Basically, the longer the bullet is compared to its caliber (of the bullet, not the barrel), the faster the twist must be to stabilize it. So for each caliber, there is a maximum length of bullet that, say, 1 1-28" twist will stabilize. If you move up in caliber, from say a .40cal Shockwave to a .45cal Shockwave, the you can use a longer bullet because the diameter has gone up.

The practical implications of this are exactly what Cayugad has shared.

As for load development, that is a much less scientific explanation -- all guns, even those of the same model -- are individuals. Some will shoot a load will and other guns of the same model will not. All you can do is experiment until you find what works best in your particular gun. Usually, start at 80-85gr and work your way up in 5-10gr increments until the most accurate load is determined. Few guns shoot accurately at their full rated charge, especially with lighter bullets.