However the pressure of a 12ga shell is greater than that of a 17, .22, 45lc, or even a 22-250.
That is entirely inaccurate on every count. The SAAMI
specs put a .22-250 at 65,000 psi and the 12 ga. at 11,500 psi for 3" and 14,000 psi for 3 1/2". The .45 LC has comparable pressures to the 3 1/2" 12 ga at 14,000 psi but .45 LC Rugers have their own pressure tolerance of 25,000 psi. Even the .22 LR and .22 Mag both carry 24,000 psi tolerances.
I don't have access to .17 HMR specs, but I will eat my hat if it's below 15,000 psi. I expect it to be above 25,000 psi. The .250 Savage to .22-250 Remington bumps up pressures by about 44% (45,000 psi to 65,000 psi) by necking down the cartridge by about 12%. The .22 Magnum is necked down by about 23% (nearly twice the difference) to get to the .17 HMR, so I would expect a significant bump above the 24,000 psi of the .22 WMR.
The pdfs at the official SAAMI website are long and cumbersome. There's an excellent chart at leverguns.com
that you can use for quick comparisons of pressures.
Why then don't they just say that only rimfire or small bore caliber barrels can be fitted.
I would expect lawyers to be involved somewhere even though some of the really old cartridges have specs in shotshell ranges. There are heavily loaded .45 LC ammo out there which would discourage a .45 LC barrel on an SB1 frame, but I am not aware of corresponding heavily loaded .44-40 or 38-.40 (13,000 psi and 14,000 psi respectively) rounds readily available, but there might also be complications on such pressures on bolt faces with smaller surface areas against such a weak and malleable metal as cast iron.
And can I fit a barrel myself?
Probably, but I wouldn't recommend it.