I get best accuracy from my 7mm RUM and the 180 grain VLD at 3184fps. Barrel length is 26". Acuuracy is .3MOA with brake, .5MOA without the brake.
Here is the math for a 30" barrel:
35fps per extra inch of barrel.
4 x 35fps = 140fps.
3184 + 140 = 3324fps.
When looking at ultra velocity 7mm's, you will need to compare the COAL's with the magazine length of your intended platform versus bullet jump. I use a stock standard M70 action (I could get the longer mag box from Wyatt Outdoor) so bullet jump is .400", thats nearly half an inch. Have a look at the various designs and see how they stack up in this regard. No doubt other posters will reply to my comments and help paint a better picture.
The one aspect of long bullet jump that is often mis-understood, is the effect on the projectile's stability factor. The longer the freebore, the more skewed the angle of entry into the lands. As velocity and or twist rate are increased, the result is increased dispersion at the target. I think that some (but not all- i.e I don't know for sure) erratic performance can be attributed to the neglected issue of freebore versus stability.
In my RUM, I have performed controlled tests to verify stabilty problems. If I seat a 168 grain Berger out long, I can load through the usual velocity ranges with acceptable accuracy- although by acceptable, I mean that groups hover around 1MOA. If the bullet is seated to fit the magazine, starting loads give 1MOA but as velocity is increased, groups spread out to 2.5MOA. The same result occurs with 162 grain bullets, including the stout jacket designs.
I use a 4 groove 1:9 twist True-Flite barrel which does the job nicely with the 180 grain VLD. My wife uses the same barrel in her 7mm Rem mag and it works well with all bullet weights. My rig works fine but I would still urge you to look into the finer details as a lapua based Wildcat/ Allen Magnum might (or might not) offer less limitations than the RUM. A short throated RUM is a bit of a waste of time as the cartridge reaches max pressures very quickly, yet suffers other pressure related problems with start loads. Suffice to say, the 7mm RUM needs a kind of release valve before reaching full acceleration (sorry- that was a horrible analogy). You will lose about 100fps with a .200" freebore reduction in the RUM and will need to use only the slowest burning powders.