The problem is that bolters, back in early 40K days, were designed to be wielded solely by two factions:
- Space Marines, who were allegedly 9ft tall (the models always had them more like 7ft-something but w/e) transhuman monster-men wearing power-armour.
For them, a 15kg gun with a fairly brutal recoil would be like you firing a 22 calibre pistol.
- Orks, who are wildly superhumanly strong and a lot bigger than normal humans.
But because bolters are cool, ever since 3E 40K they’ve increasingly started handing them out to everyone on the Imperial side (and more recently changed it so Orks don’t have bolters, they have “shooters”, which are just high-cal slugthrowers).
Also the recoil would be a crazy amount lower than you guys are thinking.
It’s a 19mm shell. So, like, barely bigger than a 12ga slug, which is 18mm. And it doesn’t need to be going fast. A 12ga slug, depending on length, is going to be going at like 1500-1800 feet per second. That’s crazy fast for a bolter - it would be fine with 20% of that - i.e. 300 feet per second - that’s the velocity a 40mm LV grenade launch throws stuff at. But let’s say it’s 50%, so 750-900 feet per second. That means we’d be looking at about 50% of the recoil of a 12ga shotgun.
Now, I could manage a 12ga’s recoil when I was 10. I don’t think I’m exceptional in this. I wasn’t particularly large. 16ga was more comfortable, but I was 10.
And we’re talking 50% of that recoil. And yeah, bolters are heavy, and they also have an automatic or at least semi-automatic action to absorb a bit more of the recoil, so I’m going to say, actually, the felt recoil would be fine.
A bolt pistol in one hand? The main issue would be the weight, not the recoil. A desert eagle weighs like 4.4lbs and even the slimmer bolt pistols look chunkier than that, so we’re probably talking a 6-8lb gun minimum. But that’s going to mean even less felt recoil.
As for “the ammo would be impractical”, again, we’re basically talking 12ga slugs - maybe they’re a little longer, but that doesn’t really impact things. So we’d be looking at pretty small magazines, yes. The Daewoo USAS-12 uses a fairly straight magazine (not a drum or strongly curved one) and it has 10 shots/magazine. I’d imagine a bolt pistol would be similar.
Actual bolters seem to have extreme wide magazines, so maybe they have 30-50 rounds in there (canonical values tend to be around there). And sometimes we see drum or box magazines which would have much larger capacities.
All in all, the lore actually states that Space Marines have heftier bolters than other people (but which fire the same shells), and I think if we assume bolters used by non-Marines are smaller and lighter (which would mean more felt recoil, but it would be manage-able, just about - esp. if the launch velocity was also lower), then it becomes just about plausible.
TLDR: Firing a bolt pistol one-handed would basically be like firing an automatic shotgun one-handed, but if all the shells had half the usual amount of powder. It probably wouldn’t be a lot of fun, but you could probably do it.