What would be the aproximate barrel life of the following three cartridges in rifles, assuming normal use.

.44 rem mag

45/70

30/06

My mate reckons there all the same, I say the 30/06 barrel is good for 3000-4000 shots and the .44 rem mag & 45/70 good for 7000 shots or so before accuarcy goes up the putt and a barrel change is needed.

It depends on your definition of accuracy and how you load/shoot the thing. In cartridges like the 30-06 it throat erosion caused by high pressure that will kill the barrel. For target purposes we are talking somewhere between 3500 and 5500 rounds. At that point the loss in accuracy will barely be perceptible at 100-300 yards. AT 500-600 yards the groups will start open up. Lots of people have gotten 10,000 rounds out of a hunting rifle.
In the 45-70 and 44 mag pressures are around 20-60% of those pressures found in a high power rifle and therefore it hard to wear out the barrel. That why handguns can last for tens of thousands of rounds with no problem. Likewise for 45-70s which in the over the counter ammo, has less pressure than most pistol cartidges. Plus in both you examples you can use lead bullets which are very foregiving on barrels.

So Kentucky I am wrong to ??

I figured that a slight increase in bore size from .30 to .429-.458 would lead to an increase in useable life of the .44 rem mag & 45/70 to 2000-3000 rounds over the 30/06, but your saying that it is more than that due to pressure levels ??. I originally though there was only two factors in barrel life bore size & powder charge.

Kentucky is right.

A 45 has over twice the bore size of a .30 cal using the formula for a radius.

If you wanted to heat steel to white hot, which is what I think erodes barrels the most, I would use high heat for a long time. Considering that the formula for temperature in a vessel (Boyles Law) is t=p/v you can see that even though firing a cartridge is a dynamic thing the smaller bore (30/06) and it's higher pressure will wear out much faster than a lower pressure large volume barrel.

I hope this helps. I have never worn out a 30/06 hunting rifle barrel so far let alone a .44. But those .22 and 6mm CF's go fast.

Thanks Don & Kentucky.