The main reason people want a gun safe that weighs more than 150kg is because they are renting. My experience has been Landlords rarely allow tenants to drill holes in the floor for gun safes. The gun safe pictured below weighs 150kg and was made in California USA. Its a Honeywell and came with a digital lock that failed. We were contracted to open it and replaced the original lock with a HIGH GRADE Lagard LG basic. Sadly I see many quality safes fitted with cheap nasty digital locks. With new heart transplant this safe should go on to give great service for at least 10 to 15 years.
Whilst there are a few larger and or heavy 2nd hand safes that would be suitable to use as gun safes. The reality is often they are just way too heavy. Its my opinion 2nd hand gun safes up to 800kg are suitable for domestic homes. We have lots of 2nd hand safes for sale. However they weigh over 2000kg and moving them becomes an expensive exercise.
It is possible to find a good second hand gun safe on Gumtree or Ebay but they are rare. Most of the safesI see dont have any warranty which is also a gamble. Safes are very expensive to open when there is lock failure. I am very keen to help if you find a safe and are having trouble identifying exactly which model it is. I can be reached by phone or email and look forward to helping you.