OK, here are the numbers of various forums. If a particular forum did not make the list that's due to membership less than 15,000:
The High Road -
http://www.thehighroad.org/
Threads: 671,552, Posts: 9,119,261, Members: 191,203, Active Members: 9,711
AR15 -
http://www.ar15.com/forums/
Threads: 4,155,769, Posts: 69,315,323, Members: 383,068, Active Members: 287,143
Glock Talk -
http://glocktalk.com/forums/
Threads: 812,294, Posts: 11,707,406, Members: 192,831
Calguns -
http://www.calguns.net/calgunforum/index.php
Threads: 800,006, Posts: 12,030,360, Members: 171,901, Active Members: 28,641
1911 Forum -
http://forums.1911forum.com/
Threads: 416,763, Posts: 4,710,942, Members: 146,410, Active Members: 11,740
Smith-Wesson -
http://smith-wessonforum.com/forum.php
Threads: 270,560, Posts: 2,791,890, Members: 136,715, Active Members: 10,813
The Firing Line -
http://thefiringline.com/forums/
Threads: 501,639, Posts: 5,656,740, Members: 134,816, Active Members: 6,419
Rimfire Central -
http://www.rimfirecentral.com/forums/
Threads: 373,058, Posts: 3,798,256, Members: 118,930
Sniper's Hide -
http://forum.snipershide.com/
Threads: 223,908, Posts: 2,830,816, Members: 97,061
XD Talk -
http://www.xdtalk.com/forums/
Threads: 238,602, Posts: 4,194,931, Members: 96,611
Gun Boards -
http://forums.gunboards.com/index.php
Threads: 327,688, Posts: 2,755,777, Members: 82,566
Defensive Carry -
http://www.defensivecarry.com/forum/forum.php
Threads: 164,838, Posts: 3,003,402, Members: 81,323
Sig Forum -
http://sigforum.com/eve/forums
Members: 77,663
M1911 -
http://forum.m1911.org/forum.php
Threads: 88,502, Posts: 920,038, Members: 69,843
PAFOA -
http://forum.pafoa.org/
Threads: 230,383, Posts: 2,585,248, Members: 64,959
M4 Carbine -
http://www.m4carbine.net/forum.php
Threads: 135,417, Posts: 1,766,112, Members: 64,253
Firearms Forum -
http://www.thefirearmsforum.com/
Threads: 115,137, Posts: 1,073,766, Members: 59,229
Firearms Talk -
http://www.firearmstalk.com/forums/
Threads: 93,083, Posts: 1,443,796, Members: 52,512
FAL -
http://www.falfiles.com/forums/
Threads: 247,708, Posts: 2,679,872, Members: 52,510
Gun and Game -
http://www.gunandgame.com/forums/
Threads: 150,283, Posts: 1,861,458, Members: 50,351
24 Hour Campfire -
http://www.24hourcampfire.com/ubbthreads/
Threads: 592,216, Posts: 843,5951, Members: 48,329
Zombie Squad -
http://zombiehunters.org/forum/index.php?sid=3da81b30f156b3ec03c41bf793566e70
Threads: 103,647, Posts: 2,164,790, Members: 48,306
Brian Enos -
http://www.brianenos.com/forums/index.php?act=idx
Posts: 2,049,102, Members: 46,005
Single Action Shooting Society -
http://sassnet.com/forums/
Posts: 730,087, Members: 44,092
HK Pro -
http://www.hkpro.com/
Threads: 77,022, Posts: 754,428, Members: 43,999
Accurate Reloading -
http://forums.accuratereloading.com/eve
Members: 45748
Taurus Armed -
http://www.taurusarmed.net/forums/forum.php
Threads: 107,022, Posts: 1,548,695, Members: 40,598
Ruger -
http://rugerforum.net/forums.php
Threads: 86,387, Posts: 1,266,930, Members: 40,930
Shooters Forum -
http://www.shootersforum.com/
Threads: 75,587, Posts: 697,765, Members: 40,627
M14 -
http://m14forum.com/forums.php
Threads: 114,576, Posts: 1,140,055, Users: 40,108
Saiga-12 -
http://forum.saiga-12.com/
Posts: 921,993, Members: 38,142
Warrior Talk -
http://www.warriortalk.com/forum.php?
Threads: 92,815, Posts: 1,312,408, Members: 35,384
Open Carry -
http://forum.opencarry.org/forums/forum.php?
Threads: 80,853, Posts: 1,388,518, Members: 33,399
Cast Boolits -
http://castboolits.gunloads.com/forum.php
Threads: 158,959, Posts: 2,235,990, Members: 32,386
FN -
http://fnforum.net/forums/forum.php
Threads: 53,606, Posts: 596,265, Members: 29,460
Beretta -
http://berettaforum.net/vb/
Threads: 98,238, Posts: 955,605, Members: 28,688, Active Members: 3,249
CZ -
http://www.czfirearms.us/
Threads: 49,107, Posts: 374,097, Members: 28,413
Sniper Central -
http://www.sniperforums.com/forum/forum.php
Threads: 35,507, Posts: 359,786, Members: 27,938
Handgun Forum -
http://www.handgunforum.net/
Threads: 33,579, Posts: 290,380, Members: 27,086
Walther -
http://www.waltherforums.com/forum/
Threads: 30,586, Posts: 311,283, Members: 26,930, Active Members: 1,864
Surplus Rifle -
http://www.surplusrifleforum.com/index.php
Threads: 124,284, Posts 974,971, Members: 25,888
The AK Forum -
http://www.theakforum.net/
Threads: 165,609, Posts: 1,516,779, Members: 24,376
Military Surplus Collectors -
http://www.milsurps.com/
Threads: 35,443, Posts: 261,081, Members: 24,221
Accurate Shooter -
http://forum.accurateshooter.com/
Threads: 59,974, Posts: 449,734, Members: 23,787
Colt -
http://www.coltforum.com/forums/forum.php
Threads: 70,259, Posts: 531,324, Members: 23,673
Project Appleseed -
http://www.appleseedinfo.org/smf/index.php
Threads: 32,909, Posts: 266,796, Members: 16,059
Uzi Talk -
http://www.uzitalk.com/forums/
Threads: 70,666, Posts: 626,968, Members: 15,772
Hipoint -
http://www.hipointfirearmsforums.com/forum/
Threads: 56,667, Posts: 744,595, Members: 15,426