The .40 super is a high pressure bottlenecked cartridge developed by now defunct Triton Ammunition Co. Basically, it's a .45 super case, using a small pistol primer, necked down to shoot .40 bullets at velocities higher than the 10mm can safely go. The idea of only needing to swap barrels, to get a very high performance round, while using your existing .45 acp mags sounded great, on paper. In reality, this is not a round that the average handloader, should mess with. Most pistols that are converted to shoot this round began life as a .45 acp. A low pressure firearm designed to shoot a low pressure cartridge. With the .40 super, you will be working at much higher pressures, bullet velocities, and slide velocities. The margin for safety, is extremely small with this round, as you are working at the utmost limits of gun design, and metallurgy. One small miscalculation, or error in judgement, could be very bad.
Also, the slide velocities from shooting this round are extremely high. Even with a very strong recoil spring, the beating the gun takes is heavy. Also it is tough to get reliable feeding sometimes because the mag lips have to be tweaked for the round. Also, the recoil spring has to be so strong to slow down the slide, and the slide gets slammed back into battery so hard that the top round in the mag will often stovepipe. For my money, I would choose the 10mm over the .40 super. Ever wonder why no gunmakers make a .40 super? At least the 10mm is available in a few factory loads, and there is plenty of reliable load data available. However, if I was to build a .40 super, I would choose a Glock 20 to start. The G20 has a slightly heavier slide than the G21. A high quality conversion barrel with full chamber support is a must. And the Sprinco Cor-Bon recoil reduction system. And some G21 mags. That takes care of the hardware. I would only use Starline brass, and I would only load them 3 times, then chuck em. Any good set of steel dies will do. No carbide dies are available for these bottlenecked cases. So you have to use a case lube. MAKE SURE THOSE ROUNDS ARE CLEAN & DRY before shooting them. Also wipe the chamber with a dry patch before shooting. Sorry this went so long. Hope it wasn't too boring. Bottom line is this.....If the 10mm can't do the job, the .40 super won't either. A 10mm in the hand is worth 2 .40 supers in the bush.