I haven't used the 30 cal J40's , but I have used the .284, 140, J36. These bullets should perform as advertised, but pay attention to the twist required. These are very long bullets and Warren told me that the 210 would not stabalize under about 3100 fps in a 1-10" barrel even at my 4200 ft of elevation. I hope this helps. You can always just give Warren a call, and he will help you out.
Vince
Vince