i have a mosin 91/30 and was lookin to get a barrel brush for it and was curious to what size to get since they don't make a 7.62x54 brush. the 30 cal. looks close but i am not 100%sure.
I use 8mm brushes because it is a bit larger than the 30 cal, I found the 30 cal didn't clean very well. (My Mosin barrel is a bit rough). The first time you use it is a bit of a squeeze, but after that it's fine.
If your cleaning rod will accept the threads, you can use a 32 cal. pistol brush. It'll be about halfway in size between a 308 and a 8mm. If not, a 8mm brush would be better as daclamdvm said. A 308 brush is really just a little small to get all the crud out of the bottom of the grooves. This would be especially true if your Mosin is wartime production as some of them got as big as 315.