a Sig 365 and a 365XL in the last couple of months and a dozen 12 rd factory mags for the two of them, in addition to a couple of ten rd.

I cannot completely fill any of the mags by hand.  Just too hard to get the last couple of rounds in.

So I bought a Maglula (like Southershooter said in the post above) and the problem is solved.

I am expecting the mag springs to loosen up with time but for the time being I am using the Magluga.

I was using a Maglula.
Even with it, it was a struggle to get that twelfth round loaded.
There was virtually zero compression left in the spring.  Consequently, it would not compress at all when trying to seat the magazine.

I've stripped, cleaned, and confirmed correct assembly.  

I'll load/unload a few times and see how it behaves.  Will take it out to shoot to verify function, as well.

Thanks for all your advice!

Only problem I've had with mine is it getting rusty carrying it in my pocket on a SnagMag.

Just got my P365 also and bought a 15 and 12 rd mag with it.  The 2 10s it comes with as well as the 15 are tight, but not as tight as the 12.  The 12 can barely be seated with the slide forward.  Use your right palm on top of the slide while pushing the mag in with your left palm, it will lock in place.  I store it like this so the mag can break in, hopefully.

It's also very hard to rack the slide w/ a full 12 rounder in, but it's possible.  The good news is that I tested it at the range yesterday and it will not malfunction, even with 12+1 in it.

I've only had a few malfunctions last week during its first outing.  I had to tap the slide a bit to get it into battery maybe 2 times.  This is no longer an issue after cleaning it and shooting it again.

Also, the maglula/uplula is a finger saver for these mags.

I have older 12round mags that took awhile to break in but are easy now.

I recently bought a magguts +2 kit for some new 12s I bought and they were fairly easy to load the 14th round. I’be only got about 200 rounds through the 2 kits I’ve bought. But no malfunctions yet and we’ll see how they handle being left loaded forever too.

Have you ever owned a Sig product? This isn't new.

Shot my new 365 today and had issues with my 12 rounders. Couldn't rack the slide once, had to download a round. Also had a couple times that the slide didn't lock open with the 12 rounders. I put about 100 rounds through it and stopped.

I've got a new 12 rd that the follower doesn't come up high enough to reliably lock the slide to the rear and will drop the slide when you push the magazine release and drop the magazine. I sent Sig an email about it the other day so we'll see what they say. All my other 12 rd magazines work just fine though.