you have what looks to be a .50 cal in the center and it is surrounded by machine gun trace rounds linked together !! i cant tell what machine gun but i think they are us bullets !!
The smaller rounds are us .30cal. The 'DEN' is the makers mark for Denver Ordnance Plant, Denver, CO, and the figures are the date of manufacture. The larger one is an armour piercing .50cal made in St Louis. Again, the numbers show the date of manufacture, in this case 1942. The largest round is a 20mm Oerlikon but I am afraid I can't make out the headstamp in your picture. If you can post what the markings are, or take a better picture I'll be able to tell you more. Take a look at the following thread for a crash course in headstamp markings.....
Ref the 20mm not having the round in front of me i can only make assumptions on it from the images posted,,but if the overall projectile colour of red is original to the round it indicates HE/Incendiary projectile + unfired primer + driving band not scored through firing = a potentially very unfriendly item to have.
I am not getting into a debate with you about this. To call them 'machine gun rounds' is inaccurate as they were used in other weapons such as the Garand, BAR and Springfield. Just because they were in disintegrating links lead you to this inaccurate comment. 'I think they're US'.......shows a distinct lack of knowledge and care that, with 30 seconds on Google, you could have avoided, (or a look at the guide to headstamps thread). You call them 'trace' rounds. The word is tracer. You further exemplify your inattention to detail in your latest post by calling them '.30cal brown rounds'. Brown??
It was obvious you have little knowledge of how to identify the origin and use of cartridges. Can I suggest you learn from the rest of us and refrain from posting inaccurate and incomplete information until you have more knowledge.
British Military Smallarms and Ammunition
Collector, Researcher and Pedant