You can use 50 and 55-grain bullets in your 1:12" barrels just fine - although you could avoid boattailed 55-grain bullets. Favorite loads for my now-gone 14" .223 were:
40-grain BTip - 22.0 grains of AA1680
50-grain V-Max - 27.5 grains of W748
55-grain BTip - 24.0 grains of AA2015 or 27.0 grains of W748
These were accurate loads that were under max in my T/C yet gave high velocities.
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