Have a 1 in 9" twist Marlin XS7 heavy barrel (26") bolt action .223 rifle. Only thing I've done with it is played with its factory trigger and made a wood stock for it more suitable for shooting from benchrest. Its one of my more accurate rifles and I often use in our private ranges monthly competition when its shot at one or two hundred yards. Each month we change the distance shot, so I go to a larger cartridge rifle with a heavier bigger diameter bullet when we shoot at 400 yds.
Anyway, seems about any bullet up to 70 grains with a variety of .223 powders shoots well for me. Some better than others. But, I'm prone to use 23.6 grs of RL15 with a 60 gr Hornady Vmax in our matches. Used a 23.6 gr charge of Ramshot Exterminator powder and Hor 52gr Match bullets this morning for a 1/4-3/8" four shot group. Ramshot TAC is pretty good for me in the same range of bullets I've mentioned. Nice thing about Ramshot TAC and Exterminator, they are very fine ball type powders and meter quite well. Also, with most stick powders, like the RL15, you might experience a minor problem dropping a charge into the small diameter neck of a .223......no such problems with the Ramshot powders I've mentioned and they burn quite clean.