going the other way??

[Rustyinfla]

  OK you can shoot the shorter shells in a 3 1/2" shotgun, but what about having a 3" gun reamed out to take the longer version?

  It seems like it would be easy enough.

[mcwoodduck]

I would not.
I Have made some hot 2 3/4" loads that were equal to a 3" load in speed and payload for pot shoots.  Shooting them in modern Side by Sides resulted in a cracked stock and OMG recoil.    the Shotgun is a Ithaca M100 (AKA SKB) that was chambered in 2 3/4" Only.
But if the gun was originally designed to take 3.5 and you somehow ended up with a 3" on it, like a slug gun then I would say yes.
Also if you want to have a stock replacement project than I would too say yes.

[Rustyinfla]

  Yes, but how easy would it be to modify the gun it's self to allow it to shoot 3 1/2" shells?

[mcwoodduck]

For an H&R single shot.... All you would need to do is move the forcing cone further down the barrel.
Ta Da your done. 
Would I do it?    NO.
the only thing the 3.5 inch shell gives you over the 3" is greater payload to shoot farther.  The bigger the load of shot the more dense your pattern will be further away from you.  It will add about 10 yards to your shots.

[Rustyinfla]

  Thanks, just curious. The 3 1/2 seems like a waterfowl gun to me. Maybe some turkey hunters would use it. My mom has killed a boatload of turkeys with nothing but a 20 ga. so I don't think it's needed there.

[mcwoodduck]

When the 3.5" Mossberg first came out, my father is an avid waterfowler and had to have one.
We were planning a goose and hopefully a swan hunt in NC.
Well I was out at the farm with him throwing clay birds for him from the hand trap so he could get used to the gun.
Before this Pump dad only had a Browning A-5 Light in 2 3/4".  And has always had problems remembering to pump my pump gun.
But Back to the hand tossed birds.
Threw a bunch for him with standard trap loads.  We then decided to toss in a couple of the 3.5"  Shells to see how the gun handles them.
I cranked back and let fly with a bird and Dad shot.  Stumbled back 2 or 3 feet and cursed a couple times.  Dad is not one to curse.
So he then asked what is the diff between 2 3/4, 3, and 3.5"  I told him payolad.  He said " not speed?"  No all the steel shot loads travel about the same speed, plus or minus a 100 FPS.  Since then I think the only time a 3" shell has been in the gun was when I borrow it.  The shorter shells have not stopped him from either hunting or shooting.
Heck since he found the Heavy shot loaded shells he has gone to his 20 Ga Red Label.  The heavy shot has allowed him to down size one or two shot sizes and still have the same penitration, also giving him denser patterns allowing him to shoot farther.  Smaller shot means more pellets = denser patterns.  the heavy shot pellets are a lot heavier than the steel and you can down size (smaller shot) from what you used to use as lead.  I have had good success with #4 heavy shot on snow geese.  If I shoot steel I use BB or larger.
Rather than modifying the gun i would look to change the ammo to give you what you want in proformance.  Yes the Heavy shot cost more but you only have a single shot and you might as well make it count.