The M109 used the same hull and turret as the M108 105mm self-propelled howitzer. The 155mm howitzer had a double-baffle muzzle brake, and a stubby bore evacuator was placed just behind it. M109 was also equipped with a stabilization spade on each corner of the hull rear.

The M109A1 used a howitzer with a longer tube than the M126 or M126A1 on the M109, and the bore evacuator had a more slender profile than that on the earlier weapons. The new M185 howitzer allowed use of the M119 propellant charge without affecting the vehicle crew as adversely as the short-tubed howitzer had. The installation of the M185 required minor changes to the turret and a modified travel lock. Bowen-McLaughlin-York (later called BMY Combat Systems) started production of M109A1s in 1974, and these were designated M109A1B.

The M178 mount in the M109A2 incorporated a new rammer and a better recoil system. The M109A2's turret bustle was lengthened to accomodate 22 155mm rounds, and a ballistic cover was mounted to protect the panoramic telescope. Other changes included redesigned door latches, engine warning sensors, and the elimination of the fording equipment.

M109A1s and M109A1Bs modified to M109A2 standard were designated M109A3.

Starting in 1983, M109A2s and M109A3s were modified to improve reliability, and NBC warfare protection was installed. Vehicles brought up to this standard were called M109A4.

The M109A6 came equipped with a new armament suite, higher-strength suspension components, and an NBC protection system with micro-climate cooling. A new turret was produced which had a full-width bustle carrying propellant charges and was protected in places by steel armor. The turret crew was offered additional protection by spall liners. The M109A6 came with a new fire control system with automatic gun laying, ballistic computation, and inertial navigation and positioning capablities. Rapid-fire bursts were made possible with the assistance of a power-assisted semiautomatic loading mechanism, and the travel lock was also remotely operated.

The M284 howitzer was based on the M185, but had a new and stronger torque key to prevent rotation of the howitzer during firing.

The M109A6 upgrade was expensive, so a cheaper alternative was also designed; it was dubbed M109A5. These vehicles were armed with the M284 howitzer in the M182 mount, and the NBC protection system from the M109A4 was fitted.