You can play an important role in your father’s health and safety by becoming involved now. It sounds like on some level he knows that he is having difficulty driving, and is afraid to go out and to do his grocery shopping.
Your father needs to be evaluated by his Doctor to determine if he has dementia or other medical conditions that could cause confusion and difficulty driving, and a Doctor is required by law to report to the DMV medical conditions or disorders that he/she feels could interfere with your father’s driving. Also, you as a family member can write a letter and report to the DMV that you are concerned about his driving ability. This will trigger the DMV to order a reexamination process that will assess your father’s driving ability, and determine whether he can continue driving. More information can be found by calling your local DMV office and asking about the Senior Driver program, in California this info can be found at https://www.dmv.ca.gov/portal/dmv/detail/about/senior/senior_top
Offer your father alternatives to driving and getting what he needs such as groceries, and research other ways for your father to travel such as buses, shuttles, paratransit, friends, etc. Now is the time to gather all your father’s resources (friends and family members) to help him.
As far as his guns, talk to the Doctor about this situation, it is also very concerning and at the very least the bullets should be taken out of the guns. Reach out to your Doctor (and other Health Care professionals your father sees) to help you with these difficult conversations which will help avoid a conflict between the two of you.