Army spokesman Colonel Sansern Kaewkamnerd said a committee has been set up to investigate the cause of the blast in the Third Infantry Battalion of the First Infantry Regiment on Rama V Road in Dusit district.
Private Bundit Wijitsak was hospitalised with deep wounds to his right leg. Also hospitalised was Private Kittipat Paijingread who suffered wounds to his left arm and hand, on which he lost three fingers.
Private Jakree Chairang was released after treatment for minor injuries to his body.
An initial report said the soldiers were cleaning and checking the weapon for routine maintenance when it went off.
Sansern said either the heat or human error could be blamed, but negligence was unlikely because the soldiers used the weapons frequently and were very familiar with them.
Maj-General Varah Boonyasit, commander of the First Division, dismissed an earlier report that an Army depot exploded at the time the soldiers were conducting a routine check on the weapon.