I have just got a L.C Smith Hammergun and have some questions

The chamber pressure stamped on the barrels is 850 bar, are there any slugs and buckshot that are safe to shoot with those pressures?
Secondly the J spring on the forend is missing and the only suppliers are in the States who either dont have any in stock or wont ship out of the country, Is there any other DIY way of securing the forend, The J spring seems to be a weak idea anyway