There would be new things to learn concerning the 650's operation, and probably new drawbacks... Like "oops I can't do that on this press".
I am generally familiar with a 550 and how it operates, but I have never used one. I will also remind you that I use a Redding expander in station 2, and a Hornady drop in station 3, so I can't speak to Dillon powder bar failsafe. I also do not have the Dillon primer early warning system installed.
That said, I think the only "drawbacks" you're going to run into in terms of function are going to involve retraining yourself to the auto-indexing. Your "oops, I can't do that on this press" moments are going to revolve around you wanting to move the ram up or down, crossing the index point, and getting it to index when you didn't want it to. Then you're pulling and moving cases around, or running three or four pulls without a full shellplate. Another PIA with this is that when the shellplate indexes, the priming system indexes whether there's a new case to be primed or not, and unused primers need to be reloaded into the primer tube periodically. If you know ahead of time that you need to index a bunch of times without priming, you can remove and replace the primer indexer quickly, but otherwise, you end up cycling new primers into an unused primer chute to be collected and reloaded into the priming system later.
It's all a minor thing once you've adjusted to it.
Do you really need that bearing upgrade beneath the shell plate ?? Is that just for some people who don't know how to adjust the mounting bolt tension, or it for people with case feeders ??
Okay, I'm not sure which of two things you mean here. There is the steel ball-bearing detent we talked about in Baldrage's thread that can knock powder up and out of the case when it snaps into place. And there is thrust bearing that people mount on top of the shellplate between the shellplate and the mounting bolt.
As to the ball-bearing detent-- Do you have to replace it? No. The amount of powder is tiny. There's no ballistic effect measurable with any tools we would be using. Given a small case like 9mm, though, and you'll start to see spillage during a run, and it's a small, cheap upgrade to reduce or get rid of it.
As to the thrust bearing, I don't know. I don't use one, and the 650 is exceedingly smooth
to me, so I'm not sure what it's accomplishing. Regardless, the answer is no, you don't need one.
What are the weak points of the 650 ?
On the 650, the case rims sit in slots at the edge of the shellplate. The amount of metal suppoerting the cases is not particularly thick. This is as opposed to the LnL, where the cases sit on thick, sub plate, and the shellplate just slides the cases around on the subplate. Under force, when the ram is driving the cases into the dies, the 650 shellplate is going to flex more than the LnL. I am not sure about how much it flexes relative to the 550. That flexing is going to give increased runout relative to the LnL, and maybe the 550, as well.
How much of a weakness is this? For it's intended purpose, zero -- because the 650 is a high volume pistol cartridge machine. It was intended for pistol. You can get the conversion kits for various rifle calibers, but that was not it's designed purpose. For pistol at normal pistol ranges, the extra variation from one pull of the lever to the next isn't going to show up on the target. I say that because I load .45 ammo on the 650 that holds 10-round groups under 4 inches. That's me and my 97BE, wrists rested on a rolled up bath towel. I'm an okay shot, but not great, so if I'm holding under 4 at 50 yards, the pistol and the ammo are getting a LOT of credit. I produce 50-yard worthy precision on a 650 without that upgrade, so I don't consider this
weakness in the shellplate to be significant so long as you're NOT loading pistol. I load rifle on a single-stage. If you wanted to load long range rifle, you might be advised to do it on a single-stage, or you might want to keep the 550.
Another small weakness would be that the priming anvil isn't adjustable. That's not really a weakness, just a feature I wish it had.
And yet another minor weakness is that unused primers go down a shoot into a little short-walled platform, where the walls aren't tall enough that the primers are guaranteed to stop there. Some of them fly off and fall to the floor. There are a number of easy fixes for it.
What additional accessories are required to make it run smoothly ?
Amada8 gave you a list that is pretty much everything.
I use the strong mount and the Dillon roller handle. I wouldn't have it any other way. When I mounted my 650, I mounted it to the strong mount, then put on the table top with the handle pulled down and the ram raised all the way to the top. I slid the strong mount and press back into the table until the lowered handle touched the table edge. I then slid the mount/press back another eighth of an inch, which pushed the hand back up just a hair. That's where I mounted the strong mount. If not mounted this way, you pull the handle and when the ram hits the top, there's some flexing going on, and variation from one cartridge to the next is dependent on your ability to
feel the exact same amount of force on each stroke so that you stop your arm, and reverse directions. I don't have that. With the way I set it up, the handle hits the edge of the desktop and stops. There's no variable flexing at the top end going on dependent on my arm consistency. The LnL has an overtravel linkage where you hit a maximum force, then it backs off, so you hit the same max every time regardless of your arm pull. I mention this here because if you want to employ this trick, the strong mount is absolutely necessary, and the handle needs to be long enough. I can tell you the Dillon roller handle is long enough, but I can't speak to others. I wouldn't do this without a roller handle of some kind anyway. It's a must have for me.
The bullet tray is nice, but not necessary.
The case feeding system is nice. VERY nice. Even withOUT the powered collator. I reload every 20-ish rounds, depending on caliber. A collator would be great, and they're actually quite easy to build yourself. There are videos on Youtube. It looks to cost about $50 to make. One of these days... But for now, the system in place with just the tube is extremely useful.
I can't think of anything else that's necessary.
If you have any specific questions, let me know.
To sum up, if you're envisioning this as a pistol loading press, go for it. If you're envisioning it for precision rifle, there might be better options -- either an LnL, or a 650 + a single-stage, or 650 plus your 550.
