This is our first bolt, scope sited gun. Definitely room for improvement, but that is the fun part.
Most of the first 1/2 hour was tuning it in. By then our eyes were pretty beat.
Sounds like you had a good time. I'd offer a couple of thoughts for your consideration about the scope sight.
First, I noticed the groups did not enlarge much when the range was increased. This could indicate parallax in the scope which can be adjusted to a given range. I'd suggest 100 yards for your rifle. It may be properly adjusted at the present time. Easy way to tell if the scope is properly set is to place the rifle on sandbags with the crosshairs on a small object at the desired range. Then, without touching or shaking the rifle, look thru the scope and move your eye side to side and up and down. If the crosshairs appear to move on the target parallax is present. The amount the crosshairs move on the target plus the maximum spread the rifle will shoot will be the smallest group you can reasonably expect on the average. Most scopes have a section in the owners pamphlet on this adjustment. The website of the company should have the information as well.
Next, you mentioned that your eyes "were pretty beat". If the focal length of the scope is properly set and the scope is in focus for the shooters eyes their should be no eye strain.
To set focal length simply pick up the rifle with your eyes closed, get in a comfortable shooting stance and open your eyes. You should see a full, clear view thru the scope. If you see less than a full field then move your head forward or back to get the full view. Adjust the scope forward or backward as needed.
Focus is adjusted in different manners on different scopes. Usually it's a matter of unlocking the rear lens lock nut and turning the lens fore or aft a bit. Be sure to lock the lens in place with the locking nut. Focus is proper when you can open your eyes behind the scope and the field is instantly in focus without your eye having to make up for it. Older eyes have more of a problem with this.
You should be able to look thru a properly adjusted scope for extended periods with no eye strain. Some cheap scopes are very difficult to bring into adjustment as the lenses are just not that good.
Fine tuning a rifle is a bit time consuming but once done it makes shooting it a lot more fun. Knowing the bullet is going to hit where the crosshairs are is very confidence building. I've helped people adjust the parallax on scopes from time to time and watched their groups shrink remarkably. Always fun.
I agree about the Marlin barrels, most of them are shooters right out of the box with little or no shooting in required. Now that "Big Green" owns them this may change.