Twist, length, and groove count applied to ballistics

[gunmutt]

How should one go when selecting twist, groove count, and barrel length during deciding what to go with?
I am guessing that twist is decided in regards to bullet selection. But also if going from a 4 groove 7.5 twist to a 4 groove 8 twist would affect pressure by lowering it in say a 6mm bullet.
When going from a 4 groove 8 twist to a 3 groove 8 twist what is affected?
Length can add velocity and add stability and allow for slower burning powders.
All and all I am trying to decide what pac-nor barrel to go with on my 6xc being able to use the Berger 105 hybrids, 107 SMKs, and possibly the

 

[LitLBoy]

7 Twist throat 90-100

 

[hunter243sgk]

I have a 28" 3 grove 8tw Pac-nor. I have not tried the hybrids but have had good luck with the 107s. That barrel is a shooter!!

 

[1shot]

That's great intel to have. I always wondered how a 3 groove would shoot. Are you seeing any pressures? What load and how fast if I may ask?
Lloyd

 

[hunter243sgk]

I have not had any pressure problems yet. Most of my loads are hunting loads. I had a few 107s I got from a member that I tested don't have my notes with me. I will hunt them up later. I have got excellent results with the 85 and 100g sgks and the 87g v-maxs. This barrel cleans really easy. Part of that I would say is the wide three groves and it was hand lapped. I need to get some more target bullets to test it better. I am just now starting to see some of them in stock for a change. Also now that a good fellow sold me a good die set I am hoping to do even better!!

 

[GrocMax]

3 groove the ramps on the lands in the throat don't appear to wear as fast or as much as say as std 6 groove. My preference for varminting.