I think it depends on what slug gun you have, how far your going to shoot it and how accurate the gun may be... Personally, I use the Zeiss Conquest 3-9x40 on both my slug guns and love them.. Also have a Leupold 2-7 scope made for a slug gun. Most "shotgun scopes" are just parallex corrected for 50 or 75 yards so if your going to zero an accurate slug gun at 100 yards then something like the Zeiss is perfect.. Good Luck...

+ 1 3-9X40 Conquest

I have a Savage 210 and 220 and they both wear the Conquests. My biggest reason is probably the etched reticles.

SWFA has the Vortex Viper 2-7X scopes on sales. I bought one of those for my 30-30 and love it. I think it is actually made for shotguns.

http://swfa.com/Vortex-Viper-Rifle-Scopes-C1410.aspx

That's also what I have on my ML. Works pretty well and for the price the optics are fairly good. Though I paid about $70 for mine at wallyworld a few years back.

I guess I'll say it again.

All we run on our H&R USH slug guns (and some others) are 4 X 32 Simmons Pro-Diamonds. Have never had a problem.

Talking a good half dozen slug guns.

Very good light gathering in the Simmons.

I have a question however .... by "slug gun" do you mean a smooth bore, using a rifled slug or a rifled barrel using ammo specifically designed for rifles "shotgun" barrels? I'll will give my opinion on both set-ups.

Set ups requiring a rifled slug are seldom truly accurate enough to be effective past 100 yards or so. I have yet to see one that would group well at that distance, and the arc is significant for sure. It is not a matter of if it will "kill" a deer at or past 100 yards, it is a matter of hitting the kill zone every shot. So if that is your set up, no need to spend $$$ on a variable that goes past about 4X to 6X. Something like a 1.5-6 would be a good choice, as well as a 4X fixed.

If however you are using a rifled barrel and "slug" ammo specifically designed for use in these set ups, the effective range is , in general out to about 200 yards. If you can manage the arc of these heavy, slow moving bullets, you probably would be served well with something like a 2.5x8 or 3x9.

As far as make ... I seldom come right out and slam any manufacturer, and will not unless I have first hand experience .... I will not buy a Simmons, BSA or Tasco brand scope. I have had extremely poor experience with products from all three of these makers. In contrast, I have had super experience with the Ziess Conquest line and the Leupold Vari-X III line. To be honest, I should have ... because we are talking 3X to 5X times the retail price.

I'd suggest a Leupold Vari-X III in 1.5-5 X 20mm, Illuminated for the smooth bore.
For a rifled barrel set up, the Ziess Conquest variable in something like 3-9 x 40 mm should do great.