Powder weight VS twist rate

lysander said:

This is the best short answer.

Medium length answer: For the same bullet weight, case, propellant, primer, ambient weather conditions, propellant temperature combination, you may see some slight increases in pressure, but it is generally not sufficient to get excited about, unless you are going from a 1-14 to 1-7 (even then I doubt any difference would show up without a pressure test rig). Big things that effect pressure are chamber dimensions, number of lands and grooves and the proportion of land width the groove width, forcing cone angle, seating depth.

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Slightly longer answer: For the same bullet weight, case, propellant, primer, ambient weather conditions, propellant temperature combination, the major difference in twist rate is encountered when the bullet is being engraved on the rifling. After being engraved, the difference in bore friction is very close to zero. So after the bullet jumps from the case to the rifling there is a slight increase in pressure, but after the entire shank has been engraved, the pressure curve will follow the same pressure curve as the slower twist barrel.

You might ask: Isn't the twist rate part of barrel friction: yes, but not in the way you are thinking. After the bullet has engraved itself on the rifling, it has been spun up to the twist rate and from here on in its spin increases linearly with the velocity it has in the bore. Thus, the difference in friction cannot be more than the added shank helical distance of the engraving compared to that part of the shank that is in contact with the groove of the barrel. Since the shank to bore is very much greater than the added helical engravement, the difference in friction is almost immeasurable.