Play game please, this situation is not common of course but happens quite often (especially with Ost units). Sometimes while 4-5 of weapon crew members can hit model and kill it. DPS based on accuracy mostly because weapon crews use same mosin as conscripts.
The "quite often" you describe has 0.0032 chance to happen when there is a single model(5 hits on same model) and 0,000005 chance for 5 hits on the same model when there are 4 models without any one being specifically at the front(like, you know, when you flank with more then single model, the chance to drop a model from weapon crews that all focused on single model with 4 models available) Also, this chance doesn't even include grens 0.91 rec acc modifier. You're more likely to get killed in your bath then to have weapon crew drop a model instantly.
OR here is another idea, if you reinforce your support weapon with conscripts or pick up weapon with conscripts, your whole 6 men crew will be conscripts, with conscripts damage.
So before using your "on paper" arguments go and play game, please.
Which goes for every single team weapon in game.
We're talking stock crews here as no one kills his team weapon first to replace it with mainline inf.