Sounds like you have some research to do. You said you want to get a Chargemaster combo then asked if you needed a powder measure for each chambering. The Chargemaster is an automatic powder charge weighing machine that serves as the powder measure, but it's a slow way to dispense charges and will be the bottleneck in your operation because you'll be standing there waiting for twenty or thirty seconds after each pull of the press handle while it weighs out the next charge for each round. For a progressive press you want a conventional volumetric measure that can keep up with the speed of the press operation, and a regular scale for adjusting the measure. Still better is a powder measure with either a micrometer adjustment so you can repeatably set it back to a setting for another powder, or with swapable dispensing tubes, like the Quick Measure with its progressive press adapter, so you can leave the tubes preadjusted.
You want to load for your friends? For liability reasons I would never reload for anyone else. Besides, if you figure a barrel will be shot out in 3,000 to 6,000 rounds, letting them run that much ammo through your guns will get expensive from a barrel replacement gunsmithing standpoint. Let them learn to take their own risks with their own weapons, though you can certainly teach them to load on your machine if you trust them with it.
If you are planning only on precision rifle shooting for yourself, I would reconsider loading progressively as you'll want to stop after sizing to do trimming and cleaning and case prep (crimp removal) before completing assembly of the round. I prefer working on the Forster Co-ax press for precision ammunition, especially for long range. I believe it's currently used by more rifle match winners shooting handloads than any other press due to the self-aligning the die holding method and the lateral mobility of the shell holder plates allow. It makes some very straight ammo. That's a press where the Chargemaster can work out in terms of timing because that's all a slower batch or serial process. But that's your call.
I don't know what the benefit of going from an RCBS sizer to the Redding Competition Seater is supposed to be, but it may be counterproductive. I'd have to measure it to see. If the RCBS starts the bullet in crooked, then the Redding may not be able to fully straighten it back out. I would just use the Redding. It's slower to get your fingers in and out from under the sleeve, but there is no conflict in alignment possible. Also, the plain RCBS seater is not bad. If you get a Lyman M die to provide a step flare that lets you seat the bullet straight in the case mouth before starting into the RCBS die, you might find you had about the same result for less tool money.
A 15% drop in plinking load powder charge may or may not be too much, depending on the powder. In some powders that could make the case fill too low and make ignition and pressure erratic. Even if it's OK, that big a drop in charge can lower pressure by more than a third, and with some powders that will make them burn a lot dirtier and less efficiently, fouling the gas systems in exchange for saving a little throat erosion. I would stay with a starting load appropriate for your case and brass. Not knowing where in the range your best accuracy loads will land, I can't say how much reduction that will be. Possibly rather less than 10%.
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