The have the analogues of constitutions that empower the federal government. Is this really so hard for you to understand?
"This is the reason why the UK has not felt the need to codify its constitution. But the UK does have a constitution, to be found in leading statutes, conventions, judicial decisions, and treaties. Examples of constitutional statutes include the Bill of Rights 1689, Acts of Union 1707 and 1800, Act of Settlement 1701, Parliament Acts 1911 and 1949, Human Rights Act 1998, Scotland Act, Northern Ireland Act and Government of Wales Act 1998. Examples of conventions include that the monarch acts on ministerial advice; that the Prime Minister sits in the House of Commons; that the Queen appoints as Prime Minister the person most likely to command the confidence of the House of Commons. These and other conventions have themselves been codified in documents such as the Cabinet Manual".
The UK is often said to have an 'unwritten' constitution, but this is not strictly correct; it is largely written, but in different documents.
www.ucl.ac.uk
"The Charter is one part of the Canadian Constitution. The Constitution is a set of laws containing the basic rules about how our country operates. For example, it states the powers of the federal, and provincial and territorial governments in Canada.
The Constitution is the supreme law of Canada; all other laws must be consistent with the rules set out in it. If they are not, they may not be valid. Since the Charter is part of the Constitution, it is the most important law we have in Canada."
This guide explains the Canadian Charter of Rights and Freedoms and its importance in our daily lives.
www.canada.ca
The point is that the countries you compare the US to have no restrictions on their federal government regarding gun control - the US most certainly does.