Sounds like your Mosin and SVT need some cosmoline cleaned out of them, or you aren't lubing the bolt appropriately. Another possibility is that you're shooting shitty steelcase or surplus that's coated in lacquer, which melts if the gun's hot, and can foul you up. The fact that you're shooting corrosive ammo means nothing in the short term; those salts will rust and corrode the barrel if you don't clean the rifle after a range trip, but they don't impede functionality during a shooting session at all. I'd expect a slight drop in accuracy after fifty rounds, I'm willing to bet you shoot them off fairly quickly. A rifle's point of impact will shift when it's hot...that's to be expected.

I've never once seen any primary source from the Boer War, WW1, or WW2 where soldiers talk about having to routinely clean their weapons midway through combat, aside from taking a dunk in the mud and needing obvious cleaning afterward.

Standing orders for the BEF during WW1 was to clean rifles twice a day, after the morning and evening stand-to in the trenches. This was a quick cleaning, with the bolt removed and a few oily patches run down the bore, with a wipe-down of the exterior and chamber. British SMLE's were typically carried with an action cover and muzzle cap when not expected to be in use. Soldiers weren't permitted to take their rifles down any further than that, anything more complex meant they needed to take it to the unit armorer.

Finally, what the heck are you doing that a field-strip and clean take thirty minutes? I can remove a Mosin bolt, swab the bore and chamber, drop some oil in the relevant spots and have the gun back together in five minutes.