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!