This weekend I installed my RFID-enabled automatic door lock (more details to come) in my new apartment.  In my last apartment, I had been using a ~$5 12 V DC car door lock actuator (Amazon, they abound on eBay as well) to turn the deadbolt lever, since more straightforward methods would probably breach the contract about not changing the lock.  (A proper electric deadbolt would be the quick and versatile way, but looking at the inside of the lock it might even be pretty simple to turn from within the hollow space in this cheap door -- making and watching a normal-looking deadbolt turn itself would be hours of entertainment!)

On this new door, it looked like the actuator wasn't going to travel far enough to lock the deadbolt.  I don't really have the parts or ingenuity to make my own, and the linear actuators available online were starting at $60, which was unacceptable.  I decided to crack open the actuator I already had and see if I could improve it.
The insides of a car door lock actuator

Not having any more sophisticated or effective methods coming to mind, I took it outside, set it on concrete and smacked it with a hammer, aiming for the seam between the two plastic parts the unit is housed in.  (I think the end where the moving part comes out is a little weaker, and the cracks traveled along the seam pretty well from there.)  Once I got it open, the mod turned out to be pretty easy, at least to get enough distance for my needs. The black thing(s) you see between the end of the gear track (?) and the inside of the casing are rubber washers that prevent the arm from moving another inch or so.  I cut them off and, as long as the unit is held closed (to keep pieces in place), nothing seems wrong and it moves a little farther.

I actually put it back together by winding waxed dental floss around it (I think the wax helps it grip.)  Glue might work too (I didn't have any on hand), but it could get in the way of the moving parts, and I wonder if it would provide enough pressure to keep everything in place.

If removing the black rubber stoppers had not worked, I was also considering cracking open the other actuator I had and "splicing" the two arms together with glue.  I imagined cutting (wider) holes in the front/back of the unit for the extra length to come out and making sure the extra-long arm was stabilized somehow (maybe some ridges/lips on top and to the side to keep the arm from pivoting up and out of the teeth of the gear or turning to the sides.)

I'm writing this in the hope that all these actuators are the same and the information might help someone.  They all look the same in online listings.  Have fun!