Just bought my first Springfield rifle today, a Saint ( base model ST916556BM ).
It was a toss up between the Saint and a Ruger 556.
It’s been 20+ years since I used an AR platform rifle so I’m a bit rusty to say the least.
Any suggestions as far as set up, ammo to use/not use, etc I know when they first came out there were issues.
I, personally, think you picked the better of the two. I bought one of the first SAINTS when they came out in 2016. They hit the market in November of 2016. In California, you only had 40 days in which you could purchase one.
A lot will depend on what you want to do with your new SAINT. .
If you don't need glasses, then the iron sights will be just fine. If you are looking at being more precise, then put a good quality red dot on it. Holosun, Sig Sauer, and Trijucon all make good red dots. Some even have solar panels on them. When I have the Holosun on it, I am able to co-witness with it.
As for ammo, if it will be for plinking, then most any ammo will do. 55 grn 5.56 is more than adequate. For distance and more accuracy, look at 62 grn or even as high as 77 grn. Ammoseek.com will be your friend when looking for good ammo prices. Before you buy 1000 rounds of steel cased ammo, try some first. I have had some of my ARs that didn't like the steel cased ammo. It would cycle no problem for the first 30 or 40 rounds, but than you might get some cases that fail to extract as the chamber gets dirtier.
Make sure that you are using a good chamber brush when you are cleaning.
If you are going to try to shoot distance for accuracy, a good quality scope would be recommended.
You will find that the SAINT is plenty accurate. I have a Holosun Red Dot on mine, and I was hitting a steel target that was only 6 inches wide at 400 yards with great consistency. I was using the Iron Sights at 350 yards on a 24-inch target, standing unsupported and hitting 90% of the time.
It is a great rifle.