I just want to list the mistakes that I, and others I know well, have made when reloading. It may not make your choices easier but it might.
1. Contaminated primers - - - - this happened to me when I was handling each primer before insertion into the case. It cause misfires and frustration. I even went to using a pair of tweezers without success. The primer feeder tubes didn't always drop a primer so I could not trust them. I went to using an RCBS hand primer that you never have to touch a primer and have had no problems since.
2. Seating primers - - - - seating primers with a press without examining them is fine once everything is set perfectly but high primers and crushed primers are never a good thing. Using the hand primer I can run my finger over each primer as I set the case in the block and know it is right.
3. Powder charges - - - - I put all my primed cartridges into a loading block and charge them all before going on to seating bullets. I can visually check powder levels compared to the others around each cartridge and see if a level is high or low. No guessing and no "faith" in a mechanical plungers accuracy. I have never loaded a double charge but I have missed charging a case and it is easy to do. If you check them all at the same time you get good loads.
4. Split and cracked cases - - - - Every step in my reloading process is another chance to find something I missed. I have found split necks on pistol brass only after they were flared - just before charging them with powder. I have found body splits after sizing that I didn't see after polishing.
(you are likely to miss subtle splits and cracks using a progressive press)
5. Bullet seating depths - - - - Just because you have used the book to set your overall length does not mean it will fit in your chamber, especially with auto pistols. You need to check the first round to see if it will chamber. If it does then you can likely load the rest and it will be fine. If you change make, type ot weight of bullets then you need to check again. Trust me, it is easier to pull and reseat one bullet a few times than it is to do it with 150 or more rounds.
6. Crimping - - - - If your gun has a tubular magazine, are shooting a semi-auto rifle, or a high recoil revolver then you need to crimp. Crimping introduces one more variable into the making of accurate ammo. Yes, neck tension will vary and can cause inaccuracy too but crimping just adds to that. Crimp when you have to but otherwise don't do it! (there is likely to be some discussion over this because some feel it evens out the neck tension. For me it did the opposite)
These are some of the reasons that I sold my progressive press and kept my Rock Chucker and Lyman presses.
