Barrel life should be about 2 rounds.

[kmitch200]

A 50 BMG necked down to .17 cal. 
If the barrel is long enough it should be supersonic halfway to the moon. 

http://www.ammo-one.com/Wildcat50to17.html

[PegLeg45]

Q: "Hey, what are those six little holes equally spaced around the center hole?"

A: "That would be where the rifling from the barrel went through...about a half-second after the bullet."

 

[TAB]

its going to depend on the powder, there have been alot of 22, 20 and 17 wild cats.      the earsplitaloudenboomer  ( or something like that, google it)   based off a old weatherby mag case is by far my favorite bs wild cat.

[kmitch200]

Q: "Hey, what are those six little holes equally spaced around the center hole?"
A: "That would be where the rifling from the barrel went through...about a half-second after the bullet."

Nice! 

[mortdooley]

I read about a guy by the name of Jug Johnson who corresponded with gun writer Skeeter Skelton and made a wildcat load like that one. He had a special press made and I believe he used the powder from fire crackers, I think his bullets vaporized before they reached the target.

[Sponsor]

[TAB]

REality is you can only drive a bullet so fast before it either comes apart of litterly melts.   every thing else is a matter of twist rate/ barrel length.

[robert69]

Reality, a 40 grain .22 bullet pushed out of a 28" barrel, cartridge was 22-250, custom rifle. HE got the loaded ammo from the guy he got the rifle from. My friend owned it (he had just got it in a trade about 40 yrs ago).
He took a shot at a coyote at about 100 yards, and it ran away.  He had me drop by to check out the Unertal scope to see if it was sighted in, and no holes appeared in the target, but viewing from alongside the shooter, I observed a definite smoke trail.  No idea what the velocity was, or the twist of the barrel. The bullet just burned up. The velocity had to be well above 4000fps.
I heard that the rifle was stolen from him a few years later.
I loaded my 22-250 to 3550 fps from a 24" barrel (a Rem BDL heavy barrel varmint rifle) with 35.0 grains IMR 4320, and a Sierra 53 grain bench rest bullet with a flat base.  With 36.5 grains IMR 4320, the velocity was 3710 fps, and it took me 2 hours to clean the copper from the barrel. What fun!