sure it's a 122mm??? Judging by the size of the gun, single rear axle vehicle, size & number of rounds carried i am inclined to believing this is a 105mm mount.
You can’t use length as a judge of bore. Older 122mm guns and 105mm even more modern types are actually pretty close in size. This is as they occupy a similar mission role of field artillery. With similar range and employment strategy. When vehicle mounted they can even mount on the same host. Example the Chinese SH2 and SH5 used the same host vehicle and are all but identical in photos. But one the 2 is a 122mm well the other is a 105mm.
here we have an American solder about to load a clearly marked 105mm shell.
And here we have a Syrian Rebel handling a 122mm shell.
generally Americans tend to have a larger build and factor in body armor but still Size wise not that different. The biggest difference is 2 part ammo for the D30 vs one piece for the American.