Sorry for the n00b question but how is it possible for a slug to spin in a smoothbore shotgun barrel?  

Mostly it doesn’t spin.   The curved ribs are primarily for show.   The Forster slug is front heavy and flies like a badminton shuttlecock.

The ribs allow the slug to swage down in tight chokes. And what Maddox said- they don't spin.

BTW those "rifled" slugs, spinning or not, get the job done at close-med ranges.

Ok thanks everyone. By close to medium ranges I am assuming within 50 yards or is it even closer than that? I guess it would be good for emergency applications like dispatching charging bears or something like that but how come these things are so popular if they have such a limited application?

Quoted: Ok thanks everyone. By close to medium ranges I am assuming within 50 yards or is it even closer than that? I guess it would be good for emergency applications like dispatching charging bears or something like that but how come these things are so popular if they have such a limited application?

yah, 50 yds is good, depending on the gun. a good slug gun can throw them farther, but they drop pretty quick. sometihng like 6 inches at 100 yds. i do know people htat have killed sevral deer at that range. i have taken deer at close range. i shot my last deer at about 15 yds running at full speed straight across me. it was about like a skeet shot, but trust me, that 3 inch mag "rifled" slug knocked the chocolate outta that deer. i would say thier so popular for a efw reasons. here in iowa, your only options for deer are bow, muzzleloader, or shotgun. shotgun is probly the easyist of those. with a rifled barrel and sabot slugs plus a scope, you can reach out and touch a deer. but in iowa, if you cant get within 50 yds of your kill, your not tryin. also very its very cheap to drop slugs into a 12 guage to make it go from a bird/ small game duster to a bad ass close range brush gun that will knock down any animal were gonna encounter short of a african safari. also defeats many types of body armor, but lets not dwell on that

My slug gun has straight lines grooved in it from the rifled slugs, I am not sure the spin?

With that said, I too use my slug gun for deer hunting on edge of town and it works wonderful.  I have shot several deer without any difficulty.  Most of my shots have been less than 50yds, but the one shot that I took at 125yds went through both shoulders and rested under the skin on far side.  1oz Federal slugs 2 3/4inch.  Many slugs travel at 1500+ fps, with a few traveling upwards of 1900fps!  

For deer sized game 12ga slugs are nothing to sneeze at.  

Quoted: but how come these things are so popular if they have such a limited application?

You're from Arizona so I would assume more open space. I hunt in the Catskill mountains in upstate New York, in a shotgun only county (Schoharie). Heavy cover. In over 25 years of hunting there, the longest shot I ever took on a whitetail paced off to just about 50 yards.
At those distances a "rifled" slug hits like a cross town bus. So for the distances involved,it is the ideal round.

I shoot rifled 12ga 3" Remington 'sluggers' in my 870.  Gets about 6" groups at 100 yards.  That's pretty good I figured, espically if they're really not spinning.  I have to believe they are to some extent.  ??

Quoted: I shoot rifled 12ga 3" Remington 'sluggers' in my 870.  Gets about 6" groups at 100 yards.  That's pretty good I figured, espically if they're really not spinning.  I have to believe they are to some extent.  ??

                                        The purpose of rifling on the pumpkin balls is to....hopefully get it to spin.....whether it does or not is most generally not as important as the shot you take.....a smoothbore with 2/34 slugs should be very effective at 100 yrds...after that.....well personally I wouldn`t take that shot......too many yokels are flailing away at deer ...often running..at 150-200 yards.....4-5 shots at a time.....hey maybe it`s cause they come out from the city......also...the 31/2 inchers are simply overkill.....but if you`re prone to those long range shots...well it may help......you know a guy who knows what he`s doing as soon as he opens his mouth.......what I have done is go to 12gge sabots with a 24" rifled and four power scope......and with that don`t really shoot more than 100yrd average.....when my eyes were better....I used a 20gge....sabots....iron sights....and took a large doe at 94 yards.....it has to be a good open standing shot.......

Some barrels shoot great with slugs, others won't hit the side of a barn.  Many times it's the right combination of a barrel and the right slug. Couple of guys use M1-90 Benelli's and they're right on at 100 yds. using the regular Brenneke's.  One took a big doe last year at 100 yds. and the Brenneke broke both shoulders and exited the far side.  Another laid a big buck flat at 85 yds.  Several years ago one took a big doe at 175 yds. using a Federal Premium Sabot Hydroshock, with a scope and the smoothbore barrel which isn't supposed to be a good shooter with sabots.  His sure shoots them though.  

I started out with an 870 in 20 ga. years ago and 75 was about max for it and Fosters.  Went to an 1100 in a 12 with smooth deer barrel and could do about the same so far as decent accuracy.  Got 1 of the 1st Hastings barrels, b4 there were any with scope mounts on the barrel.  So, put scope on the receiver, 2 or 3 layers of duck tape on back end of barrel that fits back in reciever to take the wiggles out, and it would lay 3 regular Brekkekes touching at 100 yds, 3 BRI sabots in 2 in. circle at 100.
Sabots were predictable to 150yds., the Brennekes started to keyhole at 125 out of the rifled barrel.  Federal sabots did about the same when they came out.

The Hornady H2K slugs, 300 or 350 gr. xtp in a sabot, will shoot within a 3 in. circle at 200 yds.  Used them for last 3 years and took deer at 100, 125, 150, 175, 200 yds. 30 yds was farthest distance any of them traveled, most just walked around in a circle and fell over.

If you get the right combination, you can reach out and touch them.

I know a guy who shoots rifled slugs in a rifled barrel. He claims it is no different than shooting unjacketed bullets in a rifle or handgun. It sounds valid to me. What do you guys think?

Quoted: I know a guy who shoots rifled slugs in a rifled barrel. He claims it is no different than shooting unjacketed bullets in a rifle or handgun. It sounds valid to me. What do you guys think?

                                          I`d say he`s basically right....but...I`ve done it and what happens is......much leading in barrel...and the lead slugs drop a whole lot faster than the sabots............

At 25 yards out of an 18 inch barrel with a bead sight(smooth bore) I can get consistent 2-3 inch groups (rifled slugs) but those 3 inch mags start to bust me up after a while.