120MM SOV mortar

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To not go for too long off topic, I'll just quickly provide some numbers:
All vet0 and Cons with 7 men upgrade, Conscripts have effectively 73,4 and Shocks 120 HP considering armor and RA. With 18 and 30 MP reinforcement cost, respectively, this makes merging Conscripts into Shocks at this point 1,9% more MP efficient than just buying a new shock model, meaning that for the equivalent HP you would pay 29,43 MP instead of 30. This assumes fighting "standard" small arms. MGs and AoE weapons behave differently and give further benefit to merging.
Another benefit is better weapons. This comes at the cost of your squad dropping models much earlier. E.g. merging 4 models into the Shocks lowers the total effective health from 720 to 534 or 25,9% less while saving 48 MP.
Guards have 82,5 effective HP and cost 27 to reinforce, making merging 33,5% more efficient than reinforcing.

In my opinion, merging into Shocks is not worth it outside of special cases, but surely not for saving MP. Merging into Guards is basically worth it anytime.
Merging 6 men Conscripts is not worth it at any stage.