hopdoc,

The 540gr FN is specifically designed for the 500NE. The CIP spec for the 500NE is 1:15" but even in a 1:20" twist, this bullet has a stability factor in excess of 3.5 at 2200fps. This makes it very difficult to turn the bullet away from the path it is on and makes for extremely stable transition from air to tissue.

Here are two hunt reports with this exact combination.
http://www.gsgroup.co.za/johnharris.html
http://www.gsgroup.co.za/johnharrisele.html

500grains has also done extensive work with these bullets. Nothing beats hands on field experience with a product.

Speed for the 540gr FN would be 2200 to 2300fps. At 2200fps it is a comfortable load. The 500gr .510" FN bullet is designed primarily for the 500 Jeffery. Due to the nature of magazine fed rifles, some small compromises must be made for reliable feeding. They are still formidable bullets. Below is a picture of two 500gr FNs recovered from Cape Buffalo. Both were back up shots on full going away buff. The left hand one was taken downhill and hit the spine just in front of the tail. The right hand one hit just below the tail for an authentic Texas Heart Shot. Both were recovered under the skin of the chest.

As a custom rifle smith I have seen my share of damaged doubles but I am not aware of a single gun that has suffered damage as a result of using our drive band bullets. Be aware that bullets are frequently described as drive band bullets when they are in fact grooved bullets.

Grizzly,

The 470NE is specified at 1:21" twist and, although the heavier bullets will work, a requirement is that speed must be kept on the low side to avoid turning the bullet on impact. You will see that we offer 450 and 500gr FN bullets for the 470NE on our site and both will be at stability factors in excess of 2.8. The choice between the two would be determined by the availability of powders. Ideally one would use the bullet that will require the least or no filler. Both can be run as hard as is prudent for the given rifle with no fear of having the bullet turn on impact. Once an FN has transitioned from air to tissue in a stable manner, it tends to remain on course. The 500gr FN can be taken to 2150fps easily and, with powders on the fast end of the spectrum, 2200fps is not uncommon. The 450gr FN will go 80fps faster.

hopdoc,

The vast majority of our FN bullets are used in bolt rifles but we have not encountered regulation problems with doubles. The combination of faster barrel time and less pressure seems to work out with the rotation of the rifle vertically and horizontally.