Here's the deal on '03 receivers: The early M1903 receivers were case hardened. If done incorrectly, this can leave the receiver brittle(as Noname said). These were serial numbers below 800,000 for Springfield and 285,507 for Rock Island Armory. These guns are refered to as "low number" Springfields. You can get various opinions on whether you should shoot one or not. The Army withdrew all low number M1903's from service for safety.
What happened was that Springfield was using a form of case hardening where the crafstman determined it was time to quench the receiver by the shade of glow that the receiver had been heated to. There was an upgrade in the lighting, and the receivers looked different, so they were heated to the wrong temperatures. After they figured this out they installed a thermometer system. Rock Island always used a thermometer system, and nearly all the actual blow-ups of low number Springfields in Army service were Springfield manufacture.
Rifles above the "low number" mark were first double heat treated (with tempreature measuring equipment) which gives a hard outside, but a soft inside that's somewhat elastic. Rock Island then introduced Nickel Steel receivers and those are far superior to any previous. All M1903A3/A4 are Nickel steel.
So the bottom line is low number Springfields may not be safe to shoot. There's nothing that can be done to a low number to change it's characteristics. Fitting a late barrel doesn't make it any safer. There was no govt upgrade program, or any other thing the govt did with those rifles, except remove them from service for safety. A low number '03 is a low number '03.
Any M1903 with a Serial number above those are as safe as any used gun of that age. Obviously you have to check it out before shooting, yada, yada, yada.
I have personally seen a low number Springfield that had an additional number added to the S/N to fool people into beliving it was a high number '03. You can tell this because the number will look different than all the rest, and it has to be on one end or the other of the S/N. Also since the S/N is centered with the rest of the markings, it will be obvious that something isn't right and was added. I've only seen one like this, and that was a very long time ago. You will have no trouble spotting one if it's faked. It's that obvious.
Stay away from National Ordnance recievers. They are aftermarket and cast. There's really no point in owning one, other than collecting.
Ross
Edited: How very odd. I posted this a 6:06 PM and the board read this as 6:06 AM and put my post in the wrong order. Man, this could screw up alot of threads!