It is rare that a lighter, lower BC bullet can be driven sufficiently faster than a heavier, higher BC bullet to overcome the BC deficit. Not impossible, but pretty rare. Given that, there are also the very important considerations of twist rate, chamber freebore length, and whether loading to mag length is a necessity. Your 8-twist barrel will effectively stabilize bullets up to the ~80-ish gr weight range. However, it is unlikely you can load these optimally with a COAL limited to mag length. If you aren't limited to mag length COAL, the Berger 80.5 Fullbore bullet would be an excellent choice. I have developed a solid load with this bullet in a .223 Rem bolt rifle with zero freebore by using H322, which is a slightly faster powder than you might normally choose for an 80.5 gr bullet in .223 Rem, and one that has very small kernels. I'd typically recommend something in the burn rate range of H4895 to Varget, or thereabouts. However, you need to be able to fit sufficient powder in the case, so it would help to have a chamber with slightly longer freebore to accomodate the longer and heavier ~80-ish gr bullets with these powders, which have noticeably larger kernels than H322. Nonetheless, they would be your best bet with the barrel twist rate you have, as long as the rifle freebore and necessary COAL will also work.
If the rifle is throated relatively short, and/or you need to load at mag length COAL, you would probably be better off going with a bullet at or below 77 gr. There are a few choices that seem to work pretty well, such as 77 SMKs, Berger's 70-75 gr offerings, Hornady's 75 gr BTHP, and Nosler's new 70-75 gr RDF bullets. These bullets are short enough (OAL) to be loaded to mag length with a variety of powders, yet have BCs that will provide reasonable external ballistics for your 300 yd match. All would work well with an 8-twist barrel.
The bottom line is that you need to consider any limitations in COAL (i.e. mag length, yes/no?), how the rifle is throated (i.e. from zero freebore, out to possibly as much as .062" freebore, or thereabouts), and what powders you actually have available, and then use that info collectively to determine the best possible combination(s) to try. As a general approach, I would suggest trying the heaviest, highest BC bullet you can effectively load optimally within those specific considerations. Pushing light .224" bullets fast doesn't buy you very much forgiveness at all at 300 yd against competitors using heavier, higher BC bullets, or even larger calibers, such as .308 Win. Having shot 300 yd F-TR matches for a number of years using both .223 Rem and .308 Win bolt rifles, I have a pretty good feel for what it takes to be "competitive" with a .223 Rem if there's much wind at all, and bullets lighter than 69-70 gr are really not best choice IF you can make a heavier, higher BC bullet work. If you can't, then in order to participate in the match you obviously have to load whatever you can.
