I too have used both the .30-30 Winchester and now the .30-30 Ackley Improved in Contender Super 14" Barrels. I presonally would stay away from 150gr. Bullets, especially in the shorter 12" Barrel. My reason for that statement is due to the lessened Velocity coupled with the 150's higher Sectional Density.
I have been shooting both the 125gr. & 150gr. Nosler Ballistic Tips in my Super 14 .30-30 Ackley Improved. I have run Ballistic Charts with both bullets and the difference in the muzzle velocity of these two bullets is quite a bit, and the higher Ballistic Coefficient of the 150gr. does not compensate for the lower muzzle velocity, and as such the 125gr. Nosler Ballistic Tip is traveling faster down range to better iniate bullet expansion.
In addition, in my barrel the 125gr. Nosler Ballistic Tips are quite a bit more accurate than the 150's. I too am using IMR-4198 in my .30-30 AI loads. In standard .30-30 Winchester loads I also used the 125gr. Nosler Ballistic Tip, but with AA#2230 Powder. In the 3 or 4 Super 14 .30-30 Winchester Contender Barrels I have tested these loads in, they chronograph between 2350 & 2400 FPS, and generally print 5 shots at 100 yards into an inch or slightly less. I also used the 125gr. Nosler Ballistic Tips loaded with AA#2230 for fireforming my .30-30 cases to .30-30 AI. Velocity was about the same as in the standard .30-30 Winchester Barrels, as was the accuracy.
The Sierra and Hornady Single Shot Pistol Bullets are designed to Expand and Penetrate at velocities possible with Handguns. Just so happens that the 125gr. Nosler Ballistic Tip's performance falls into this catagory also.
I have been taking Whitetails with Speciality Handguns since 1982, and in all of those years I have never ever had to track a Whitetail due to lack of penetration, but have had to track a few when the wrong bullet was choosen and it did not expand.
Good luck
Larry