I’ve been given an Interarms Mark X rifle which looks to me to be based on a Mauser 98.

Is this how the safety is supposed to work?:

Pull the bolt back, push the bolt forward, this rifle is cocked. (I can see it’s cocked because there’s a part protruding from the back of the bolt that slams forward when the trigger is pulled.)

Flip the safety clockwise to the vertical position which basically jams itself between the protruding part (I’m sorry, I don’t know what the protruding part is called. On a handgun, I’d call it a striker.) and the back of the bolt thus preventing the protruding part from moving forward.

To take the safety off, flip the safety lever counter clockwise from 12:00 to 9:00.

The rifle will now fire when the trigger is pulled.

Correct?

I’m asking because when I take the safety off, the protruding part slams forward by itself without pulling the trigger. I’m assuming this isn’t a decocker of some sort since the only way to recock the rifle would be to operate the bolt and eject the unfired round. That makes no sense.

I’m assuming a sear is suppose to catch the protruding part and only release it when the trigger is pulled. And, for whatever reason on my rifle, operating the safety causes the sear to disengage.

That isn't good. Moving the safety should never give the same result as pulling the trigger.

Time for a visit to a gunsmith IMO.

This sounds clearly like a typical malfunction after someone tried to "adjust" the trigger letoff.

The classic three position safety on an original Mauser 98 action should always function as follows:

Safety lever all the way to the left = safety is off.

Safety lever all the way to the right = safety is on, bolt is locked.

Safety lever in the upright middle position = safety is on, bolt can be opened for safe unloading, this position is also needed for disassembly of the bolt.

With the 98 action, the safety can only be applied, when the firing pin is cocked. This is visible by that "protruding part", you've mentioned.
This part is called the "Schlagbolzenmutter" (= firing pin nut) in German. It is connected to the back end of the firing pin and captures the firing pin spring. The underside of this part has the contact area that is held by the sear, when the rifle is ready to fire.

On an all original correct working action, applying the safety lever will cam back the firing pin nut back out of contact with the sear and at the same time will block the firing pin from moving forward. Pressing the trigger now, will pull down the sear (as it will when firing the rifle), but the sear has no contact to the firing pin nut anymore (the firing pin is held back by the safety lever). Upon releasing the trigger, the sear will return to its uppermost position.
Taking off the safety (no matter if the trigger has been pulled in between or not), will return the firing pin nut ever so slightly forward to its contact point with the sear. The firing pin is now held back by the contact between the firing pin nut and the sear again.
In order to safely function together as described, the working parts (firing pin nut, safety lever and sear) must be within their intended size tolerances.

Attempts to get a "cleaner" letoff with less creep and/or lighter trigger pull "feeling" often lead to someone tampering with the working parts.
One way to do this is polishing or removing material off the firing pin nut on the contact points with the sear.
Another way to achieve a smaller contact area between the firing pin nut and the sear is to slightly(!) bend the complete firing pin upwards (a somewhat spread method by Swedish sport shooters with their Mauser 96 rifles, I've seen several Swedish Mausers with these bent firing pins, quite annoying when assembling the bent firing pin in the "wrong" alignment). Sometimes, the safety levers are polished, in order to make them turning more easily.

Overdoing any of this, which is easily done by amateur "gunsmiths" who do not know the dos and don'ts of working on a Mauser 98 action, may result in malfunctions like yours.

Applying the safety will put the rifle on safe. But it will either not cam back the firing pin far enough from the sear so that it won't be caught again upon taking the safety off, or, in the case of a bent firing pin, may put the firing pin nut "out of reach" of the sear. Taking the safety off will release the firing pin at once and may result in an unintended discharge, if the rifle was loaded.

You should let your rifle be checked by a competent gunsmith, who may find indications of modified working parts. Good on you, that you've tested it before trying to shoot it!


_____________________________________________________________________________________________

A Schlagbolzenmutter. That was going to be my second guess.

Thank you for confirming my suspicions. I‘m lucky I caught it. I was showing it to my sons, explaining how it works, and that’s when I realized it wasn’t right. I’m going to add function checking the safety to my prefiring check list.

A Mauser action is cock on open. When you raise the bolt, it pulls the striker back. Yes the thing sticking out the back is called the striker.

The 1903 is based on this action, but you get a knob to grab to cock the rifle if you want to re-cock it without lifting the bolt handle. It can also be used to de-cock it. (I would not recommend) It's best to raise the bolt, pull the trigger and gently lower the bolt.

https://youtu.be/1tGyxwYdqxc

This guy explains how it works

During the rotation from 1200 back to 0900 you will see the striker move forward just a bit..and that is normal.