here's a complete list of accessory barrels for the Remington 870 from their current catalog,
no listing for a 20 gauge cantilever barrel.
MODEL 870 EXTRA BARRELS
3-inch Chambers (All barrels have Vent Ribs except Deer Barrels.)
Order# Barrel Gauge Length Choke MSRP
24557 Wingmaster® Deer Barrel Fully Rifled Cantilever 12 20" CL Fully Rifled $304
24558 Wingmaster Fully Rifled Deer Barrel w/ Rifle Sights 12 20" RS Fully Rifled $256
26227 Special Purpose Deer Barrel Rem Choke Cantilever 12 20" CL Rem Choke° $256
24545 Wingmaster Deer Barrel w/ Rifle Sights 12 20" RS Rem Choke¹ $197
24495 16
26229 Special Purpose Deer Barrel Fully Rifled Cantilever 12 20" CL Fully Rifled $304
26203 Special Purpose Deer Barrel- Fully Rifled Cantilever - Fully Camoflauged with Mossy Oak Break-Up 12 20" CL Fully Rifled $335
24551 Special Purpose Super Slug Deer Fully Rifled Cantilever 12 23" CL Fully Rifled $313
24549 Special Purpose Deer Barrel w/ Rifle Sights 12 20" RS Rem Choke¹ $197
24611 Express® Deer Barrel Fully Rifled w/ Rifle Sights 12 20" RS Fully Rifled $187
24622 Express Deer Barrel w/ Rifle Sights 12 20" RS Imp. Cyl. $140
24590 Special Purpose Barrels - Vent Rib 12 30" VT Rem Choke² $219
26225 28" VT
24592 26" VT
26215 Wingmaster Light Contour Barrels, Vent Rib (Bradley Front Sight & Middle Bead) 12 30" VT Rem Choke² $252
26217 28" VT
26219 26" VT
24576 Wingmaster Standard Contour Barrels, Vent Rib (Bradley Front Sight & Middle Bead) 12 30" VT Rem Choke² $219
24580 28" VT
24582 26" VT
24589 Express Barrel 12 30" VT Rem Choke³ $179
24609 Express Deer Barrel Fully Rifled w/ Rifle Sights 20 20" RS Fully Rifled $187
24578 Wingmaster Deer Barrel w/ Rifle Sights, LW-20 20 20" RS Imp. Cyl. $153
24606 Wingmaster Barrels - Vent Rib
(Bradley Front Sight & Middle Bead) 20 28" VT Rem Choke² $219
24608 26" VT
VT=Vent Rib RS=Rifle Sights CL=Cantilever
Wingmaster Barrels- Polished Blue Finish | Special Purpose barrels - Matte Finish
Express barrels - Matte Finish
NOTE: Model 870 Extra Barrels with 3" chambers will fit Super Magnum, Magnum, and 2-3/4" receivers. These barrels will handle both 2-3/4" and 3" shells when used on Magnum receivers. Use only 2-3/4" shells when these barrels are used on 2-3/4" receivers.
0. Cantilever Rem Choke deer barrels supplied with Improved Cylinder and Extended Rifled choke tubes.
1. 12 and 20 gauge Rifle Sighted Rem Choke deer barrels supplied with Rifled and Improved Cylinder Choke tubes. 16 gauge Rifle Sighted Rem Choke deer barrels supplied with Improved Cylinder Choke tubes.
2. Supplied with Full, Modified, & Improved Cylinder choke tubes.
3. Supplied with Modified choke tube only.
Hastings barrel company
does make a barrel that would met your needs:
1359H40 M-870, Std. Wt. 20 GA., 2 3/4" & 3", 24", Scope Mount
Here's the link to their web site:
http://www.hastingsbarrels.com/20para.html