Police Minister Paul Papalia says 280 firearms were surrendered, sold interstate or overseas or altered to become compliant by the July 1 deadline.
The state Labor government announced in February it would outlaw very high-powered guns capable of shooting over a long range and piercing armour plating, in response to a request from WA police.
More than 50 types of firearms and 19 calibres of ammunition were banned.
The government had initially expected to buy back 248 licensed guns but a further 32 were identified following communication between police and gun owners.
"All of the firearms that we sought to remove from the community have been removed," Mr Papalia told reporters on Friday.
"It's basically a regulation change where police determined these very high-powered firearms represented a risk to police and public safety.
"They are very high-powered, they are able to fire a round with great velocity over a long distance and it'll arrive with incredible accuracy and a great amount of power."
While some payments were yet to be processed, Mr Papalia expected the buyback to cost less than the $1.5 million set aside for the scheme.
The most valuable firearm was worth about $18,000.
Mr Papalia revealed authorities had knocked back a plan to potentially send some of the weapons to Ukraine to help its military fight invading Russian forces.
"I did enquire with the federal government as to whether we might be able to send them to Ukraine to be used because many of them are military-style firearms and sniper rifles," he said.
"It doesn't look like that will be able to happen, so they'll be destroyed."
A planned rewrite of WA's gun laws, which date back almost 50 years, is expected to come before parliament by the end of the year.
The government has flagged making it tougher for people to get firearm licenses and revoking them for people convicted of serious family violence offences.
"This buyback and the ongoing rewrite of WA's firearm laws will ensure community safety comes first and make an already tough job safer for my officers, who are out on our streets day and night protecting our community," Police Commissioner Col Blanch said.
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