I'd imagine that it's a vehicle, 20mm would be a bit much for person...maybe good for a large creature. Usually 20mm rifles are used against material targets, but no doubt in history someone has used them on somebody.
Since this was a commission piece the details as to target would lay with "Goliath".
Almost all armies, before and during World War II, experimented with 20mm or slightly larger anti-armor, shoulder fired rifles. The recoil must have been astronomical. Your weapon is an excellent design, I'd like to see more of it.
Your young lady sniper is outstanding. I'd really like to see more of her.
That rifle seems realistic to me. Anti-material/anti-armor rather than sniper, but still realistic. Similar in concept to the South African Mechem NTW-20 (available as a 20x110mm anti-tank cannon or as a 14.5x114mm anti-tank rifle).
It would be a lot of fun to fire a 20mm rifle, but ammo would soooo expensive! And I'd hope the rifle gets braced well, or it would beat the hell out of your shoulder( firing my 30.06 for an hour at the range does that to me!).
The rifle is almost pure fiction, with parts from several real world weapons being referenced.
You may be confusing "Recoiless rifles" with old anti-tank rifles. Trust me there's plenty of recoil from an AT rifle. Sure the frame, spring, and gas tube suck up some recoil, but ahh...just some.
I've fired an old Boys AT rifle and that thing had a fearsome kick( even anchored well), and I've seen an mighty Lathi 20mm be fired...yikes!! Still...the Artraccoon dreams of somehow getting a Solothurn S18-1000 20mm. My buddy wants either a Soviet PTRD or a PTRS AT rifle.