Re: press question
Been using a RL550 for many years to load precision ammo, but I also use a number of single stage presses as well. Thgey've all got their places. The troublesome word in your question isn't "progressive." Progressives and precision can go together quite well. The word that concerns me is "fast." Fast and precise don't go well together, and something usually has to give.
There's another point that I'd consider here, and that's that you're loading for an AR, which is by definition and advanced form of handloading. The problem with a progressive in this application is that it takes a fired case that's inserted into the shellholder, and gives you back a loaded round with every pull of the handle. Not really a good thing when you stop and consider that there's no place for case inspection, verifying headspace, trimming, chamfering/deburring, and a host of other operations that may (or may not) be needed between the time the brass comes out of the tumbler, and it goes back in to boxes as loaded ammo. I do use a 550 for some of my match loading, but I do it in a way that doesn't really match up with most people's idea of a truly progressive press. I use a two step operation, that results in sized, tumbled and sometimes primed brass, being charged and having bullets seated. I sometimes use the 550 for sizing as well, but in that instance, sizing and depriming is all that's done before the brass comes back off the press for inspection, trimming or anything else it needs before being charged and having a bullet seated.
Even the charging and priming operations are done a bit differently when I use the Dillon. In the charging, I use a Redding BR-3 or BR-30 measure, mounted to the adapter that Dillon sells. I like these measures much better, and they generally give much better results than the Dillon measure when using the extruded tubular powders I favor. As for the priming, I will sometimes use the Dillon press and priming system, but ALWAYS employ a followup QC check to ensure that all primers are seated below flush with the case head; an especially important check when loading for an AR or other military style autoloader.
However, once you get a system in place and think through some of the potential pitfalls, the Dillon will definately make it easier to produce large amounts of match quality ammo in less time.