I'm thinking the 358BFG being based on the WSSM will have too much breech thrust, just depends on the pressure it's loaded to, no one offers any short mag chambered Encore barrels for that reason, loaded to pressures lower than 65kpsi it may work ok in a Handi, I've worked with a 300WSM on and SB2 frame which didn't work out, to much action flex with the big .555" case head even at 30-06 velocities.
If they go to a 1.8" case length, how about a short necked 35 Remington? There wouldn't be much neck, but crimped bullets may work fine, I push the 180gr SSP in my 22" 35 Rem Handi over 2450fps.
Tim
http://stevespages.com/page8d.htmTrajectory for Hornady .358 dia. 180 gr. SP-SSP Interlock at 2450 Feet per Second At an Elevation Angle of: 0 degrees
Ballistic Coefficients of: 0.248 0.248 0.248 0.248 0.248
Velocity Boundaries (Feet per Second) of: 1440 1440 1440 1440
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 Humidity: 70 Percent Pressure: 29.53 in/Hg
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 2450.0 2398.7 1.96 -0.18 0.37 23430.58 0.000000000
50 2280.2 2077.7 1.82 -0.76 1.67 0.0 0.063465324
100 2117.1 1791.0 1.69 -3.19 3.16 0.0 0.131738660
150 1960.8 1536.4 1.57 -7.57 2.71 0.0 0.205364303
200 1811.9 1311.9 1.45 -14.21 0.0 0.0 0.284951608
250 1671.1 1116.0 1.34 -23.5 -5.37 0.0 0.371167756
300 1539.6 947.3 1.23 -35.92 -13.85 0.0 0.464705669