I have used foam filler with R15 very successfully in my 500NE. Federal uses R15 in their 416 Rigby factory load but they also use their exclusive Federal 216 hot mag primer to fix any ignition issues.

I have used slower powders in the 416 Rigby in years past which of course filled the case quite nicely with 400 grain bullets. At this time since I use and have more than a few pounds of R15 has anyone used R15 and used a foam or dacron filler in the 416 Rigby? If so, what kind of loads have you found to get the magical 2350-2400 fps out of 24" barrel. I was thinking of starting up a load of 80-82 grains of R15 with 1/2"filler.

I have used R15 a lot in my 416 Rigby but never with a filler. Ditto my 500NE.
Peter.

Peter what kind of load/bullet/barrel lengh/velocities are you getting out of your 416? If you want to send a PM that's fine also, thanks

OOPS! I LIED! just checked my folder and all I settled on was the following:
IMR 4064 78 grs. GS Custom 380 gr. FN 2406/2424 fps 2 bullets touching at 100 yards.
IMR 4350 95 grs. Hornady 400gr. 1" group, no velocity (I show 93 grs. as giving me 2430fps but accuracy was not as good as the 95 gr. load)
IMR 4350 95 grs. Woodleigh 410 gr. SP
H4831 105 grs. GS Custom 380gr. FN 2531 fps.
This was in my CZ 550. I shot the above loads at 200 yards and they still gave good accuracy. These were my Africa loads (which never happened!).
My apologies for raising your hopes.
Peter.