It will shoot a 100 yard group under 5/8" when I can.
Accuracy of the Contender seems to rival a very good bolt rifle. Operator error tends to be the biggest question, at least in my case. I got a LOT of different bullet weights at a blow out sale for $3 a box. Bought something like 18 boxes, every one they had. Some were 110 grain, and they even shoot extremely well.
The Maximum is just a little shorter than the Herrett. In the real world, I suppose the difference is very little. Remember that a 10% increase in powder typically gives a 2 1/2% increase in velocity.
The Herrett is a .30-30 necked up to .35 and shortened .200. It's been a long time since I chrono'd mine, but IIRC I was getting about 2,200 with a 160 grain. I'd have to check my records to make sure. These were about middle of the recommended load range.
In any event, there are a number of rounds that will work. There is no one right answer, but a number that will work fine. Right before deer season my local shop gets 25-30 boxes of .35 Rem, and we can't hunt with rifles here. They're all for Contenders. You could find .30-30 anywhere.
I don't know to much about rifle calibers and would'nt be reloading myself . I would just be buying loads off the shelf .
I know some guys don't want to reload, but you might consider starting. I'd get a nice simple single stage for rifle. You can pay for your entire set-up with the first 2 boxes (40 rounds) of ammo you'd buy retail. We go through that easily in one sitting. As you can see, a lot of the guys here reload.