Bryan,
I did extensive testing with the 200 gr ULD RBBT Wildcat Bullet in 7mm in my 7mm Allen Magnum. This has relavents because its built on your J-4 jacket and in earlier testing, it behaved quite similiarly to the 180 gr VLD you make.
Originally when we started this project, Dan Lilja made us the barrel he recommended for this new bullet which turned out to be a 1-7 twist 3 groove. They shot extremely well to start with but we ran into problems as the barrels got a bit of wear on them and the 180 and 200 gr bullets started to come apart after clearing the muzzle.
This occured after around 200-300 rounds down the barrels and it did not matter what velocity we were shooting them at once the barrels got to this point, they all came apart.
Then I was contacted by Mike Rock who had heard about my problems and said he had the cure. He sent me one of his 1-8.7 5R barrels and said it would solve everything. It did not, in fact, it was worse then the Lilja, only bullet that landed on paper was the very first shot through the oily bore.
This baffled both of us. Then I started looking at some of the old projects I had built in the 7mm AM. THere were several with 1-9 twist barrels. One in particular had a 34" 1-9 4 groove Lilja. I called up that customer and he was having great results and had far more then 200 rounds down the barrel??? He also was able to push the 3350 fps level harder with good accuracy....... Got me thinking, this had to be a twist issue, but why did the Rock barrel perform so poorly with it was a much slower twist then the 1-7????
Got to looking a bit deeper into the issue. This brought me to baring surface compression. I figured the baring surface compression(BSC) % of all the barrels I had tested and compared that to the actual results shooting the rifles.
The Rock barrel had roughly 31% BSC meaning that the surface area of the rifling compressed 31% of the baring surface of the bullets body.
The Lilja 1-7, 3 groove barrel was around 28% BSC.
The Lilja 1-9, 4 groove was right at 22% BSC
The Lilja 1-9, 6 groove was around 21% BSC
I contacted Mikes barrel maker and asked if he could make me a special thin land 5R barrel with something in the 19-20% BSC range. He did and I tested that barrel along with several other 1-9, 6 groove barrels, all with BSC in the low 20% range. All performed extremely well.
With the higher BSC %'s, anything much over 3250 fps would cause the bullets to come apart in flight.
With the low BSC % barrels, accuracy was pretty good up to 3300 fps. Over that and up to around 3400 fps accuracy dropped off but the bullets did not come apart AS MUCH, in fact much less, only once in a while but still enough to cause a problem.
My conclusion, with a heavy, long for caliber bullet using the J-4 jacket, for best accuracy, you need a barrel that produces less then 22% BSC and also velocity in the 3250-3300 fps range and no more in most cases.
I have run the 180 gr Berger up to 3500 fps in my 7mm AM. This did show higher pressure signs but I could easily get 4-5 firings per case in my testing of chamber pressures. That said, I had to shoot through a large piece of paper protecting the chrono from muzzle blast and had to put the chrono 1 foot off the muzzle to see enough of the bullet to get a reading on. On target, there was nothing. At 1 foot, the bullet hole appeared to be perfect.....
I believe BSC has as much if not more to do with getting these bullets to produce fine accuracy at high velocity then twist rate. In spite of conventional wisdom, I have found at these higher velocity levels, slightly less twist then you would expect to use is generally the best bet.
The 175 gr SMK on the other hand would take anything I threw at it. These two were driven to 3500 fps in testing and accuracy was 3/4 moa. Not as good as it was with them at 3300-3400 fps where they shot 1/2 moa consistantly but still pretty good and when I tested them at long range, their consistancy was much closer to 1/2 moa then 3/4. This was at 800 yard testing.
Simply tells me that at 100 yards, the severe stress imposed on the bullets has yet to allow the bullet to go to sleep and spin around its own center of gravity, but they never did come apart.
Just to be clear, I am not saying 3500 fps with the 175 gr SMK or 180 gr Berger is a proper load, they are very high pressure but this was just for test purposes and education on the parameters of my wildcats performance levels.