Hi,
I am unfamiliar with these actions and looking for info to give me some idea of what I might be in for.
I am considering buying a complete rifle for just the action to base a build upon.
How good are these actions ex factory ? Do they feed and cycle perfectly ? Do they run rough or smoothly. Is some work usually necessary to bring them to standard for a tough, used-hard hunting rifle ?
Any info would be very much appreciated.
Thankyou.

In case you are not aware Zastava 375H&H actions are a 30/06 length action that has been modified. It is factory modified and it is safe, but all the additional length is gained by milling away the front ramp. A local shooter had his opened up to 375 Weatherby and it’s still in one piece, so they seem safe enough.

The front of the magazine is also milled away and a relatively thin strap of metal is welded or brazed to the front of the mag. It really looks crude. Apparently this is the way Zastava made the Interarms 375 actions but have not seen one myself.

I bought a LH model for a project and when my smith examined it he suggested I sell it and buy a 30/06 length action instead. He said he would modify that by taking most of the additional length required from the rear. Some metal would still need to be taken from the front, but not nearly as much as the factory modification.

Also look for bolt slop in the receiver when it is open and try to find an action that has relatively tight tolerances. Some are so sloppy that you might need to bush the rear of the receiver to make an acceptable action.

I ended up abandoning the project and selling off the rifle. They are a good option for a lefty but check them over and try to find one that went through some QC.

Answering some of your other questions, I have 2 rifles built on Zastava actions and after some work the actions were slick and the feeding was perfect. The trigger is adjustable and works fine.

You can buy a lot of aftermarket parts for them, even for LH models. I swapped out the factory push button release bottom metal for one from an earlier Interarms action that have the more traditional guard bow floorplate release. This was purely for looks. Aftermarket bottom metal for 98’s can be quite expensive. The side safety is fine, but you can buy an aftermarket LH 3 position safety (Gentry) like the one on the Win M70.

The main issue is the factory quality control. If you can find a good one they make a very acceptable CRF option for a lefty.

I have an original Mark X .375 H&H action (RH) and has been 'opened up" front and back for the long magnum cartridge, not just at the front as stated above.

Zastava 375 actions are opened on both ends and have the magazine extension on the front. This is exactly what they did with the Whitworth Interarms
375.