When it comes to heavier bullets in the 35 Remington, practical realities make themselves known.
Ideally, the bottom of the bullet would be no deeper in the case than the shoulder/neck junction in order to make as little intrusion upon powder capacity as possible. Since the bullet diameter is relatively fat, increases in seating depth results in less powder capacity. The rate of capacity reduction is greater than that of a smaller caliber bullet per a given increment of increased seating depth.
Most 336's have short throats, so the only way to get more bullet in the case without case capacity being lost is with a bore riding type bullet, wherein the full diameter section of .358" is relatively short, and the nose of the bullet which bore rides would need to be about .350" give or take slightly. A cast bullet might fit this description with a relatively blunt nose to insure greater possible weight in a given length.
The problem then becomes one of bullet balance. I have an RCBS 200 FN mould, as well as a lengthened clone 25 grains heavier intended to optimize the cartridge in terms of increasing bullet weight. Since full caliber bearing surface is identical, but the heavier one has more nose length, the shorter RCBS bullet has more accuracy potential and has repeatedly proved it in live fire. It's better balanced......more of its weight is located near the full caliber bearing surface. Since bore riding bullets (both the RCBS and its clone qualify) often don't perfectly bore ride, a little "wiggle" occurs as the bullet goes down the barrel. In a perfect world, the bullet rides exactly on top of the lands and is provided guidance with little loss of potential accuracy compared to an all body or Louverin design. In the real world, and especially in Microgroove barrels, the tolerance often is more loose than that, and the front end of the bullet is not firmly prevented from side to side movement on its journey down the barrel.
This is oftentimes why all body bullets shoot better than bore riders, especially in Microgroove barrels. To the frustrations of many, who supposedly "slugged" their bore and matched the diameter needed with their cast bullet, only to find their long bore rider still doesn't shoot well. This is most of the reason why.
The longer the unsupported nose length is in terms of being imperfectly guided by the rifling in a bore riding bullet, the worse the bullet will shoot. So a cast bullet mould of heavy weight intended for a microgroove 336 .35 Remington would have to be carefully designed indeed. I'd go to a more helpful .352" diameter for the nose of the bullet.
A two diameter jacketed bullet is unlikely. Very few have been used over the years in any caliber, one of the few being the .264 Winchester magnum factory load. Bullets with shorter noses and full caliber bearing surfaces have proven on average more accurate.
Stabilization would be a final and maybe insurmountable problem. Since nearly all .35 Remingtons, to my knowledge, have a 1-16 twist, the relatively low velocities possible with a 300 grain bullet might result in a loss of accuracy and maybe also a loss of penetration. If a bullet has precession (tilts as it flies through the air) short range penetration is often compromised even if the bullet is heavier. The bullet may tumble instead of penetrating straight, especially at close ranges where the bullet has not stabilized, or "gone to sleep." A marginally stabilized bullet flies further before it stabilizes, or it might not fully stabilize at any distance, become progressively worse and eventually tumble as it goes through the air and loses velocity. Velocity loss might be greater than a shorter bullet adequately stabilized even though the longer bullet has a theoretically higher BC.
John Goins (Beagle. The same fella who popularized the term "beagling" a mould) has a few comments about the worth of an extra heavy .35 caliber bullet here. You might want to pay attention to his comments about stabilization, and note that the blunt bullet he speaks of does not quite meet your 300 grain criteria, but it comes quite close. A 300 jacketed bullet, even with a blunt nose, would be longer yet. Longer isn't helpful in a 1-16" twist, and since most bullets used are from 180 to 220 grains in weight, manufacturers like Marlin have little interest or incentive to produce a fast twist 35 Remington.
http://www.castpics.net/subsite2/ByCaliber/The 358009.pdf
The problem with hard cast bullets in the 35 Remington is velocity at the expense of penetration. The lead bullet composition similar to the "hard cast" bullets of heavy weight such as the 300+ grain bullet in the .44 magnum don't hold up well at the 2000+ fps velocities of the 35 Remington. Even BHN 33 bullets, which are considerably harder than most "hard cast" .44 magnum bullets, don't hold their shape, undeformed, at 35 Remington impact velocities at close range. The front end of the bullet shears off and the mostly unexpanded shank, a little over two to two and a half calibers long, remains to penetrate.
The penetration is a little better than the 220 Speer at close range, but only about 10 percent or so.
For those that wish to "emulate" the penetration of a hard cast 44 magnum slug fired from a rifle, it is difficult to do so with the hard cast higher velocity .35 Remington due to this bullet deformation. In actual practice, this is really no great handicap as the bullet still penetrates very well; well enough for any practical purpose. Since it penetrates less, tissue damage is greater in all instances where the internal organs are close to the point of initial bullet impact compared to the hard cast, undeformed .44 slug. So the loss of penetration is compensated for in terms of more damage to what you're shooting at.....asssuming you hit where you're supposed to, and not in the south end of a northbound animal.
I realize some of this will be argued about by the .44 fans, but in a practical sense a .35 Remington rifle is a different animal than a .44 shooting a very heavy bullet. Lower strike velocity of the .44 bullet makes the bullet performance different and the bullet may penetrate relatively unchanged in shape. "Hard cast" doesn't hold up to higher impact velocities of the .35. If the velocity is lower with a heavy bullet in 35, say your 300 plus grains in a non deforming bullet, stability may be a problem and penetration less than expected even if the bullet is going slow enough to avoid any appreciable expansion.
The 1600 fps threshold is often mentioned. Above this velocity bullet deformation even in hard cast is more likely. I've outpenetrated hard cast 225 grain bullets at 2200 fps using a 158 grain bullet at 1600 fps intended for a 38/357 pistol when both are fired from the 35 Remington. While damage was much greater with the 225, the non deforming 158 penetrated quite a bit further.
When we give up bullet deformation to obtain more penetration, we often give up some amount of damage to vital tissues on a well aimed shot in exchange for that penetration. This is not often discussed by the "penetration above all" advocates.
It should be.
In jacketed bullets, it may be that a thick jacketed bullet weighing around 220 grains with a reasonably broad meplat to ensure impact stability driven at 2200 fps would penetrate as well as is reasonable in the cartridge while also having the potential for greater soft tissue damage than a slower bullet that penetrates further. Since that would be a bullet with very low demand likely it will never be made.