There's always a slight difference for projectiles oal. Lead tipped are the worst followed by hollow point. Plastic tips less so.
As you are doing Base to Ogive that is the correct and only way to measure. Cartridge OAL only comes into play to ensure your loaded cartridge fits your magazine.
Just to muddy the water a bit sometimes the Base to Ogive measurement can differ from lot to lot. Sierra used to be very bad for this.
I always check.
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