Surplus rifles definitely are outclassed, in terms of firepower, by modern rifles designed for combat. 30, or even 20, rounds of ammo in detachable magazines is far superior to 5, 8 or 10 rounds of ammo on stripper clips. Establishing and maintaining firepower superiority on a battlefield is often a vital component of victory, especially if you are outnumbered. We must all assume that, if we are attacked, we will be outnumbered at least 2 or 3 to one. Wolves usually operate in packs.
Surplus rifles are not always chambered in ammunition that isn't commonly available. The Lee Enfield in .303 is certainly not common, and the ammunition is not cheap. However, by contrast the Mosin Nagant's ammunition, while no longer used by police and military forces, is extremely affordable and is also used by hunters, collectors, etc. in the civilian sector. The M1 Garand uses the still-popular 30'06, and is a very reliable, accurate, and semi-automatic rifle capable of making accurate shots several hundred yards away and still retain killing power.
Regarding the philosophy that you'll use your milsurp rifle to obtain a better rifle...well, let's think more on that, shall we? If you are most likely outnumbered (already covered this point), you probably will not be able to just run out and grab the rifle from the first guy you shot. If you are fortunate enough to be behind actual cover (not just concealment), the people who are attacking you likely did it at a time where you are at a disadvantage. Perhaps you were asleep when the attack began. Perhaps you were separated from the main building they constantly see you around, the building where your supplies and ammunition happen to be, and all you have is that one bandoleer of ammunition for the entire firefight.
3 guys with modern rifles, each with at least 60 rounds (not being generous here, because there's no guarantee they'll be well supplied, either), makes a minimum of 180 rounds incoming to your position. Count in tactics used by the opposition (assuming they think on the level we do here - do you really want to assume they don't?), and I seriously doubt you'll be able to do much besides desperately fend off flanking maneuvers. So many things have to go right in order for you to be able to actually get that rifle. So, you had better be Carlos friggin' Hathcock, or you had better be extremely clever about defense, because if it's just you vs. 2 or 3 similarly skilled guys with superior firepower, your ass is probably toast. Milsurp rifles are good for two scenarios, from what I've read: 1) Hit and run/guerrilla warfare, and 2) Long-range engagements (100 yards, minimum). Anything other than that, and I don't see how you'd be likely to survive against people of similar skill.
Which brings us to my essential point: if you only have a milsurp weapon as your primary, your only real hope is to truly know your weapon. Train with it. A lot. This means, preferably, get one that fires readily available and cheap (or at least somewhat affordable) ammunition. So, avoid the Lee Enfield-type rifles that are cool collectibles but are no longer practical for prepper purchase. Use the Nagants, and possibly the Garands or SKS's. That seems to be the logical decision to make.
Again, my opinions are just from things I've read. Your mileage may vary, and I'd love to read other opinions.
