You seen White Oak Precision's recommendation here:http://www.whiteoakprecision.com/info-reloading.htm
Also Sierra's recommendation for Reloder 15 is spot on in my White Oak 1/7 National Match upper. http://accurateshooter.net/Downloads/sierra223ar.pdf
I load 24.1 grains of REL 15 with RP 7 1/2 primers using both RP and LC cases. Overall length of 77 grain Sierra MK's or Nosler Custom Competition both at 2.250-2.255".
The 80 grain Sierra's you know are single loaded and overall length of the loaded round is 2.400" plus. You need to measure the distance to the lands in YOUR rifle with the 80 gr. SMK's My WOA with 80 SMK to touch the lands is 2.472 oal to just touch the lands. WOA says they do best .005-.015" off the lands. My loaded length for the 80 grain SMK in my rifle is 2.459". It's strange that the 77 gr load is 24.1 while the 80 gr load max listed is 24.3. The 77 SMK extends well into the case while the 80 grain doesn't. Some shoot the 69 grain out to 200 300 yards and some shoot 77's while the 80gr SMK has shown to be best at 600 yards. Notice in White Oak Precision's info that they mentioned Florida HighPower and New Jersey Highpower site and they both have discussions and loading info.
The CMP forum has some good info as does NationalMatchus forum but you have to sign up on it.
I like the Reloder 15 for 77 and 80 grain loads. A lot of shooters load Varget for the 69 gr. SMK. This link has a bunch of highpower forums: http://www.iowahighpower.com/hplinks/forums.htm
Good luck and have fun.