anachronism

My MA decapper solved a lot of problems for me. I'm damn impressed that you managed to break one. How'd it happen?

From Speers website: https://www.speer.com/performance/gold-dot-performance.html Speer has developed a plating process (Unicor) that allows them to vary the thickness of the plating by it's actual position on the bullet, and the copper they use is more structurally durable than the cheap copper wash that many cheap plated bullets do. This also allows them to make rifle bullets in the same pattern, just with a thicker copper sections. The Gold Dot name came from the little spot of the copper outer layer that is left in the center after the hollowpoints are formed in the bullet after the plating process is complete. It's really an injustice to call a Unicor bullet "plated", but technically it is plated. Features The bullet begins with an alloyed lead core. Uni-Cor electrochemical process virtually eliminates core/jacket separation and guarantees uniform jacket thickness for superior accuracy. Nickel-plated brass cases for smooth functioning and durability. Select, clean burning propellants deliver optimum velocity while ensuring consistent chamber pressures. CCI primers for highly sensitive and reliable performance.

I've never had the desire to neuter my 357 to 9mm, so such aftermarket conversions mean little to me. The OP was asking for upper end loading data for 357 SIG and you expect him to buy a new barrel in a lesser caliber to suit your idea of what he needs instead of giving him what he's asking for. I'm not even going to address your suggestion of shooting 9mm through a 357 barrel.

You do understand that 357/.40 slides are different from the 9mm, don't you?

What manuals have you checked so far? The real performers in 357 SIG tend to be AA #7 & #9. Stay away from plated bullets since most of them are only good to around 1100 fps or so. I use RMR, and tend to load heavier bullets. Many 9mm bullets can't handle the extra velocities and are not recommended for use by their manufacturers. Most notable in this class is Hornady 124 gr XTP. Most ammo and bullet manufacturers download .357 to keep their bullets in the velocity ranges they were designed to perform best at, and this is the main reason you can't find much for hot loads listed. Underwood 357 ammo seems at its hottest when loaded with lighter weight solid bullets if that's any indication of the situation for you.

Not all bullets of a specific catalog number from any manufacturer are identical. Each bullet forming die is a tiny bit different from the next one. This leaves us with slight ogive profile variations, nose length variations, overall bullet length variations, and so forth. Ask any high power rifle shooter about this, even the very best bullets have some minor variations, even from the same factory box. If the accuracy and reliability are there I wouldn't sweat it. The variance are enough to be measured, but the gun rarely know the difference.

My P320 prefers .357 sized cast bullets, and seems to feed any bullet design I throw at it without a bobble. My favorite powders are AA #7, AA #9, and 800X (now discontinued). I size the cases first by pushing them through a gutted LEE carbide Factory Crimp die to make sure we have full-length sizing, then sizing a second time in a standard Hornady sizing die to that care of the neck. It takes longer to tell about it than it does to actually do it. So far it's pretty standard stuff, and I also use LEEs collet type crimp die as described above. Look for 9mm bullets with the greatest parallel shank length since 357 prefers short nose bullets for feeding. My personal favorite cast design would be hard to find if you don't cast your own, it's a design called a "Ranch Dog" 135 gr that has a pretty large meplat and has a well deserved reputation for accuracy.

Go to the grocery store and buy a package of copper Chore-Boy pot scrubbers, they come two in a package. There are also stainless steel Chore-Boy scrubbers. Do not buy them for this. Don't buy another brand because it's cheaper, many of those are stainless steel with a copper wash over them. It must be the copper Chore-Boy, stainless steel will damage your barrel. Chore-Boy is a woven pot scrubber that you can snip a small section off and wrap it around an old bore brush, try to get a few strands out of it about two inches long. Wet the bore with your choice of bore cleaner and push the brush with the copper Chore-Boy through the bore, back and forth a few times. The copper strands will literally cut the leading out of the bore, and works just as well on leaded cylinder throats as well. Repeat as many times as you wish, then finish with your standard cleaning routine.

And....?

Armor All Wash and LemiShine have worked best for me, and my brass never tarnishes with this combo. Honestly, I just run them a few hours and don't change cleaners or anything. Too much hassle

You may need to check your platform alignment: To do so, you'll need this tool: https://www.dillonprecision.com/13713 And/or it may be necessary to adjust your indexer cam position:

Wadcutters, or SWC?

I shoot a lot of 357 SIG, and have this problem with Speer brass. I ended up punching the old primers out with my RCBS decapping die, then drilling the flash hole out with my flash hole deburrer. It's a pretty quick, permanent solution for me. For 9mm? I'd probably end up tossing them into the scrap bucket.

I've done a number of SRHs, GPs, and SPs for local people. I always start them out with the middle weight spring (#10), and the only ungrateful revolver was an SP101 that should not have left the factory in the condition it was in, lots of rough edges and poor finish work in the action. The owner didn't want to send it back so he ended up paying me to do an action job on it. He left really happy with it afterwards, and later swapped in the 9# spring and it still worked perfectly. The only other issues were with a couple of guys using magnum rifle primers in 357 mag. They ended up with 12# hammer springs.

Sand sticking to grease. Impossible!

My first thought after hearing that this situation is only happening to cops was to wonder what holsters they were using. There are serious penalties for cops who have unintended discharges, so of course it must be an equipment failure, since all cops are perfectly trained, seasoned professionals who never have accidents or make mistakes. From there it's "we have used brand X holsters for the last thirty years, so it can't be their holster". I don't think we're being told the whole story. BTW, I have three P320s, and intend to keep them.

To add to the concern, I recently pulled a large number of older cast 9mm loaded ammo to recycle the components. I found that although all identical cast bullets were seated to the same depth, the inside taper of the cases don't appear to have any sort of standardization as far as the inner taper goes. So some bullets came out with slight bevel bases, some had more pronounced bevel bases, and some weren't bevel based at all. All from the same mould, same alloy, same sizing diameter, everything. Seating depth was the same, and the bullets were not bevel based when they were installed in the case. This was one of those eye-opening moments for me. I had never considered the angle of the inner base wall taper before. My plan now is to watch my seating depth much closer, and perhaps gauge the upper case walls with locked calipers to get a rough idea of max seating depth available in the cases. most cast bullet manufacturers don't have much for seating depth info. There were never any concerns with overpressure with these loads, and I don't believe there is any sort of realistic safety concerns here either. The ammo got broken down because accuracy was abysmal, and the pullet bullets still measured their original sized diameter, at least the upper portions of them did.

I think the current "why didn't I think of that" system is offered by Chambers Custom. Here's a link to their site: https://www.store.chamberscustom.com/products/chambers-custom-rdms-plate Joes price is just for the adapter if I read it right, machining and installation by one of his authorized installers. I'd really like to see someone make an mount that works with the std low mount Bomar cut, but I see potential pitfalls there.

I'm patiently stalking the auction sites, waiting for the best opportunity. I did find a full size 357 complete slide, but then I have to spend another $200.00 or so sending it out for an optics cut. It's amazing how fast we can spend money on this sort of thing. To give a little more background on this I have one stock Compact in 9mm, one in 357 SIG, plus a 40 barrel for it, then I have another Compact in 9mm that only have very few OE SIG parts left in it. At least everything runs really well for me.

I thought P320 slides were different for .40 & .357 vs. 9mm. I can't interchange barrels on my Compacts at all, the larger caliber barrels are larger at OD than the 9mms. The only real option I've found is to find a .40 x-change kit, SIG seems oblivious to the 357 & 40 guns anymore.

Wow. I haven't shot pins for years. One load I remember best was my 38/357 revolver load, I cast Lyman 358627, a 215-ish gr GC design with dual grooves an a Keith type nose. My load back then was 3.5 gr of 700X for slightly better than airgun velocities. The pins seemed impressed by this load. 230 gr. cast LBT LFNs for 45 ACP in my 1911, and 124 gr cast FNs for 9mm. My 44 mag Redhawk was actually downloaded to hot 45 ACP levels. After several years, it just wasn't fun anymore, we had a crowd of IPSC shooters move in with race guns and optics who kind of ran the more casual shooters off. There were no equipment rules, and a guy shooting a single action revolver could often find himself up against one of the race gunners. I guess I'm saying that the equipment race ended my interest there.

Late to the party as usual. Red River Tactical may have what you need. https://rrtholsters.com/product-category/holster/

I'd suggest OP looks long and hard at target level wadcutter loads. Most HPs won't reliably expand from a 2 inch barrel at standard velocities. I suggest you check out the Lucky Gunner Lab tests for 38 /357. Note that the only bullets that reliably expand from a two inch revolver are specifically designed for that purpose, and a lot of them don't appear to be offered for sale to reloaders. Also take note of some of the tradeoffs between expansion and penetration. Anyway, enjoy: https://www.luckygunner.com/labs/revolver-ballistics-test/

What powder were you using OP? I've had some episodes of powder bridging in Dillon powder funnels. Once the problem was caused by a few grains of really fine ball powder figuring out a way to jam between the powder funnel and the powder die. In another, I had ball powder cause a micro-momentary hang up in the powder measures slide travel. By the time the powder dropped, the case wasn't there anymore. Then there was the time the bushing in my powder measures pivot arm broke and gave me a number of late drops, some of them sort of trickled out. That was a pain to figure out. So many possibilities

As noted above, Apex supports the FN509, and it's available in a few different sizes.I have an FNS 9mm, and the trigger wore in pretty decently, but the FNS Series has NO aftermarket support at all. The FN509 appears to to have a much beefier fire control system compared to the FNS as well. Still, my FNS has never missed a beat, ever. A friend just bought a 509 Mid-size and likes it well enough, but it's taking a bit of time to wear in. His groups out of the box were atrocious, but after perhaps 1000 rounds they're half the size he started with, and the gun shows enough promise that I'm considering buying one myself although the new generation M&P CORE should be about ready to ship to the general public any time now.