I'm 76 so I suppose I can speak on behalf of the grouche o"phart generation.
But first, let me say that over fifty years ago when I was near your age and fresh back from Korea, I experienced the very same thing that you experienced often enough. But after seeing each other a few times and gradually becoming acquainted, those old geezers (mostly WWI guys) became friends and mentors. There is and always has been a generation gap and often it is very wide. And the greater the age disparity, the greater the gap in most cases.
Elvis was just breaking on the scene back then. You can't imagine how offended the older generation was. Rock and Roll was hardly a step removed from devil worship. It hurt the ears, it caused young girls to become nymphomaniacs. Preachers condemned it from their pulpits, city officials sought to ban it. It couldn't be played at school proms.
And guess what. Today I feel about rap and hip hop just the way those geezers felt about Elvis and Jerry Lee.
The generation gap isn't just about music, of course, it is about everything, a complete value structure that includes manners, mores, behaviors, tastes, dress, language, you name it. So obviously, these two old boys are sitting over there quietly, firing a single shot every few minutes, having a zen moment with their rifles, and you show up and fire more rounds in a six second burst than they have fired all afternoon. See, to them you are just turning money into noise. They don't understand that you could possibly be practicing and even having a few zen moments of your own. To them you may be violating everything they believe shooting is about.
Besides, they are already predisposed not to like you much anyway. After all, you can still effortlessly pick up your brass, you can still see your toes when standing, and you don't have to ask your wife to tie your shoes. Moreover, you still have hair, and, crushingly, pretty women may look at you with some degree of interest. Something they will ever experience again. And finally, they are also aware that the high point of your day is not likely to be a really good BM. They have a lot of reasons not to like you too much.
Unfortunately, wisdom is not always a hand-maiden to old age. More often, the attitudes of elders become less flexible. They are less willing to "see the other side" of an issue. They are less likely to change their minds about anything. They have arrived at all the answers and your facts will only irritate them. And they will be impatient with you for not seeing and understanding that which seems so clear and obvious to them. This is why we call them grouchy old men and old fuddy duddies.
None of the foregoing excuses rudeness. But bear in mind that you and your friends see the world from a very different perspective than these old men. You are looking forward and they are looking backward so though you are traveling the same path, the view is quite different.
Regrettably, many old men, as well as many young men, are simply a$$holes, and neither is worth cultivating as friends. But friendships across the generation gap can be very rewarding for all concerned. So don't hesitate to be the first to extend a hand across the gap. You know where the old man is coming from. He has no idea where you come from.