I understand double rifles are usually regulated at 50 yd.If the barrels are crossing slightly(1/4"),is this reason to gnash teeth.What is an acceptable value if there is one,also a rifle crossing at 50 yd. by 1/2" about what will it be doing at a hundred. Thank You.

Acceptable is relative.

If you are getting a quarter inch at 50 yards, and your kill zone is an 8" circle, than I deem that "acceptable". Heck, I'd deem it "acceptable" out past 200 yards.

Likewise, if all your shots are less than 25yards, than your crossing an eigth inch, again "acceptable"

Good possibility that adjusting your loads or bullets will affect regulation also.

Frankly, I'm working on a double with barrels set about an inch apart. Acceptable for me would be that the bullets strike an inch apart, regardless of range.

the left barrel is "pointing" low and right relative to the POA and the right barrel is "pointing" low and left realtive to the POA (thus, the barrels are starting out crossed)

under recoil, the barrels want to move "up and out" away from the centerline (POA)

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under recoil, the LB swings up and left, with the bullet exiting when the barrel is parallel to the POA

under recoil, the RB swings up and right, with the bullet exiting when the barrel is parallel to the POA

(the two shots should be theoretically parallel to infinity)

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if your load is too slow, the bullet will exit AFTER the barrels pass parallel and the two shots will be wide apart (but not crossed)

if the load is too fast, the bullet will exit BEFORE the barrels pass parallel and the two shots will be wide apart (and crossed)


Remember (!) : the paths are divergent , the distance between the crossed POIs will get greater and greater with increasing range. Crossing is never okay.

draw a box (like below) and map the shots. you need to slightly reduce your charge if they're crossing.









LB too slow_______________RB too slow




......................LB RB................. <------ just right



RB too fast______________LB too fast




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standing, off sticks


standing (freehand) ad 50,75,100,100 yds

Whether this is good enough is a personal decision. It would make me nuts. Not from the is it good enought, but is it how it should be. Just a personal problem that a little therapy might help.
There are more expert folks here than me, but I've done a lot of things wrong with a double and learned from them. WRF is giving good advice, I would add the words 'most of the time.' I've had slower bullets cross by a couple inches at fifty yards. How you are holding the rifle counts, as does the burn rate of the powder you are using, bullet weight, and ...
Make sure you are holding the rifle with your hand, shoulder and face. No bags or rest. If you use a rest, only your hand should touch it. No part of the rifle should touch anything but you. Took a lot of frustration and ammo to get this into my head.
I suggest a chronograph and a box of the loads the rifle was regulated with. Get their velocity. Get the same weight bullets and load data. You are attempting to get the same velocity with the same bullet weight. Sometimes a faster powder will cross at the veloctiy you are looking for. If so a slower powder might make it happen.
The correct load should place the bullets the same distance apart that the bores are, that is the barrels should shoot parallel, not converge. If you shot a group using several rounds from each barrel, you should have two groups that kind of overlap. Kind of oval like.
There are a bunch or real experts on the Accurate Reloading and NitroExpress forums that have helped me.
Hope this helps
Bfly

If the group is crossing that is undesirable as they will get further and further apart as range increases..The correct option it to load a grain less powder at a time until they do not cross..In your case that should only be a grain or perhaps two..then you will have a proper regulation and a gun that is suitable out ot a couple of hundred yards...

What Ray said. Recoil starts at the first movement of the bullet, and if it is slower, more movement of the rifle up and sideways will occur before the bullet leaves the barrel. You may need to add to the front sight or file down the rear, also.