–re: no lead-free…many lead-free primers have higher failure rates than standard primers, and it’s been suggested that lead-free primers have been to blame in failures of a very widespread service sidearm. ATK does not offer lead-free primers in any standard LE duty cartridge as far as I know for this reason.
–re: .357SIG penetration…the Secret Service used this caliber for a long while, so to me that helps address concerns of ‘overpenetration’ from the cartridge, or as a CQC concern in general. I’m no physicist, but cartridges like the 9mmP +p+, .357SIG, etc. have a higher envelope of ‘piercing’ power in a few instances, whereas heavier projectiles from popular service cartridges like the .40SW and .45ACP have more consistently deep tissue/simulant penetration in FBI protocol tests. If an agency wanted reduced penetration, Speer has offered such products in the past. They could also load a standard (non-bonded) lightweight pill and get very shallow penetration by modern standards.
Anecdotally, in my own testing for my agency, even the relatively small weight difference of 25gr made a noticeable difference in penetration in the ‘car door’ sheet metal test event, and was born out in further shots on actual car doors (165 Federal Bonded Tactical vs 180gr HST; the cheaper heavier non-bonded pill actually won in this instance).
If memory serves, the first USSS .357SIG pill was the Ranger Partition, followed by the Gold Dot (both ‘deep’ penetrators). I also recall part of the initial cartridge justification from US FAMs was the ability to penetrate the headrests/seats in airliners and still do the job terminally. The fine print on the terminal effect test performance requirements definitely toe the line for FBI standards (12-16" ideal, 10-20" on both far ends). My main prob with .357SIG in LE is platform longevity, perceived recoil/blast, and cost, esp. since a good 9mm will do all or most of the LE .357SIGs job.
–re: SMGs…the 10mm Mp5 was/is a thing of beauty in my opinion. I’ve only shot one a few times, but the cartridge kinda comes into it’s own in the carbine/SMG platform. Last time I was in Brussels I recall drooling over the UZIs carried by the folks at the airport…and hearing how our heavy LE use of shotguns was considered odd OCONUS.