If you got to have something near a .416, how about a 414 Supermag or a 405 Winchester. No where near the expense of the 416 Rigby. Both would be much more fun to shoot.
The 405 Win works well with a 28" Target barrel, I get well over 2400fps with the 300gr Hornady SP, the biggest problem is bullet availability, pretty scarce in .411", I've thought about sizing some .416" jacketed bullets down, have read of others doing the same, but haven't tried it yet. In the mean time, the 300gr Barnes X is my next load, I've had them loaded up for quite a while, just have had other rifles I wanted to shoot instead!!

I also have some Beartooth .413" cast bullets to try, but I don't think they'd be the best for any longer range shooting, dunno what their BC is, but it's likely fairly low compared to the X.
Tim
Trajectory for Barnes .411 dia (405 Win) 300 gr. X at 2400 Feet per Second
At an Elevation Angle of: 0 degrees
Ballistic Coefficients of: 0.401 0.401 0.401 0.401 0.401
Velocity Boundaries (Feet per Second) of: 2400 2400 2400 2400
Wind Direction is: 0.0 o'clock and a Wind Velocity of: 0.0 Miles per hour
Wind Components are (Miles per Hour): DownRange: 0.0 Cross Range: 0.0 Vertical: 0.0
Altitude: 0 Feet with a Standard Atmospheric Model.
Temperature: 59 F
Data Printed in English Units
Range Velocity Energy Momentum Drop Bullet Path Wind Drift Time of Flight
(Yards) (Ft/Sec) (Ft/Lbs) (Lb-Sec) (inches) (inches) (inches) (Seconds)
0 2400.0 3836.3 3.20 0.0 -1.5 0.0 0.000000000
25 2347.4 3670.0 3.13 -0.19 0.43 0.0 0.031598238
50 2295.5 3509.4 3.06 -0.78 1.96 0.0 0.063907908
75 2244.2 3354.3 2.99 -1.77 3.08 0.0 0.096952416
100 2193.5 3204.5 2.92 -3.2 3.77 0.0 0.130756417
125 2143.4 3059.9 2.86 -5.08 4.01 0.0 0.165345629
150 2094.0 2920.5 2.79 -7.44 3.77 0.0 0.200746673
175 2045.3 2786.2 2.72 -10.29 3.04 0.0 0.236987032
200 1997.3 2656.8 2.66 -13.65 1.79 0.0 0.274095122
225 1949.9 2532.3 2.60 -17.57 0.0 0.0 0.312100379
250 1903.2 2412.5 2.54 -22.05 -2.37 0.0 0.351033273
275 1857.3 2297.5 2.47 -27.13 -5.33 0.0 0.390925219
300 1812.1 2187.0 2.41 -32.85 -8.93 0.0 0.431808384
325 1767.7 2081.1 2.35 -39.22 -13.18 0.0 0.473715471
350 1724.1 1979.7 2.30 -46.29 -18.13 0.0 0.516679541
375 1681.3 1882.7 2.24 -54.09 -23.82 0.0 0.560733884
400 1639.4 1790.0 2.18 -62.66 -30.27 0.0 0.605911795