A case bulge is usually caused by over crimping or crimping / seating in one step ...
Overcrimping, roll crimping where there is no crimp groove will buckle a case. Seating and crimping in one step does not. Unless you have screwed something up, that is.
At times the crimp is applied while the bullet is still moving downward ... that buckles the case .
This is entirely incorrect. The bullet is not moving, it is stopped by the seating stem, and the case is moved up over it, and crimped in the final bit of movement.
The .450 Bushmaster is a straight, tapered case. Like all other straight cases, you should be using a 3 die set, a sizer, an expander and a seating/crimping die.
The sizer die resizes the case to slightly BELOW min spec. Generally this leaves the inside of the case slightly too small for the bullet. Seating a bullet without using the expander die will show a bulge in the case due to the bullet diameter. This will happen even when the bullet is seated straight.
The expander die stem returns the inside case neck dimensions to the proper diameter to accept the bullet without bulging the case. Normally there is a "step" or tapered portion at the upper end of the expander that is larger, and it flares the case mouth. It should be adjusted to give enough flare so you can set the bullet in the case mouth, straight. It is possible to over flare, and under flare.
The body of the seater die "straightens out" the flare, and usually has a crimp shoulder in it so that it can be adjusted to crimp the case as well.
In the .450 Bushmaster, it should be a taper crimp, which is usually not an easily seen "step" the way it is with a roll crimp die.
The proper way to load the ammo is to size, flare, then seat & crimp.
The seater die can be adjusted to seat and crimp in one step. Its really pretty simple, and once done (and locked in) it works well. HOWEVER, your brass needs to be of a uniform length. Roll crimps are very picky about that, taper crimps a bit less so, but not a huge amount.
Take a factory round, and with the seater die backed out an inch or so, (and the seating stem also backed out in the die body, run the loaded round up into the press and then screw the seater die body down over it, until you get a firm contact with the factory round's case. Snug that down with the lock ring, then turn down the seating stem until it contacts the bullet.
Snug the stem down, and then load a round or two of your reloads using that adjustment setting on your die. Rounds should come out proper length and crimp. Sometimes a tiny bit more adjustment is needed, sometimes, not.
Take your reloads and test fit them into your gun. IF they fit, you're good to go. If not, make SMALL changes to the seater die so the rounds do fit your chamber.
now, about this....
But my questions are.....
how much more pressure does it create to shoot a hotter load than what the book advises? How do I know my barrel can take the increase?
The answer to this depends on several factors, primarily what powder is being used, and how much of a change is made to the volume of space the powder is in.
Can I shoot safely 40 grains of lil gun out of my barrel safety?
What does your load data say? If it doesn't list 40grs, then the answer is "no".
How can I find out for sure how much pressure is created when the bullet is pressed deeper than spec?
Again, this depends on the amount of change to the area the powder is in. There are ballistic software programs that can calculate this, giving the needed inputs.
My barrell is rated for 38,500.
No, its not. 38,500 psi is the SAAMI max working pressure for the cartridge.
This is the max safe working pressure, a level that can be used day in and day out without putting any excessive stress on your rifle or its ammo.
Your rifle is proof tested to a much higher pressure level, so it is known to be safe at the standard max working pressure.
DO be aware that each rifle and ammo combination can be slightly or even more than slightly different. Some guy on U Tube shooting 40gr of powder in his rifle does not mean yours will do so. It might, but only careful testing, working up to that level will prove if it will, or not.
DO NOT EXCEED published data. It has been tested. Going beyond that, you're off the map and "there be dragons here" wake one and they could eat you!
(meaning you could suffer damage to your rifle or even yourself!!)
Good Luck, we're here to answer your questions and give what advice we can.