Personally I would choose either a 7.5 or 7.7 twist for use with the 90 grain bullets. An 8 twist might work ok with the shorter Sierra 90 grain MK (1.165 in) if they hold together, but some have found that it is insuffucient to get good accuracy with longer JLK (1.233) and Berger 90 grain bullets.
There has also been some debate on whether 6 or 8 groove barrels are kinder on the thin J4 jackets, but I havent yet used either to be able offer an opinion.
My .224 Clark has a Krieger 26 inch 6.5 twist, 4 groove barrel, and it is still work in progress. The 100 grain Wildcat ULD (1.300) and 105 grain Little HPBT (1.305) which were both built using J4 jackets, came apart at velocities above 2800 fps. The 80 grain Sierra MK came apart at about 3450 -3500fps.
Garry Little a local Oz custom bullet maker then made a variety of 85 - 110 grain bullets for me to try that were made on 257 and 277 cal jackets, and they all held together at maximum pressure velocities. However, they were made with a 6 cal ogive die, which was less than ideal.
Subsequently, Garry made some 10 ogive 100 grain HPBT's (1.250) using a 25 cal J4 jacket with a soldered core, and these have held together at velocities up to my maximum pressure load (56.0/H870) of 3,220. I am now testing to see what accuracy can be obtained with them at ranges out to at least 600 yards.
I believe that a 7.0 to 7.5 twist barrel would probably be sufficient to stabilise these bullets, so my 6.5 twist is adding to the stress on jackets. However, when I ordered the barrel I was expecting to be using the Wildcat 107 ULD which was 1.350 inches in length, but that didn't occur.
Hope that the above is of some help. Brian.
