The AC130 has a 105 Howitzer; it shoots a shell. It’s the same shell fired out of the 105mm used by light infantry. The cartridge case is shorter, but the projectile is identical.
Armament configurations of current AC-130s, although the H model has been officially retired, it is still flying missions and the J model will be online in the coming years:
AC-130H Spectre
1× 40 mm (1.58 in) L/60 Bofors cannon
1× 105 mm (4.13 in) M102 howitzer
AC-130U Spooky II
1× General Dynamics 25 mm (0.984 in) GAU-12/U Equalizer 5-barreled Gatling cannon
1× 40 mm (1.58 in) L/60 Bofors cannon
1× 105 mm (4.13 in) M102 howitzer
AC-130W Stinger II & AC-130J
1x 30 mm ATK GAU-23/A autocannon
‘Gunslinger’ weapons system with launch tube for AGM-176 Griffin missiles and/or GBU-44/B Viper Strike munitions (10 round magazines)
Wing mounted, AGM-114 Hellfire missiles, GBU-39 Small Diameter Bombs (SDBs) and/or GBU-53/B SDB II
http://foxtrotalpha.jalopnik.com/the-u-s-air-forces-new-ac-130-gunships-are-really-bomb-1584518199
6/01/14
I guess you need less propellant if you are only shooting DOWNWARD.
The cartridge can be shorter because there is no need for it to gain altitude. lol