I posted this on Wyoming Site, but I would appreciate input from here also. 
I have had bullets in a 25/20SS , .32 Ideal, & 32/40 appear to be not stablized when penetrating a target. Sometimes they seem to show promise in group size. I have always returned to Greenhill's formula and checked twists. I have had a urge to increase velocity ( powder charge) as an answer. It often has not helped. I try to shoot ( cast ) bullets a .001-.002 over bore dia. I breech seat. I use Emmerts and SPG lubes. I have considered casting harder alloy. Most of what I shoot is 30/1 or 25/1. 
In a 25/20 SS with 7.5 grs of AA #9 and 86gr 25720 I get around 1427 FPS. I have considered using a #2 alloy. I am an old x-course Highpower shooter, and yawing is generally a product of loss of stabilization due to a loss of velocity in the appropriate twist. I would appreciate any comments. 
Kurt

Bob:

You are shooting a light weight or short bullet for a quarter bore. However the lack of twist resulting in tipping is usually due to pushing too long of a bullet. You checked your twist and bullet length using Greenhill's formula; so what did it tell you?

For my .25 RKS I am shooting a 122 grain Jones of 1.047" length with 20:1 alloy with 12.2 grains H108 or AA#9 with Alberta Schuetzen lube without gas checks at 1,810 fps. Shooting this fast with plain base bullets will result in immediate leading unless you wipe the barrel with oil prior to the first shot. After that the bullet lube from the first bullet coats the barrel and you can shoot about fifty shots without significant leading. Not sure how much you can load up your case before you might get pressure signs. An adjustable breech seater will give you a bit more space to increase your loads but I am not sure what the maximum load would be for your case.

I am shooting a RKS barrel that was specifically rifled with Greenhill's formula for this specific bullet and I have no tipping or yaw.

Kurt,

Talk to Russ Weber about your seating depth, you might try seating your bullet a little deeper, to where it engraves almost to the base, but call Russ first and see what he thinks.

Bob

If those 25 cal holes were brought up about one minute of angle I see six or seven 25's --two or three 24's the other hole is a mystery. I'll take some of that yawing any day--seriously I don't see any yawing. When the shots are clustered like this I don't care if they go through side-ways.     leadball

I have a twenty five that shoots very well but tips just a little. I can get rid of it with a .1 grain more of powder, but the accuracy doesn't appear to improve. I did loss a match once because a double was called a tip, leaving me 9 shots (in bench).

Kurt,

  From my experience with the .25/20SS I've found that the original twist rate will not handle a bullet over 75 - 80 grs. This is also the conclusion a friend has come to with his rifle. There are some rifles that are exceptions to this rule.

  My main competition .25/20SS has a 1-10" twist and I shoot a 100 gr. Darr bullet in that. I mention this to point up that my powder charge is 7.0 gr.s of AA#9 so I don't think more powder is your problem. 7.2 grs. and the group opens up by about half. Your current MV should be in the accuracy range. I wouldn't go much over 1500 fps. You can up the charge a bit but I've found that even tho you can improve the roundness of the bullet holes at 100 yds., you will still have tipping at 200.

  You said you have checked your bullet and twist to the Greenhill formula. What is the twist of your rifle? The experience around here is that the 50 constant does not work all that well with the velocities in the 1400 - 1500 fps range. We use a constant of 25 or 30.

  I use a 1-25 Tin/Lead alloy, so don't think going harder will improve things. Also use Emmerts lube.

  Yawing is a product of to LONG a bullet for to slow a twist. I think your breech seating to engrave half the base band is right on. The back stop should have nothing to do with a bullet tipping.

PETE

Kurt,

 With the twist you have you should easily be able to get stability with the bullets you have,  especially with your 1-10" twist. Altho I haven't gone to that heavy a bullet I've been told the 1-10" twist will handle up to 120 grs. So you might try a little harder alloy. Say 1-20 to start. It could be that pushing the bullets at 1600 fps, as you mention, might be causing nose slumping, which would easily cause tipping.

 Just on the off chance there might be something wrong with your barrel there are two things I'd check. The first is the crown. Make sure there aren't any "dings" around the mouth. I've seen amazing changes in accuracy when this was corrected. The other that most people don't check is to make sure your barrel is smaller at the muzzle than the breech. If your barrel was installed by a reputable gunsmith he should have checked this, but you never know.

  I sure hope you're having better luck with Bertram brass  than I and many others have had. Considering the price of the cases a 20% reject rate was to expensive for me so make my own now, altho I'd recommend Rocky Mtn. Cart. as they seem to hold up well.

 I haven't personally tried Lil Gun but a friend says it works great with jacketed bullets in the Hornet. If he sees this post he can give you the details.

PETE