Patrick,
I think the reason your pellet jammed is this..All the vintage Benjamin air rifles and pistols make good power,and this is because they have very large holes in the gas/air valve and TRANSFER PORT to aid rapid gas flow. The transfer port can be up to 4mm wide in a .177 and 5mm in .22, so you see it is very easy for the nose of a pellet to drop into the transfer port......then when you try to close the bolt on it........well you know the rest.
When I load my Benjamin 352 or 312 rifles I point the muzzle verticaly at the ground, pull back the bolt then drop the pellet into the breech. I then have to give the barrel a tap so that the pellet drops all the way in without snagging on the edge of the gaping TP..
You learnt it here first.
Pete.