A £150 Lanber, bit tatty, but feels tight! (Bit like my first bird

)
Think its an old one and I need a bit of advice regarding load sizes I can shoot with it, the man in the shop gave me some Victory 1 1/4oz , 35gm 4's to shoot a Fox we are having trouble with (9th down in game loads on this link
http://www.victorycom.com/GameTable.html )
Are these too heavy for the gun to cope with? I don't know model name but there are marks on the barrel as follows:
an X in a box /shield with smthing on top
BP or HP on top of an oval in a shield
CH in a shield with 2 swords behind it
1200Kp/cm2 on both barrels
1660 gmos on the bottom barrel
18.4 on both barrels
A star on the top barrel
3 stars on the bottom barrel
A2 on the bottom barrel.
Any advice would be very much appreciated!!!
PS, Also, how hard does a cartrige have to be hit for it to go off? If i drop one will it go bang?!?!