Why does minecraft lags?

i downloaded a minecraft mod called SDK GUN MOD. i downloaded and i can see the guns since i have TOO MANY ITEMS. but for some reason whenever i use the gun it lags so much. the game only lags when i put the bullets in the gun. it doesnt lag when i have a gun with no bullets though. i can drive the ATVs with no problem and throw grenades too. snipers lag but when i use the scope it doesnt lag anymore. please help me. i really like this mod and i dont want to give it up. thanks