I took mine to the range to compare 3 different types of ammo and see what kinds of 5 round groups I could get. I was shooting with a front rest, with a fixed 3x optic. Conditions were excellent with bright sunlight and no wind. All groups were fired from 100 yards.

I shot a group with each brand, changing for new targets in between. From left to right they are: Barnaul steel case, IVI 55gr, and Hornady 72gr.

The IVI is what I normally shoot, and the MR1 likes it just fine. This was my first group of the day, and it printed well under MOA if you discount the one off to the right. It was probably me.

This was my 3rd grouping, shot with the barnaul stuff. It's another tight 4 within 3/4" and a loner down low. Shooter error? Likely, I didn't have high expectations for this ammo and I was very surprised.

The flyers might have been mitigated by a proper lead-sled, but this set up was what was available and suited my needs. You might be wondering: How did the 72gr Hornady rounds fare?
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Total bust. This was my 2nd group of the day, and I put every bit as much effort into this one as the others. Considering this ammo was $30 for 20 rds, I won't be firing this through anything else with a 1:9 twist rate. What a fail!
I fired a few more groups in rapid succession after the 3 you see above, and experienced shot stringing. Groups opened up to 3 or 4 inches when the barrel got too hot to touch. Whoever said the MR1 likes heavier rounds, my own experience suggests otherwise.