Why cant we use jacketed bullets?

Pellet rifle barrels are made softer than traditional rifles. Ive stripped rifling of cheap airguns using copper bbs. its a bad idea to use something that can scratch a barrel. Atleast with the guns ive used. 

The other issue here is that the swaging taking place as a bullet enters into ANY rifling is using up energy. If you have energy to spare(powder burners) it's not really of consequence, however with airguns there really can be no "extra" energy that you would want to lose into the swaging process. Unless of course you want to muzzle load each projectile to delete the swaging process from the firing process. This is in one way or another the idea behind fx's smooth twist barrels, and thats being done in an effort to lessen the swaging process of a pure lead projectile. Now add a hard copper jacket over the lead and the energy needed to swage into the rifling jumps by quite a bit. I won't bother running through the equations that mathematically prove this (search elastic limit of materials) Someone with a bit more grasp of this will chime in and clear it up for us.
If you'd like to test the theory, simply shoot a jacketed or plated projectile through a chrony, and then muzzle load the same exact type of projectile, fire it through the chrony as before and watch the velocity jump. That's the extra energy showing itself in the form of higher velocity due to reduced friction and pressure required to start the projectile moving down the barrel.

P.s. turn back the clock 100yrs and every projectile was pure lead.......the speeds obtained by the changeover to smokeless powder drove the move to copper plating and copper jacketing because without it the lead would be a ball of deformed liquid poo by the time it got to the end of the barrel.

I believe replacing the barrel for a harder one may be a bad idea. I bet they use softer barrels for a reason and it would void any warranty on your guns and if you have a big bore 357 i wouldnt try its an expensive mistake.

You mean like a revolver round, jumping from cylinder to forcing cone? I wouldn't change the geometry of the leading edge of the rifling, I'm sure a lot of engineering has gone into those particular angles in respect to the helix angle of the rifling itself. It may very well be set up to "cut" the rifling into the projectile rather than deform it to the shape of the bore. Some guys get insane about this stuff and would shoot you for pulling a brush down their barrel 1 time....... especially serious rimfire target shooters, they're fanatical about that portion of the barrel in which the rifling engages the projectile........which is kind of ironic because again your talking about low power, small bore rifles shooting soft projectiles at RELATIVELY low velocity. Interesting.

As to seating the projectile against or started into the rifling, yes.......that's the way professional shooters/reloaders using "collet style" dies seat their projectiles, even going so far as to use only "fire formed" brass for competition ammo. In other words, if the brass case hasn't already been fire formed to fit THAT particular rifle chamber it will not be used in competition. On top of that, they put an index mark on the round so that it is loaded oriented to the chamber the exact same way every time. Like I said, it gets crazy. Crazy.....but they can shoot through the same hole all day long.

Never tried jacked bullets and do not thing I ever will. The copper jackets of most bullets has a BHN of around 35 and the lead we shoot is between 6-10 BHN. The avg. chamber pressure of a 45/70 round with a 405 gr. bullet is around 16500 psi. We shoot 460 gr. bullets with a 10-16 BHN out of to Western big bore Bush buck by A.O.A. at 3800 psi and it leaves the muzzle at around 750 fps. Even with a 4000 psi fill the chamber pressure when shot most likely does not exceed 2000 psi. So with that being said the problem I see is we do not have enough pressure to push a 35 BHN jacked bullet into the groves and lands and out the muzzle without getting stuck some where in the middle.

The jacket doesn't have as much to do with long-range ballistics as the bullet shape. Long, pointed bullets require a higher twist rate to stabilize. An airgun barrel has a twist rate in the neighborhood of 1 twist in 18 inches. A powder burner of similar caliber (say .223 Remington) has a twist rate of 1 in 12 or even 1 in 8. And the spin rate is a function of the twist rate and the projectile velocity, so even if you had a .223 barrel fitted to your air rifle, long bullets would still probably be understabilized. Air rifles get away with such a low twist rate because diabolo pellets are self-stabilizing. The rifling is only there to ensure that if the pellet has a slight defect, it will make a tight spiral instead of veering off in one direction.