My son purchased NIB Ruger Heavy Varmint 22-250 with 26" stainless 1:14 barrel. Like Bergers, but per Berger site only the flat base bullets perform well in 1:14. Would like opinions on 60+ gr. bullets that will perform out to 5-600 yds. Also need input on non-temp sensitive powders particularly the IMR Endurons for this caliber. Thanks