I have actualy seen a sow hold the door open on the "one way pendulum" type doors. she was letting the little ones out that ran in and tripped the door. I believe the guillotine type is the best. and best if there is some kind of mechanism that locks the door in the down position.

On my traps I took a piece of flat bar ran it the length of the gillotine door and put a spring pin that the door goes past and the door and the issue is solved!

I like the spring loaded repeating doors that allow multiple hogs to enter!!!

i tend to lean twards saloon doors over spring side door for 2 reasons

1 wide open access into trap, no root under or push through for FIRST hog in trap

then competition for bait  and presence of first hog  entices other hogs into the trap thats why i like to build a 10' hinged trap, more room for more hogs once the first one is in!   see picture below...

2 spring loaded saloon doors mantain contact with the hog on both sides as a new hog enters, after the trap doors have been tripped by the first hog, thus making it very difficult for a hog in the trap to get out

the spring side door can let out a hog if they are facing the new hog coming into the trap on the side away from the hinge

like bs_hntr said root up pendulum doors can let smaller hogs out if a bigger  one stands in the door.

guillotine doors are great if you want to catch a few  hogs  at a time like running hog dogs but once tripped thats it!

Just my 2

After you Choose Hog Maulers Rugged hog traps now, you have an an excuse.. er duty  to be in the woods outside of deer season!

Mine.

I have a door design that is gioutine, but positions the animal before firing. Very open. Hogs body only has to be in trap 24" to get caught. Testing now. Plan manufacture soon. Can be used as a pen or single trap design. Have made patent application. Should make alot easier to catch trap shy hogs. Similiar prototypes work extremely well.