For plinking and blasting, you'll find the most affordable stuff in the 55 gr range. Whatever is cheap should be fine for plinking around with it.
I'm not entirely sure what twist rate those barrels come in. Is there any way you can look at the barrel and see if the barrel is stamped.
It'll say something like 1 to 9 or 1-9 or 1-8 or 1-7 forward of the gas block somewhere. If it's not there, maybe it'll be listed in the owners manual or on the box.
The faster the twist rate (1:7 being the fastest) tend to prefer heavier weight bullets (like 75 gr Hornady for example) better at longer range when precision is wanted.
I've ran tons of Wolf Gold over the years. It's a great 55 grain full metal jacket round.
For chasing small groups at 100 yards look at 55, and 69 grain "match" ammo from Hornady, Nosler, and Federal. Might have to try a little of each to see what the gun likes.
You are getting good advice here. 55-gr FMJ is a good choice for blasting, and it is proven to be reliable. Some of the Federal XM193 is even excellent for self defense ammo and its can be found at good prices.
For accuracy at 100 years it will just take finding out what your rifle takes well to. It can be frustrating but buy small batches of varying brands and see what makes her sing.