Way back when the Marine Corp started working at making the M16 competitive in across the course HP matches, the S69MK was the heaviest production bullet available. However, it didn't take long for good HP shooters to realize that they needed considerably heavier bullets with much better BCs to keep them competitive at 500-600yds. I've always liked the 69s, but don't shoot them out past 300yds regularly - there are better bullets available - notably the Sierra 77MK - for AR mag length loads.
When it comes to bolt rifle mag length loads, there are also better bullets than the 77MK, which was made to be deep-seated in AR mags. Hornady's 75 AMax has always been a good bullet in my bolt 223s out past 600, and though I haven't seen any of the new ELD 75s, I can't imagine that they'd be any less effective than the older AMax. The AMax has shorter bearing surface than the 77MK, as well as a longer ogive, so can easily be seated out farther than the 77MK, which helps maximize the limited case capacity of the 223. I built myself a 223AI bolt rifle with a Krieger 1-7tw sendero on a M700 several years ago, putting it into a McM A5 with APA DBM. I use AI poly 10rd 223 mags, and can seat Berger or JLK 90VLDs out so that I get excellent accuracy out past 1000yds with them. Not having a M700 with the internal box mag, I don't know what the max mag length in that action nor in Howa 223s, as I also use the AI mags in my Howa custom 223. But I'd think - even if you're limited to shorter lengths than what will fit in the AI mags - that the Hornady 75 AMax/ELD would still be a better bullet than the 69s.
