Still trying to learn what I can about doubles in case I ever decide to buy one.

It seems the LOP is generally longer. Why is that? Why would shooting a double with open sights be different than a bolt rifle without a scope as far as needing more LOP?

Assuming the stock style is the same it will be the same. Open sights require your eye/head be lower to aquire a sight pictre. So an open sighted double or bolt will have the same LOP assuming the stock style is the same. If the double or bolt have a scope the LOP will need to be shorter to allow the eye/head to sit higher to aquire the scope.

Brett

An example:

My LOP for my open sighted double (used in warm weather with minimal clothes) is 15"
For an open sighted bolt (warm weather) 15"
For a scoped bolt for warm weather 14"
For a scoped bolt for cold weather (more clothes) 13.75"

I'm probably being anal retentive, but that's me.

Brett

When I get back you can try some of mine out and see what you think feels best for you.

I do find the LOP in a double to be longer than a bolt. You have a finite eye relief measurement on a bolt (with scope) which may be a reason to need a shorter stock, not the case with open sights however take into account what LOP is... A double will have the front trigger about 3/4 - 1" in front of the rear. A bolt rifle will typically have its single trigger placed close to where the rear trigger is on a double within the trigger guard.
The LOP may be longer to the front trigger on a double by about an inch but very close to the position of the rear trigger.
The grip on the stocks may also have something to do with the lop as typical production bolts have a pronounced pistol grip that may cause you to position your grip further forward where as most doubles have a more open grip allowing your palm some latitude back toward your shoulder.
It might be worth considering comparing the LOP to the rear trigger of a double to the bolt.
My Heym double has a 15" LOP and is perfect for me.
I agree with the above statements regarding as clothing can mandate a change in stock dimensions as well, so can eye relief on a scope.
I am pretty picky on LOP as a long eye relief scope on a bolt allows me a natural 14" LOP and a short one 13 3/4"-13 7/8". Pretty much the same as Mr. Barringer's.

EZ

Most people seem to be best served by a LOP about one inch longer on a double vs a bolt rifle.

They have a longer LOP because they have two triggers. My theory is you want to keep the hand pulling the triggers further away from your face so it doesnt come back and belt you.
With the two triggers, it is very important if you order a double that you specify what trigger you want the LOP to be calculated from.
If in doubt, always order it on the long side. Easy to cut down, not easy to make it longer.

My double needed about an inch longer LOP than my bolts.

Most folks need about an inch longer LOP on a shotgun than a rifle as well. As you lean your head forward to meet the stock, the distance from your shoulder to your face increases. So a longer LOP is needed to keep the grip out in front of your face.