Nominally it's 7.92 mm (.312"), about the same as the .303 British Enfield (as opposed to the .308 used for calibers like 30-06 and, naturally enough, .308 winchester or 7.62 NATO)
I would not really get into the cost thing, handloading 7.62x54 is really fun and can make your Mosin EXCEPTIONALLY accurate, also very much so with the SVDs (also considering the SVDs operate on light ball, heavy ball hammers the action to death, but light ball is being blasted up by the case by Mosin owners, leaving the SVD guys with little recourse but to handload).
While the diameter is same as .303 and similar, DO NOT USE .303 DATA!! There are MANY sites that offer the data, in fact, Hodgdon is even offering data for the cartridge. Keep in mind however, that the cartridge was originally designed for blackpowder, modernized later. While the action could handle some roughing up, the cases certainly cant, and those brass casings can get expensive!
Handloading offers the shooter very good accuracy. Milsurp ammo is HORRIBLY inconsistant, leaving people to believe that their Mosin "aint no tackdriver". BS! I handloaded 7.62x39 (while not the Mosin cartridge, it illustrates my point) for my SKS. Saw 1-1.5MOA results at my buddy's orchard. They were loaded lighter so the bolt carrier wouldn't slam into the reciever, causing undue vibration. Mosins with good bores or Finnish Mosins (keep in mind some of those are .308 in diameter, potentially requiring a different expander ball, but are exceptionally accurate).
Bed the action (look here: Mosin Nagant ), handload, use a decent scope on a GOOD scout mount (S&K mount, good scout scope, NC Star and Barska doesnt count ), you have an EXCEPTIONAL rifle, particularly if you use a M44, shorter barrel SHOULD help harmonics a smidge.
Some will say it costs too much for a rifle that is under $100 and ammo for less than $100 a case, but that is not the point. Mauser rifles were treated like this, but they have the best actions in the world. Let's continue the tradition of making GREAT rifles by modifying and sporterizing military rifles!
i think he just wants to play with it and see what knid of accuracy he can get i doubt he will ever scope it, he said he would buy whatever i needed if i would do it he usto reload so i told him to tell me exactly what he wants as far as powder bullets and seating depth and i would do it. to be honest im just looking at it as a opportunity to get to do some reloading and it not costing me a dime and as a newbie it wount hurt me to practice espically free pratice. i told him a really light bullet may or may not shoot well in his rifle. he is wanting a 120-135 gr bullet and i cheaked barnes and speer and nosler and berger and none have one that light so i was hopping mabye someone could point me/him in the right direction. hell i just want to do some reloading on more than my 243