Guys... I have a 52 lb rd longbow, 28.5 draw length, and need to slightly stiffen arrow spine...
I'm shooting 400 spine Gold tip traditional shafts. Can anyone clue me in about how much say 1/2" will stiffen shaft dynamically...is it pretty noticable...  btw,I havex100 grain inserts,and 200 grain heads and do not want to lessen weight

The only way you will know for sure is to bare shaft tune, but I realize that many people are reluctant to go down that road.

Cutting 1/2" off the shaft will stiffen it considerably.  Probably as much in your case as reducing the point weight by 50 or so grains.  I normally trim in 1/4" increments, and there have been times when I have agonized over whether to take another 1/4" off, because if taking another 1/4" off makes it too stiff, I have no easy way of adding it back on, because like you, I usually don't want to change my point weigh.

You can always add wieght to the back of the arrow as well.  There are several ways to do it. If you're just testing,  I'd recommend using tape.
Cut an inch of duct tape,  and weigh it,  now you know how much it weighs per inch.  Now you can decide how many grains/ inches of tape to add to the back of the arrow to stiffen up the dynamic spine. 

Moved here from Shooting Form Forum. 

Don't guess - test!  Get a test kit and shoot through some paper.  Look up Stalker Stickbows information on tuning or the Ranch Fairy has a video on tuning carbon shafts with a traditional bow.

I have a pretty easy time of it for the most part.  Example my 55lb bow I can shoot my 340s with a standard aluminum insert and a 150 grain points.  Drop that same arrow down to my 50lb bow it still tunes out using a 175 grain points or I can drop to a same length 400 spine and shoot 150 grain points.  Gives me a lot of versatility.
If I trim a half inch it's usually about 25 grains of difference to me.  But I shoot my shafts 3 inches longer than my draw length.

To quickly get a general idea...play around with a dynamic spine calculator (Stu miler, 3Rivers, etc.)

Most carbons react more quickly to a change in length than a change in point weight. A quarter in might be a lot particularly on a 52#R/D with .400's. I would not cut over a quarter at a time and drop to 1/8" when getting close.

the ranch fairy vids are VERY informative!

If you are using ADs, cut them on the front end, as the taper is where the Magic is.

Those shafts are the most versatile I've ever seen if the ability to shoot from a wide margins of bow weight. Don't worry about the weight loss, it wont make a hill of beans.

I used them for over 20 years, however, my guy that hand selected mine retired and I refuse to work with the owner.

Depends on how close to tune you are, nothing wrong with 1/8" cuts several times if needed. You can't put shaft back on. 1/2" is a lot and will move bareshaft several inches. As long as it is not a tapered carbon just pull the nock and cut from back.

I always remember a point OL Adcock made about tuning, verify with point weight reduction first.

I am with McDave on this. In my own experience a 1/2" of carbon shaft lenght is around of 50grs point weight