Only when loaded, unloaded seats fine. I later thought it might be a safety function? Maybe the gun wasn’t on ‘safe’ and it’s designed to not seat with rounds in the mag? I haven’t had a chance to test that theory and thought I’d just cut to the chase with the experts!

Is it possible you crammed 31 rounds in the mag?  That could do it?

I've had no issues seating a 30 round mag.  So I'm curious what is causing your issue.

If it is happening with fewer rounds, I would wonder if it were an out of spec mag.  Does this happen with all your mags?

Start with the easiest tests first:

- Try a different mag
- Try downloading by 2 or 3 rounds

Post results.

I would bet brand new mags + loaded to capacity on a closed bolt.

Sorry, should have added more info.

I tried Lancer 300bo specific mags as well as Magpul; they both seat fine unloaded. I did try just a couple of rounds in the mags and they won't work. It's really strange. I wonder if the mag release needs some adjustment, filing....something?

Really hate to take it to a gunsmith for something seemingly trivial and have reached out to the PO to see if he had issues like this hoping he has a fix or explanation.

I did do all testing with a closed bolt: didn't think about an open bolt and trying that. Will advise.....

It won't seat with a solid smack?

IMO if it won't seat with only a few rounds in a 300blk Lancer Mag, Sig will make it right. Could be an out of spec mag catch or (less likely) an out of spec lower.

So it will seat with an open bolt. I'll be happy to hear from y'all that I'm wrong but I do believe that's unacceptable. I am the 2nd owner and have contacted the PO to see if he had the same problem; I plan on keeping it and not making it an issue with him; just curious in finding a solution. I tried loosening and then tightening the mag release to no avail.
Before sending it off to Sig, I'm going to look for a replacement mag release that is oversized; planned to do this anyway so perhaps kill two birds.

If anyone has other advice, I'm happy to hear it!! Thanks.

Called Sig and they were as mystified as I am. He said to send it in but I'm gonna take it to a local gunsmith first; hope it's something easy to fix. Barring that, off to Sig it goes.

Just send it to Sig. They should warranty it no fees no questions. Let us know if they don't. I checked out one at a LGS and when you pulled the charging handle back the dust cover wouldn't flip open. Nor when you released the charging handle unless you smacked the gun. I passed on the rattler and went with the Q although more so for other reasons, Sigs QC certainly didn't help

I loaded it with about 20 and jammed the hell out of it into the mag well. Finally worked and with subsequent seatings, it became easier. I’ll just keep at it, looks like it’s a break-in issue.