Can I use the same dies for both?

No. These need completely different die sets. 357 SIG uses bottleneck dies closer to what a rifle cartridge needs, while 40 S&W runs standard straight-wall pistol dies. Case mouth diameter alone differs enough to make the sets incompatible, 0.381 inches for 357 SIG versus 0.423 for 40 S&W. You'll need dedicated sizing, seating, and crimping dies for each.

Is formed 357 SIG brass as good as factory brass?

Properly formed brass performs close to identically for most uses. The real gap shows up in case life; expect 1-2 fewer reload cycles than factory brass, with neck splits arriving a little earlier due to the work-hardening from forming. For range ammunition and even defensive loads, formed brass holds up fine if you follow proper technique and inspect it regularly.

Which feeds more reliably in a semi-auto?

357 SIG generally has the edge, thanks to its bottleneck shape acting almost like a funnel guiding the round into the chamber, which reduces sensitivity to magazine geometry and small OAL variations. 40 S&W feeds reliably in a well-built pistol too, but shows a bit more sensitivity to bullet profile and seating depth. Both still need proper magazine tuning and ammunition within spec regardless.

Can I shoot 357 SIG through a 40 S&W barrel?

Absolutely not. Even sharing a slide and frame, these cartridges need dedicated barrels with different chamber dimensions and bore diameter, 0.355 inches for 357 SIG against 0.400 for 40 S&W. Firing either cartridge through the wrong barrel creates a genuinely dangerous pressure condition and real risk of catastrophic failure. Check barrel markings before you load a single round.

Which powders actually fit each cartridge?

357 SIG wants medium to medium-slow pistol powders, Power Pistol , Longshot , Blue Dot , or Accurate No. 5 , that build pressure efficiently in that small bottleneck case, and full-power loads there often call for a magnum-rated primer like CCI 550 . 40 S&W does well with a broader range, medium-burn options like WSF or Universal work cleanly, and its larger, lower-pressure case tolerates a genuinely fast powder like Titegroup far better than 357 SIG ever will; don't carry that assumption over between the two. Unique works adequately in both without being the standout choice in either. Pull multiple current manuals and find the powder with the deepest data behind your specific bullet weight rather than assuming one list transfers.

Does 357 SIG actually offer a meaningful edge over 40 S&W?

Depends entirely on what you're optimizing for. For barrier penetration and outright velocity in a defensive scenario, 357 SIG delivers a real, measurable gain, typically 200-300 fps over a comparable-weight 40 S&W load. For range practice, competition, and general versatility, 40 S&W wins on component availability and cost. Neither dominates across the board, and that's exactly why both still have dedicated followings among serious handloaders.