I have a single shot 12 gauge that we bought back in the 60's. 2 3/4" chamber. It shoots low-brass loads fine. If you shoot a high brass shell or a slug, the action opens upon firing. It doesn't fly open; its more like it comes unlatched. It's been like this for years, and its been sitting unused. I realized it is unsafe.
I would like to fix this gun and use it. Any help would be appreciated.
Also, It is a full choke. Assuming I could fix it, could I then shoot steel shot from it?

obviously,you have some worn parts.why not call savage and see if they can fix it?

I have a Handi Rifle in 45/70 that will pop open in free recoil with double loads of Ruger #1 max loads. This happens after the bullet has exited and the gas pressure is low.

If the gun can recoil backwards fast enough, parts of the latch are pulled open by inertia.

If you put the butt up against a wall to stop that, it may break the stock wrist.

You can increase the latch spring force.

Max,

Your description is somewhat vague but the problem is understandable... sort of. I once had an old Savage O/U that did the same thing. Made in the thirties it'd sat broke open in a closet for God knows how many years before I bought it. The return spring for the top snap lever was weak assumedly from all those years in the open position. Sounds like you have a similar problem. Any smith worth his screwdrivers should be able to fix it. Now the question is whether the cost of repair is worth it? I'm guessing a minimum of 1 hour + parts. Probably around $50 if he's benevolent, possibly more.

This reminds me of my brother over 50 years ago. He picked up an old single shot Cresent.
When he fired it it came open, the barrel was recoiling up and the forend came off. The barrel
went up about ten feet and came down on his head. No injury but he found out why the gun was
so cheap.
Good Luck!

Thanks for the input.
I believe I will dissassemble her and see if I can't replace the latch springs with stronger ones. I'll also look for worn mating surfaces.
I've had these types of actions apart before. I just need to know what I'm supposed to be looking for.

If the parts were just worn, you could pull it open.

I don't think they wear that much.