Wolfepack01 wrote:I have not been able to find one that advertises that capability but does Remington make an 18.5" barrel that will shoot 3.5" shells? (preferably smooth bore and rem choke) Here's what I'm considering if it shoots 3.5":
http://www.midwayusa.com/product/516311 ... uctFinding
The barrel you linked is an Express Tactical barrel, and it only has a 3" chamber. I'm not aware of any factory 18.5" 870 HD/tactical barrels with 3.5" chambers. Maybe there's an obscure special-order part or something from an aftermarket company, but Remington doesn't offer the 870 SuperMag in a combat/defense configuration; even 3" magnums are usually considered to be overkill for social work, so there's little advantage or appeal to having the more-complex 870 SuperMag action in a combat shotgun. About the closest I know of with factory SuperMag barrels are those from certain models intended for turkey and/or predator hunting. They have Rem Chokes, and rifle sights or front beads similar to what you'll see on combat barrels, but they're more in the area of 20 - 23" in length, and often have camo finishes.
Wolfepack01 wrote:(2) I currently have a 20" barrel that is advertised as having a 3" chamber and on the barrel it has stamped 2.75" and 3", but the packaging from Remington the barrel came in says 2.75", 3.0", & 3.5" (with Super Mag receiver). I have cycled 3.5" shells through it with no problem but I'm not sure if the pressure is a problem. Does any one know if the packaging is correct and I can shoot the 3.5" shells through this barrel before I ruin my gun?
You're probably looking at common packaging that is used with a variety of different barrels. Always go by the engraving on the barrel, not by the label on the packaging.
Remember that advertised shell length - 2.5", 2.75", 3", 3.5" - refers to the
unfolded length of the shotshell after it's been fired. Before firing, a shotshell will almost always be shorter due to the crimped front end. Shotgun chambers are sized to fit the shell in its unfolded state to provide the the payload with a straight path into the barrel, so there'll be extra room in front of an unfired shell that's the same length as the chamber.
- shotshell and chamber.JPG (35.41 KiB) Viewed 5448 times
When crimped, 3.5" shotshells are usually around 3" long, so they'll fit in a 3" chamber, but if you fire the shell, the crimped portion of the hull will be forced to unfold into the forcing cone (the tapered section ahead of the chamber that transitions between the wider chamber and the narrower barrel bore), partially obstructing the path of the payload and potentially causing dangerous spikes in chamber pressure. This isn't helped by the fact that 3.5" 12ga shotshells have a higher maximum pressure limit than other 12ga shotshells; standard 870 barrels with 3" or 2.75" chambers aren't designed for those pressures even without the added effect of obstructions in the forcing cone.
Barring some life-or-death, hail-Mary scenario (say you're cornered in a Wal-Mart sporting goods department by zombies or terminators, and 3.5" ammo is all that's left on the shelves), never use shotshells that are longer than the chamber in your shotgun; each shot carries a very real risk of turning your barrel into shrapnel.
If you really want 3.5" capability, and you just can't find a factory or aftermarket 18.5" SuperMag barrel, you can always cut down a longer barrel, and have it re-threaded for chokes. Vent-rib hunting barrels are very convenient for this, because you can shorten the rib along with the barrel, and avoid having to re-mount sights.
Wolfepack01 wrote:With the 20" barrel I have a factory Remington +3 extension. The extension is about 1/2" longer than the barrel which is driving me nuts! I'm ok with giving up 1 round to make the gun more compact and satisfy my OCD, so if I were to switch to an 18.5" barrel would the +2 extension be more flush with the end of the barrel?
Just about all +2 magazine extensions are designed to line up with either an 18" or 18.5" barrel. If you have a specific extension in mind, we may be able to help you figure out whether it'll be flush with an 18" barrel, or an 18.5" one.
I'm ok with giving up 1 round to make the gun more compact and satisfy my OCD, so if I were to switch to an 18.5" barrel would the +2 extension be more flush with the end of the barrel? 
Maybe I'll try to find some pics and see what I think. Your thoughts?