what are some good optics that can have variable magnification to be effective from 100yds out to 600yds?

good optics in my term are rugged, takes you actually trying to lose zero, sharp/clear picture, etc....

preferably under $600.

the optics will be mounted to an A3 flattop 24" barrel.

You can get a Leupold VXIII 3.5-10X40 LR/T M1 on e-bay for about that price or less NIB...

That is one fine scope.  I've got one.  Get one.  The eye relief is very long so you'll likely need an ARMS S-EX rail so you can mount it far enough forward.

i have a Leupold VXIII 14.5-14x50 LR/T mildot w/a 30mm tube and side forcus that i bought used for $485.   great eye relief and enough clicks to go to 1k yds.  i have badger ordnance rings and 20moa canted base.

what does the canted base do?

Quoted: what does the canted base do?

It gives you more elevation for long range shooting/extra clicks on your scope.

I think the real effect of a canted base is as follows:

It ensures that when the reticle is zeroed at extended range (~1,000 yards) there is still adequate WINDAGE adjustment left in the internal mechanism to accommodate the windage adjustments necessary for long distance shooting.

The number of clicks of windage available in an internally adjusting scope is a function of the elevation setting.  The farther off center (higher and/or lower) the reticle is, the LESS windage adjustment is available.  If your reticle is centered when you are your 100 yard zero and then you come up 20 to 30 MOA to shoot at 600 to 1,000 yards, you will likely run out of available windage adjustment.  

With a canted base, your 100 yard zero may be in the lower 1/3 of the elevation adjustment range, with a somewhat smaller amount of windage available, but that's okay because at 100 to 200 yards you don't need much windage adjustment (unless you plan to shoot in a hurricane).

The above discussion assumes you can even GET to a 1,000 yard zero with a scope whose reticle is reticle centered when zeroed for 100 yards.  

Also consider that you might need a 20 MOA cant just to get a 1,000 yard zero if you have a 1`" diameter scope body.  These scopes have much less elevation (and windage) available as a consequence of the smaller 1" versus 30mm) tube.

Having said all that, in effect, a canted base buys you a greater range of adjustment in WINDAGE.