
Here an excerpt of the penal entity blueprint. The crits the unit receives when at 0 HP:
| | | critical_group: {
| | | | critical_info: [
| | | | | info: {
| | | | | | 0x9F66C8E7: {
| | | | | | | 0x6C96A366: true;
| | | | | | | 0x9657F82A: true;
| | | | | | };
| | | | | | applicable_weapons: {
| | | | | | | ballistic: true;
| | | | | | | explosive: false;
| | | | | | | flame: false;
| | | | | | | small_arms: true;
| | | | | | };
| | | | | | critical: "soldier_killed";
| | | | | | weight: 75f;
| | | | | };
| | | | | info: {
| | | | | | 0x9F66C8E7: {
| | | | | | | 0x6C96A366: true;
| | | | | | | 0x9657F82A: true;
| | | | | | };
| | | | | | applicable_weapons: {
| | | | | | | ballistic: false;
| | | | | | | explosive: true;
| | | | | | | flame: false;
| | | | | | | small_arms: false;
| | | | | | };
| | | | | | critical: "soldier_killed_death_intensity_100";
| | | | | | weight: 5f;
| | | | | };
| | | | | info: {
| | | | | | 0x9F66C8E7: {
| | | | | | | 0x6C96A366: true;
| | | | | | | 0x9657F82A: true;
| | | | | | };
| | | | | | applicable_weapons: {
| | | | | | | ballistic: true;
| | | | | | | explosive: true;
| | | | | | | flame: false;
| | | | | | | small_arms: true;
| | | | | | };
| | | | | | critical: "make_casualty";
| | | | | | weight: 25f;
| | | | | };
| | | | | info: {
| | | | | | 0x9F66C8E7: {
| | | | | | | 0x6C96A366: true;
| | | | | | | 0x9657F82A: true;
| | | | | | };
| | | | | | applicable_weapons: {
| | | | | | | ballistic: false;
| | | | | | | explosive: false;
| | | | | | | flame: true;
| | | | | | | small_arms: false;
| | | | | | };
| | | | | | critical: "burn_death";
| | | | | | weight: 90f;
| | | | | };
| | | | | info: {
| | | | | | 0x9F66C8E7: {
| | | | | | | 0x6C96A366: true;
| | | | | | | 0x9657F82A: true;
| | | | | | };
| | | | | | applicable_weapons: {
| | | | | | | ballistic: false;
| | | | | | | explosive: false;
| | | | | | | flame: true;
| | | | | | | small_arms: false;
| | | | | | };
| | | | | | critical: "burn_death_out_of_control";
| | | | | | weight: 10f;
| | | | | };
| | | | | info: {
| | | | | | 0x9F66C8E7: {
| | | | | | | 0x6C96A366: true;
| | | | | | | 0x9657F82A: true;
| | | | | | };
| | | | | | applicable_weapons: {
| | | | | | | ballistic: true;
| | | | | | | explosive: true;
| | | | | | | flame: true;
| | | | | | | small_arms: true;
| | | | | | };
| | | | | | critical: "soldier_flamethrower_explode";
| | | | | | weight: 25f;
| | | | | };
| | | | ];
| | | | max_health_percentage: 0f;
| | | };
Look at the last one, right at the bottom.
If you want you can even work out the chances. Basically take a look at all parts where it says "true" for a class (ballistic, explosive, flame, small arms), sum their total weights up and then calculate the relative weight for the critical

Small arms is 25 / 125 I think, so 20% chance.