I think AlwaysAisle nailed it - your calrose type rice would need to be piping hot if you want to shape it for anything other than norimaki/temaki.

Every now and again I’ve had to make large batches of onigiri (without nori) for groups of little people with not quite optimal rice. If you don’t use piping hot rice for this, the onigiri shapes disintegrate in a child’s hands - the mess is horrendous and the kids go hungry as they don’t get their snack.
Some people have skin like asbestos. Not me! Handling hot rice is painful!
Solution? Sarin wrap! (You can re-use it). If you have some food handling gloves, use them too. The more layers between you and the scalding rice the better.

If the rice isn’t blisteringly hot, the temptation is to exert too much pressure. Packing onigiri too hard also affects the taste.

Another scenario where it’s important to use well made onigiri that are resistant to disintegrating is yaki-onigiri. Vital to make these with very hot rice also, otherwise they just fall apart in the pan/BBQ/under the grill.

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On Saturday we had lunch in our tiny garden, the weather was beautiful and we enjoyed Okinawa inspired Taco Rice in the sunshine.