Pelengas has made a lot of progress in a relatively short time and has produced a number of models, moving into larger guns which once would have been thought too big.as 800 mm would have been considered long. The Pelengas Magnum 90 Profi is their Sten in a sense, but has a 12 mm inner barrel as against 13 mm and is also stainless steel tubing. Due to inland waters often having suspended silt the alloy barrel guns can have abrasion problems and wear out, but stainless steel provides protection against this. Like the Sten it has a see-saw sear lever or an "Italian rocker" as they refer to it. The Pelengas guns all began as vaumm barrels and this gun is the same. A vacuum barrel also helps with keeping abrasive stuff out of the inner barrel. In the marine environment it rarely gets gritty so we don't need stainless steel inner barrels. The Magnum has no easy loading dual power system, so it is single power. The gun does come with a spring stainless spears as they offer a consistently smoother surface to the vacuum seal making it last longer, but the guns must be loaded when wet and that includes the muzzle. Salvimar only offers that spear type on their Vuoto models for the same reason, any light rust on the spear will ruin the vacuum cuff.
I haven't used the gun much as I really bought it to help the company out as warmonger Vladimir Putin wants bragging rights for conquering the country. Hopefully that doesn't happen and the Russian public wake up to that schemer selling them a bucket of lies and get rid of him, he is just a troublemaker.
Going back to the inner barrel size the cross section of the barrel determines the force level for a given pressure in the gun, the bigger the cross section then the more propulsive force as pressure is force per unit area, e.g. pounds per square inch. If you want more force in the gun, and make it harder to load, then you put the pressure up. My view is whatever you can load against you put in the gun, provided you can still ram the spear in without bending it. A straight push is required in a controlled manner, eat your wheaties!