I've never worked with anything shorter than 20". Expect an average of 25 fps loss for each inch shorter. Most factory ballistics are from 24" barrels, so 100-150 fps slower than advertised on factory loads.
It is impossible to say for sure because every load and barrel is going to be different. I load for 7 different 30-06 rifle, 2 have 20" barrels. Depending on the load the 20" guns are 50-75 fps slower than the 22" and 24" barrels. The fastest barrel is one with 22" barrel that shoots the same ammo faster than the one with a 24" barrel. There is a 135 fps difference between the fastest 22" barrel and the slowest 22"barrel.
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Also, is there a faster 30-06 powder more suitable for 18.5"?
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This is a myth. If you hand load, which ever powder gives the best speed in a longer barrel, will still give you the best speeds in a shorter barrel. I'm using H-4350. From my 20" guns my handloads are still faster than factory specs for a 24" gun and much faster than a 308 loads from 24" guns. You will get more muzzle blast. It is not that bad from a 20" gun. Another 1.5" less, I can't say. But as far as perfomance on game. You will still probably beat 308 specs and that ain't a bad thing.