Does an increase in the twist rate have much effect on the pressure?I know it's a hard question to answer with all the variables in barrels.

The quick answer is yes, faster twists will raise pressure do to greater force required to make the initial turns in the barrel. I E overcoming the minimum resistance necessary to make one revolution. This of course assumes all other factors to be equal.

I thought it would,common sense.Real question is ,I guess , how much per inch of rate increase, a lot or within the the normal variations.Loading a touchy cartridge can be a bit unsettling at times

Logically the answer is yes. However, from my experience with the .223 I haven't been able to prove that. I've built several .223 rifles with barrels from 1-9 twist to 1-14 twist and logically I should have been able to achieve better velocity with the 1-14 twist barrels.

Statistically that hasn't been the case......so far...

Or at least I haven't been able to measure it.