Re: short range varmit cartridge
The original .221 FB would be hard to beat for short range varminting. I shoot the 40gr B-Tip (moly) bullet @ 3,600 ft/sec in my 21" barrel. It has accounted for MANY groundhog kills out past 300yds.
The only drawback to the .221 would be noticed during windy conditions. However, you were talking about 150yds, at that range the little bullet doesn't get blown around all that much, epecially with launch speeds of 3,600 ft/sec.
If you just want something different than I guess you could use the 300 Whisper (.308-221). Fairly quiet and production dies should be available. Another benefit would be that you could load the 240gr Matchkings, thus making the little 221 a very effective 1,000 yd elk (maybe deer) rifle, should the opportunity present itself.