I've been working up a 200 gr RCBS GC boolet load in 35W. As I increased from 8 to 17 grains, the point of impact rose a couple of inches (peaked at 11 grains) and then fell back to about the starting point. (50 yards, smallest groups were 11 and 17 grains.)

Hadn't seen the rise AND fall before, just thought it worth noting.

As you change the rate of spin on the bullet by changing velocity, it changes the precession when it leaves the muzzle. Most times it will change the POI left and right also or give a circular pattern.
Then you have to add in the barrel vibration changes that will put the muzzle in a different position when the bullet leaves.
Then add in the differences in recoil that can also change the muzzle position.
And don't forget the flatter trajectory from a faster bullet along with reduced barrel time.

The group size went from large to small, where the POI rose and then fell.

Why did it rise and then fall? What was happening as the charge & velocity increase to cause the poi to rise and then still increasing the charge and velocity caused it to fall?

There was no significant left-right movement of the group.

The groups got smaller while rising 8-9-10-11 gr. of Unique (200gr RCBS FNGC hard cast). They opened up a bit and stayed about the same (between group size of 10-11 gr charge) 12-13-14-15-16-17 with 11 and 17 being smallest groups.

Repeating 17.0 17.5 18.0 and 18.5 gr charges showed trend getting to smallest at 18.5 with group falling slightly between 17 and 18.5 gr.

Depending on the nature of the vibration the size of the group will be affected - depending on whether the end of the barrel is at a node or annode of the oscillation. (If it is at the quiet point (node) the group size will be smaller.)

Sometimes it is hard to figure. Small increases of powder can increase recoil and barrel rise without gaining enough velocity to counteract it. Then when the velocity goes up high enough, the bullet can exit before the barrel raises too high. Then, where was the muzzle in it's vibration mode?
You are right about the vibration. The only thing I can add is to stick with the tightest shooting load and adjust the sights.

Right now it's 1.2" average size for 18 or 18.5 gr at 50 yards. I'd like to get to 1" at 100. But that's going to take limiting the flyers.

First, be very critical of your shooting. Are you able to call the flyers? If you can, just discount them to human error.
I read a long time ago that if you have a lot of flyers, the gun does not like the bullet. I don't know how true it is. Cast boolits can give a flyer if it has an air bubble off center in it. A hard lube that flys off of one side of the boolit and not the other will throw a boolit out of whack and is a big reason for most of them. A damaged base can be bad.
I do not trust any lube that has to be heated to put on a boolit. I have the best luck with soft, sticky Felix lube. I have seen lubes that if you put some boolits in a box and shake them, half the lube falls off. You sure will not get groups with them.

What kind of powder are you using that allows you to go from 8-17 grains!!

Unique

One thing that I have found to be critical in getting good groups when shooting cast bullets is follow though. it is much more important than when shooting jacketed bullets. be careful with your follow through and your groups should improve.
Paul B.

Paul, I can't see how you can say that! Is it not as important with both? The jacketed bullet does not correct for poor shooting form!

I agree with you on follow through, as barrel time is longer. I'm using a sand bag under the barrel just in front of the action and another under the butt of the gun - very stable. Glass bedded the stock and lightened the trigger to about 3 lbs - will go a little less soon. 4-12x scope, good sight picture. Paying attention to breathing - 3 breaths, exhale part way, squeeze. I'm used to competing with M-14 standing at 200, sitting at 300 and prone at 600 yards (iron sights).

Yes, if you shoot light, slow cast loads the barrel time is longer, but are not slow, light loads with a jacketed bullet the same?
In fact, if you load two bullets of the same weight to exactly the same chamber pressure, the cast boolit has a shorter barrel time then the jacketed.
I say follow through is just as important no matter what you are shooting from 800 fps to 4000 fps.

A good study of trajectory is order. Yes, barrel time is a factor, but sight settings are more a function of trajectory than recoil. Being that a faster bullet typically produces more recoil, we would think it would strike higher, not so. Typically, a faster bullet will strike lower, not because of barrel time, but becasue it has a flatter trajectory. If the sights are regulated to a slower load, the bullet will strike lower. An extreeme example of this is a 12 grain powder charge that pushes a 335 grain bullet from my 375 JDJ at about 900 fps. That load stikes a full 8" higher than the same bullet traveling at 1600 pushed by 35 grains of 4895. I assure you the latter load produces more recoil.

High speed stuff (2500 fps +) doesn't show such dramatic changes as slow speed stuff

I've had good results with the accuracy of slow speed stuff at 50, but it does begin to fall off at 100, depending on the bullet. In 35 caliber, the RCBS 35-200 has performed better for me over a broad range of velocities and distance than pistol bullets.

Barrel time IS the factor. Trajectory to 50 yd's is nil with any boolit or load. A very heavy boolit will hit higher. A light but slow boolit will hit high. As you increase velocity with any boolit, the POI will be lower. The time it takes the boolit to exit in relation to recoil and barrel rise is the deciding factor with a handgun.

Back from the range. Ran 17.5, 18, 18.5, 19 and 19.5 gr of Unique behind the RCBS 35-200. Did it a 100 yds this time, 5 shot groups, walk to the tgt each group.

Groups verified the 50 yard groups in size and location. 17.5 - 18 - 18.5 grains were smallest groups at both ranges ( 1-1/2 to 1-7/8 at 100 yards; .85 to 1.03 at 50 yards) Both ranges had the 18.5 group as the lowest.

That to me indicates two things for location, one involving recoil and the other a function of barrel vibration (because it rises, falls and again rises while charges are incrreasing).

Good observations and discussion - thanks!