The 75 grainers, and the Hornady HPBT will work just fine loaded to mag length in an AR rifle. Buffalo Bore uses the 75 grainers in thier Ultimate Sniper ammo I think. It might be the 77 grainers. Either way several reviewers liked the way they perfomed in thier AR platform rifles.
Again I will recomend trying the Varget since you already have some.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.