i know i am asking too much of one cartrige but do you think there is a good compromise in a varmint cartrige that will reach out to 500yds with varmint type bullets but yet not burn barrels that fast that you are unable to have a play about on the range with them? i am thinking about 22-250 with 55g ballistic tips and 6br with 55-70g ballistic tips your thoughts would be very much appreciated. this would be a rifle to get my wife started and will only be used for shooting crow sized varmints!

Either of those two, among many others, would work fine for what you are contemplating. Keep in mind that burning out barrels is more due to firing a rifle while it is too hot than probably any other issue. If you allow cooling between groups, as most varmint hunts allow (except for maybe a good prairie dog town), you should be fine for quite awhile with you chosen rifle.
I've used my 22-250 and 223 for long shoots and haven't noticed any drop off in accuracy after several years of doing it.

Shooting crows at 500 yards will test the ability of any caliber. I don't know how big your crows are, but around here that would required a rifle capable of around 0.25 MOA, and a cartridge with a PBR of less than 4" from muzzle to 500 yards.

When I go after the little critters I carry 5 different rifles with me. A .22 rimfire (out to 50 yards), a .22 MRF (50 to 150 yards), a .22-250 (150 to 300 yards), a 6mm Remington (300 to 400 yards) and a .25-06 (400 yards to forever).

By doign this you have a rifle sighted for a specific range, and by selecting your targets you also give them time to cool between shots.