Nice, my 708 has an 18" barrel. I'm using H335 and a 110gr TTSX getting 3000 fps with 45gr.
2208 is probably the best powder for 140gr, irrespective of barrel length. The lighter the projectile the faster the powder, within reason.
Neck diameter to case diameter ratio also has an effect. The smaller the neck diameter to the case diameter the slower the powder needed.
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