First off, if you're reloading, you NEED a good scale. You need to try the bushings and see how much powder they are dropping. Weigh several drops - chances are there will be a few tenths of a grain deviation which is normal, and won't cause a problem unless you are using high pressure loads (which you shouldn't be anyway for most purposes - anything under 10,000 psi is adequet)..
Second, you must have a reloading book or chart with recipes from the powder company and you must follow these exactly!
You MUST follow the loading recipe EXACTLY - HULL, PRIMER, POWDER BRAND and AMOUNT, WAD, and SHOT charge. Do NOT deviate from the recipes! Different wads cause different pressures in different hulls! Different primers ignite the powder differently and cause different pressures and velocities! Different powder burns at different rates and pressures! Do NOT mix different powders! Each time you buy a new batch, weigh the drops and make sure it meters the same! Read up and know your load and each component and do not substitute anything not published!
Find the one you want and try different ones to see what works best in your shotgun, and stick to it!
For my traploads, I use a #32 bushing, that drops 17.4 grains of Red Dot (Alliant Powder) ... OR ... Clays (Hodgdon Powder Company). I switched to Clays because it burns clean. I use 12AASL wads, and Winchester W209 primers, with 1 ounce of shot (7 1/2's, 8's, or 9's), in a AA hull or Remington unibody/STS/Nitro.
Feel free to contact me offline if you want more advice and to go into detail, or www.trapshooters.com forum can help you out.