My 5th ed. Hornady reloading book does not list IMR-4320 for their three 139gr bullets for the 7mm Weatherby Magnum. From your description, your friend has the 139gr Spire Point (the other two 139-grainers are boat-tailed). I've used the spire point in my 7mm Remington Magnum with IMR-4350 with impressive results (Pa whitetails).
Sierra, Nosler, Speer, & Barnes list loadings for their 140gr bullets using IMR-4320 in 7mm Weatherby Magnum.
There is no reason on earth to pursue loading 4320. There are many fine modern loads with easily available powders in any reloading manual. That Weatherby ammo was loaded by Norma for Weatherby and they used Norma powder, Why they ever thought 4320 was a good choice is beyond me as it is clearly in the realm of other magnum loadings. H4350-IMR 4350 H-4831-IMR 4831. Lots of choices rather then trying to fit a square peg in a round hole. Those 7MM's Weatherby or Remington are not picky. Jeff
I can't believe any body at Weatherby told your friend to use 4320, in the late 50's and early 60's I spent a lot of time at Weatherby in South Gate testing loads at their 100 yard underground range. First we never told the upstairs what we where working on, and I'm not going to tell you. Sarge has it nail down very well.Do yourself a favor go to, www.reloadersnest.com and you will find a real good load. I've taken over 250 big game animals with a 7mm Westherby so I think a have a little input. A load you might think about, Combined Technology, 140 Gr. Ballistic Silver, with 74-75 gr. of IMR 7828. Now this is going to get some responds,but I can handle it.
NO, NO, NO do not use 4320 in a Roy W. gun. If you must, there are some 4350 Loads, but 7828 is the proper powder to reach the full potentiol of this caliber.
Also please be sure to make the cartridge OAL to book specs. Trying to seat the bullet into the lands will create unsafe pressure levels.
Weatherby guns have a goodly amount of freebore to attain the velocity that they do. If you ware going to seat the bullet further out reduce charges by at least 10% and work up carefully. A pierced primer is no fun.
I have reloaded for the 7mm rem mag for around 25 years and I have tried different powders and bullets for this round. The best powder I have found for this round is IMR 4831. My guess is this powder will work great for the Weatherby mag and this would be the powder that I would try first. Aside from the rounded shoulder the two rounds is almost the same. Hornady Handbook 3rd edition list the 139 gr spire point load data for IMR 4831 as; min powder weight of 67.4 gr (2900 fps) and the max powder weight of 73.1 gr (3300 fps). The manual also states the test rifle used showed excellent uniformity and accuracy with IMR 4831. The manual list the load data for IMR 4320 as; min 55.2 gr (2800 fps) to a max of 60.9 gr (3100 fps).
IMR 4831 is a slower burning powder than IMR4320 which results in more powder to get the same speed from your bullet. More powder in the case means the case has more powder in it and less air which allows the powder burn more consistent which means better accuracy.
One thing that I have found from reloading the 7mm cal is this cal likes bullets weights in the 160's this has to do with the ballistic coefficient and sectional density of bullet weights in this range for the 7mm caliber. The 160's gr bullets are more accurate than anything I have use. My current load is IMR 4831 and 162 gr boat tail hollow point bullets. The load will put 3 shots thru the same hole at 100 yrds when I do my part and is graphed at 3000 fps.