These are reloading variables we must work with and ways to optimize for accuracy or compensate for when using mixed range brass:
Bullet selection - As many posted, I think bullet selection is one of the more important factor for accuracy. For me, Montana Gold/Remington/Speer/Winchester etc. jacketed bullets have shown less than 1 gr weight variance compared to 2-3+ gr variance of plated bullets. With lead bullets, I have seen 5+ gr variance depending on the brand/caliber weight. As others posted, lead bullets that are sized .001"+ groove diameter of your barrel and/or of soft enough alloy (12-18 BHN) to match the powder/charge used will better deform the bullet base to seal with the barrel to produce more consistent chamber pressures. But using bullets with longer base has limitations as powder compression may limit powder selection.
Many prefer bullet types with longer base for accuracy (like 9mm 124/125 gr vs 115 gr, Hollow base vs solid base, SWC vs RN, JHP vs FMJ, TCFP vs RN, non-step RN vs stepped RN) as having more weight towards the back of the bullet/longer bearing surface to engage the rifling may produce greater rotational stability in flight. But using bullets with longer base has limitations as bullet seating depth and powder compression may limit powder selection and charges used.
Brass - Unless you use new brass or once-fired same headstamp brass, you are going to get different chamber pressures from varying degrees of brass quality and condition. Also, your resized case length will vary with mixed range brass and for semi-auto calibers that headspace on case mouth, variation in case length will produce different amount of bullet sticking above the case mouth. For greater consistency, you can sort "resized" brass by length -
http://www.thehighroad.org/showthread.php?p=9606640#post9606640
Neck tension - Same as above, work hardening of brass will vary depending on the headstamp and number of firings and will affect malleability of brass/neck tension. If you are using mixed range brass, this variation is a reloading variable you must work with. You can maximize neck tension by using bullet types with longest base/bearing surface, minimizing expander/flare, and using deeper bullet seating depth (if you are using bullets with short base like 115 gr FMJ/RN). Some plated bullets have softer lead alloy core and bullets like RMR HM plated bullets with harder 11-12 BHN alloy may produce greater neck tension consistency.
OAL/Seating depth - Using longest working OAL/COL will reduce high pressure gas leakage and will produce more consistent chamber pressures. If you are using progressive press, resizing brass in separate step may reduce OAL/bullet seating depth variation from shell plate tilt/deflection.
Crimp - Although I use match barrels with tighter chambers, I prefer not to use excessive taper crimp to prevent reducing bullet diameter. Often taper crimp of .021"-.022" added to the diameter of the bullet will be enough and will freely chamber in even tighter barrels. Depending on the bullet type used, some reloaders won't even use taper crimp as they will simply seat the bullet with minimal flaring of case mouth to not require any taper crimp.
Powder/Charge - For me, drop to drop variance of less than .1 gr is preferred and is the requirement for my match loads. Some large flake powders like Unique/Red Dot/Promo will meter with .2+ gr variance but still produce accurate loads. Some denser powders like Titegroup, .2-.3+ gr is the full start/max charge load range and will require more precise metering.
IMO, slower burn rate powders than Unique/Universal will obtain optimal accuracy with high-to-near max load data. So if you are loading lighter recoil practice/match loads below high range load data, you want to consider using faster burn rate powders that can produce accuracy with lower powder charges. Fast burn rate powders like Bullseye can produce accurate loads even below start charges that require lighter recoil springs to cycle the slide.
Barrel - Many factory barrels have twist rate of 1:10 while match barrels have 1:16, 1:18, 1:20 and even slower twist rate. IME, KKM barrel with slow 1:20 twist rate has produced greater accuracy than 1:10 factory and 1:16 Lone Wolf barrels with faster twist rate.
There are other reloading variables ...
