Theoretical question: what powder in mortar booster charges?

[svt-40]

What powder is used in 120 mm (and other) mortar round boosters - donut shaped plastic bags, hanged on the tail of a mortar round to increase the reach?

 

[Leonidas]

Bp?

 

[svt-40]

Doubt that - no smoke. It's smokeless, but is it supposed to be fast or slow?
It appears to be extruded cylindrical powder, and knowing the Soviets as, well, the kingdom of scarcity, I suspect it could be identical to one of their rifle, carbine or MG calibre powders.

 

[Leonidas]

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