Shotguns are not as easy to get hold of as you think there is still a lot of requirements and it takes up to 8 weeks to get licence and selling the shotgun on is not so as simple as selling a gun privately in the US.
https://www.sportingtargets.co.uk/shoot/the-shooting-club/applying-for-a-shotgun-licence-.aspx
As for how many pass the background check incorrectly which trukinthumper correctly pointed out isn't even a licence, quite a few.
https://www.google.co.uk/amp/s/www.nbcnews.com/news/amp/rcna36391
Now I know ABC news is just another untrustworthy media outlet but there is a link there to the actual report.
And I dont believe everything the american media says as like all media companies they lie, but the morgues don't.
Ok, I will say it again. The media lies, they are adding stuff in there to exasperate the falsities from truth.
Of that 1 million, I will bet there is a 99.9% reason of corrupt data, incomplete data, or is a duplicate entry.
There are more, what they call "delays", for reasons of same names, data entry errors, or something else that does not indicate a "prohibited person".
I will provide a link to the Form 4473 that we have to fill out (with some exceptions this is not needed) at the time we purchase a firearm.
It basically wants to know your readily available Name, age, race, etc.. Our Social Security number is one given to use at a young age, and is the ONLY thing the federal Gov't has to make us unique. Tracking

. It is NOT required and some folks are tinfoil hat nervous about this, so they opt to leave it out. How many John Smith do you think exist in the USA?
Tens of thousands. You can guess why that may cause a delay in the right circumstance. Makes sense.
Now, we run MILLIONS of 4473/background checks per year, 2021 there were over 38 million alone.
That is one person, with a minimum of one gun per data point. It could be 1 or 100 guns, but ONLY one person.
To have 1 million delayed is very understandable as a margin of data point errors. More so when dealing with an incompetent govt agency. Pretty sure the UK is just as bad..LOL.
Out of that 1 mil that very well may be a known felon? I would say one in 5 million. They are very anal about putting felons on the data records, and part of that Delay up above. They will accidently put it ON more people than NOT putting it on the ones they should.
Before you bring up any of our more famous "mass shootings", these are failures AT A COMMUNITY level. So, the local dont give a shit if they are protecting a nut job, and those types are.....well.... you will have to do the math.
Use this link to see total number of 4473/background checks since 1998.
https://www.fbi.gov/file-repository/nics_firearm_checks_-_month_year.pdf/view
CHEATER:
Last 24 years, there were 427,444,071 BG/4473 ran.
In the last 24 years, I dont think we have had 12 of those mass shootings.
The ratios are something beyond my math skills , but I am sure people have a better chance of hitting the 100 million lottery than they do getting killed by a mass shooter.
I will take those odds all day long. As for the day to day shootings we have, well, that TOO is a local problem and not country wide. Just a small compact area where leadership fails to maintain peace. It is very confusing to people who do not live here and I understand your sentiment, but I am happy to give you the truth. No hard feelings at all. Truth should never make one mad.
Like I said, I HAVE to pay attention to the guns laws and news as people want to make me, and every other law abiding gun owner, a criminal. Think about that.
So we know the truth/facts, and love to share it.
Link to the 4473 form
https://www.atf.gov/firearms/docs/4473-part-1-firearms-transaction-record-over-counter-atf-form-53009/download
If you are buying at a private sale, this is not required, but you can take the gun to an FFL and have it done there.
Both you and the buyer would pay $25 or more each to the FFL.
Once this is filled out, it is sent into the the FBI national criminal database (all computer driven). Within an hour it is either good , denied. Sometimes that answer can go out 24 hours. There is also a number of days it can go unanswered before being considered "good to go". I would like to say 5 days. I can hunt it down if you would like.
I have a concealed carry permit, so my 4473 goes straight to the storage bin and bypasses the check. I have already been....well..... licensed to bypass that check.
Cost me about $15 per 5 years. It also means I can pack a pistol on me as I go about my daily life. If I have on pants, I have a gun. LOL.
Not that I am scared, but for the same reason I wear my seatbelt. Never know when it will come in handy.
Ok, next question