I know we're discussing powders but also consider how the physics of the powder burn affect the bullet at various points within the rifle as well. Pressure in front of the bullet is probably negligible, I don't really know but haven't seen any evidence of it causing a problem. However, I have seen beaucoup problems from too much pressure on the bullet's base at launch.
Ever wonder why copper jax foul the first six inches of the barrel so much worse than any other part? It's mostly because the bullet didn't cork the bore up as it was getting engraved and some of the jacket got washed off. It takes pressure to move the bullet and a lot more to engrave it, but the bullet's resistance to being engraved (together with its mass resisting acceleration) can be less, or more, depending on its design and the way the burning charge is pushing it. The factors of pressure rise and bullet resistance within the first half inch of the barrel are intertwined and make or break you in a variety of ways. If you "jump" the bullet you will have some blowby. If the powder is burning slowly, the pressure will be low and the blowby less, also less if you have a long column of powder and almost nil if you have a buffer to seal up all the gas behind the bullet. If the powder is burning on the faster end of the spectrum (making gas pressure faster due to less deterrent), then the bullet pops out quickly and corks the bore quickly. If the bullet has to "jump" to the ball seat, it has an inertial running start and engraves with relatively less needed pressure, also leading the expanding powder and creating more space....which puts more X and less Y into the burn curve...which does less damage to the bullet base as it transitions into the barrel.
Now consider if the bullet DOESN'T jump and is parked firmly into the ball seat with all matching fit etc.: The powder pressure must start the bullet from a standstill, against a load. Think air compressor unloaders and air conditioning system time delays for compressor re-start which keep the compressor from having to start up against heavy head pressure. When you start the bullet against a load with a fast powder, the bullet base will rivet if the force required to get it started squeezing through the throat is more than the bullet's base can withstand without permanent deformation. Also think about engraving force (with or without jump, and with or without a positive ball-seat shape match) and how that can be reduced through strategic placement of crush zones, alloy manipulation, and jacket material properties. My goal has always been to get the bullet (any kind of bullet) going up the middle of the pipe and the base to the end of the ball seat BEFORE the pressure needed to get it there exceeds the bullet's ultimate compressive strength. Hard to do when all this happens in microseconds and the peak pressureswe're working with are easily quadruple that of the alloy's strength, or in the case of most copper jacketed bullet cores, more than five times the bullet's strength. Peak pressure in a typical bottlenecked rifle load at 100% of the velocity/pressure rating of the system typically occurs about two inches up the barrel, so you might think you're easily safe not compressing the bullet during the engraving portion of the trip, but you may not be. If the bullet has excessive resistance to moving (remember, force vs. TIME here) then its strength can easily be compromised before it even gets out of the neck, and even worse, if some of the gas has somewhere to go other than up the barrel (it always does as the neck expands and gas flows to fill the tiny space around the bullet driving bands and any extra room in front of the case mouth and throat) just as the nose slams into the ball seat and goes "OOOMPH!", that short burst of gas flow will wash out the base band of the bullet. So, how your pressure rise, powder column, and engraving force curve interact with each other in the first half inch of bullet travel are critical, and you can't just look at one part and hope to see the big picture.
From the peak pressure point to the muzzle is a zone with a completely different challenge. Say you got the alloy, bullet design, seating depth, and propellant all worked up and sorted for a HV launch. Great. But right before you get where you want, the bullets start scattering all over the place. Why? Well, either you actually DON'T have your launch sorted anymore at that point and the bullet is getting damaged before it even gets fully into the bore, or something else is happening farther up the bore. Some of us have dug up the old formulas and done calculations on land stress during acceleration and compared that to alloy strength. The initial finding is that in most cases the alloy should be strong enough not to smear and wash out. However, it does. WHY? Heat and unsuitable alloy is why. Lead alloy at 150 degrees is half the strength of 70 degrees. The land/engrave interface generates enough surface heat to weaken the bullet so much that it begins to lose metal, leak gas on the trailing edge, and when it reaches the crown, uneven gas pressure slings the bullet out yawing willy-nilly. You see this at shots 3,4 and 5 or near the end of a ten-shot string as barrel heat increases. So is that "walking" tendency the barrel corkscrewing or is it just getting hot and losing the bullet? Many possible misconceptions here, particularly with cast bullets. By the way, PAPER jackets have demonstrated to me a virtual immunity to the heat fade, similar to copper but better. Powder coat is in between copper and plain, and not as far down toward plain as might be thought from its physical properties. Going back to powder for a minute, if the powder is burning hard through the whole barrel, muzzle pressure, heat, and muzzle velocity will be high. If the pressure peaks early and peters out quickly, the "heat" is off and the drive splines of the bullet can handle a lot more heat (longer, faster shot strings) before breaking down. Hot, long-burning powders give a nice gentle launch, but can fry your bacon closer to the muzzle. There are limits to everything and finding success is all about understanding how all this stuff works together as a complete system, so try to throw all of it in the blender when ruminating over the stuff you put in the case and how it will affect launch.