Because this gun is nitro proofed I would use 2.75" 20 gauge ammo with 7/8 ounce of shot. I would avoid magnum loads at all costs to protect the wood. It has been shown time and time again that chamber pressures are not much more when going from 2.5" to 2.75". If you can obtain 2.5" shells and afford them then great. If not, 2.75" will work fine.
Qualifier-I don’t believe it has been shown time and time again in anything other than 12 gauge. I seem to recall that sub gauge guns run at higher pressure, and a bit of caution would be well advised.
A 7/8ths load, at 1100fps, would likely be fine. 7/8ths at 1300fps, likely wouldn’t be fine.
Best,
Ted
