They were talking about the 110gr Sierra Varminter which has a large hollow point compared to most rifle bullets. If loaded to a short OAL it may have some feeding issues in a 300W/300BLK AR. The longer case of the 7.62x40 (by about .2") won't allow it to be loaded short enough to cause problems as far as I know.
The .30 carbine bullet you pictured will most likely work in both but you may have to load long in the 300W/300BLK like the 110gr Sierra or you may not. You would just have to try them.
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