Not sure why you would have put the slide on sans barrel and RSA. But pulling the trigger is how you get it off as you found out

 

I don't remember exactly but it made sense at the time. Apparently it's more common than I thought because after I posted this thread I found several youtube vids discussing it. The secret is to get the slide to normal battery position, THEN pull the trigger and it comes right off.

 

It doesn't hurt anything..

 

The first time I ever did it was when I wanted to verify exactly where my slide was impacting my frame when shooting hot 10mm loads. Took some fiddling to get it back off the first time.

 

Pulling the trigger to remove the slide in the example described above does the same tbing as pulling the trigger before removing the slide during normal disassembly.

Forward movement of the slide is halted when the firing pin lug encounters the "kick up" at the rear of the trigger bar cruciform. Pulling the trigger forces the trigger bar kick up down and out of the path of the firing pin lug, allowing the slide to be fully pulled forward from the frame if the slide lock has also been pulled downward to free the barrel lug.

 

If there's no barrel in the slide, just cycle it all the way rearward and pull the trigger. If you ever get really messed up, cycle the slide back and remove the striker assembly.

 

The first time I ever did it was when I wanted to verify exactly where my slide was impacting my frame when shooting hot 10mm loads. Took some fiddling to get it back off the first time.

That sounds familiar.
Believe it or not, an aftermarket place was selling Gen 3 slides for Gen 4 frames. After I started a conversation about how bad an idea it was, the ad disappeared from his web site.

 

I have a Gen4 33 complete slide assembly (barrel and RSA) on my Gen 3 26

 

Check if gun is empty
Pull trigger
take slide off

That is the procedure. YouTube will show you this and much more.

 

Yes Yes easy fix push slide forward to its normal keep thumb on rear po pistol down and press trigger surprise little trick armores use job security

 

I've done this several times. As William Scott stated, the trick is to have the slide forward to the same spot it would be at when the action is closed with barrel and RSA. Then pull the trigger. Proceed as usual.

 

The first time I ever did it was when I wanted to verify exactly where my slide was impacting my frame when shooting hot 10mm loads. Took some fiddling to get it back off the first time.

I'm one of the guys that pulled the slide plate down and removed the striker insteads of using the trigger.