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GRT isn't a drop chart type app its an application that calculates muzzle velocity based on case design powder charge seating depth projectile etc.

It's for reloading

I use it on my desktop.
I'd be surprised if it worked on a phone. Quickload definitely doesn't and came on a CD.

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Yeh I think the level of overbore seems to be a huge factor that and obviously physical case capacity for the some Bulkier powders. Like H1000 in the likes of the 243, 7mm Mag, and 300 Win one doesnt...

Things I have found when reloading for short barrel rifles (14-17 inch 308s and 7mm08s mainly) is the following:

Higher pressure loads burn powder efficiently
Heavier bullets typically help...