sly, I bought my MR1 used and assume it's had at least 1000 rounds put through it by the original owner. I don't plan to baby it, if it can't cut it, it can't stay.
So far I've fed:
1. Chinese Norinco yellow box, marked .223 Remington on the box, headstamp is a "C" at 10:00 o'clock, a "J" at 2:00 o'clock, and 93 at 6:00 o'clock. It's the Canada Ammo 1600 round stuff.
2. Some reloads from a surplus store marked Cedar Brass Co. with varying headstamps and even different bullets in the same box!
3. Some other surplus store fodder marked 5,56 mm Ball-55 Gr LOT FNM 82-1 on the box, and FNM 82-1 on the headstamp.
I am trying to see if this rifle is worthy or not, so the idea is to see if it will jam or do something untoward. So far I just get tired after several hours and hundreds of rounds. I've had no jams using either the stock mag, or a borrowed aluminum aftermarket 30 rounder with a rivet in the back. Avoid these as the rivet makes them a friction fit in the magwell. One Norinco round refused to fire even though the primer is well dented after being the fifth round in at least 3 mags.
At the next range session I will be testing a 30 rnd Norinco and a 20 rnd ProMag.
As a word of caution, with any different ammo, especially spooky reloads, start by putting just two rounds in the mag. When you fire it keep your face away from the action and try to pull the trigger with a hook. YMMV.