Hi Jeff.
It depends on how you define "heavy" bullets. 77 SMKs and 75 Horady Match bullets can be seated at mag length and will shoot fine. Nearly all service rifle shooters use one or the other of these bullets and run them from the mags. Lots of folks load Berger 82's and 80 SMKs a bit longer than mag length.
I too have one of the older long action .223 Savage rifles. The original barrels on most of these rifles have slower twist bullets that will not properly stabilize a longer bullet. They shoot fine with shorter bullets, but will throw keyhole groups with 77 or 75s.
I hope this helps.
Good luck.
JS
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