I reside in Oregon where most anything is legal to hunt with .22 or larger; handgun, rifle, doesn't make a difference. And I just arbitrarily went with 25 yards because there's some thick Cascade brush out here, and most shots are close.
I grew up hunting whitetails in Oklahoma and Texas. I’ve lived in western Washington and hunted blacktails in the Cascade brush that you describe for over 30 years. I promise you that there’s far more to the question than which cartridge to use.
The biggest issue is that it nearly always rains here during deer season and blood trails can wash away in minutes. You want that deer down RIGHT NOW.
So how do you make that happen?
Conventional wisdom says that the 45 ACP is adequate for broadside shots on small deer at short range. Cascade blacktails run small but broadside shots are rare. You need more penetration on those raking shots and the 45 ACP may not provide that. To do that with a factory 45 ACP load, I‘d want a premium 230-grain +P JHP or Buffalo Bore’s +P WFN load. In a handload, I’d push a 230-grain cast flatpoint as fast as I could make it go, probably about 950 fps. You can reach 45 Super levels in some autos and in most revolvers. That’s not a bad idea, but it’s also not a 45 ACP.
Range is not always close, and even then it comes in a couple of different flavors. I've come to expect three types of shots.
1. The deer is above you looking down from a rocky outcropping. The distance and geometry are about the same as standing on a suburban sidewalk looking up at a streetlight across the street. These deer are nearly always lying down, so you have to hit the neck or throat and break the spine because you can't see anything else. A high-velocity rifle bullet will nearly sever a blacktail’s head at this range. That’s a decisive hit, but handgun bullet must hit bone to stop the animal, and that’s maybe 2” wide. Your other choice is the brisket. A center hit here is essentially the same as a neck shot since the animal is lying down and you’re looking up at it, but the spine is wider farther back and a low hit will take out the heart before it ranges up into the spine. Hitting a bit to one side can wreck a shoulder, which damages more meat but will anchor the deer instantly. If the deer is standing because it’s about to run or is sneaking away, then shoot for the shoulder. Almost anything will work for these shots since precision is more important than raw power. In an accurized gun the 45 ACP has enough of both, but you have to be an excellent shot to make it work.
2. The deer is 75 or 80 yards through the trees at a target on about the same level as you, usually across a heavily-wooded ravine or streambed. Shoot for the shoulder. The 45 ACP is powerful enough (barely) for this kind of shot, but trajectory makes it hard to get hits at that distance.
3. You step out of the brush and see a deer across a clearcut hundreds of yards away. For this you need a bolt action rifle chambered for a high-velocity cartridge. A 4x scope will easily reach that far and will handle the closer shots easily if you practice with it like you would for an IDPA match. The geometry of a Remington factory stock or a Rimrock custom stock is perfect for speed up close.
Speaking of speed, it's also a factor with blacktails. They’re far easier to spook than whitetails. They decide whether you're a threat very quickly and they do NOT stick around afterwards, so you need a handgun that you can shoot well in a hurry. To break it down for a non-hunter, you must be able to:
1. Spot a blacktail in the brush, which is no small feat.
2. Decide whether to shoot that particular animal.
3. Decide whether your weapon can penetrate to the vitals from the angle that the animal presents BEFORE it moves. You can skip this if you already know the answer, which is why the 41 Magnum, 44 Magnum, and hot-loaded 45 Colt are so popular. Same goes for most long guns. With a 45 ACP, you have to decide on a case-by-case basis, which takes time, experience, and no small amount of luck.
4. Present the gun, align the sights, and press the trigger.
For the first or second type of shot described above, expect 3-5 seconds max. 1-3 seconds is usually more like it. I usually have to do this from an awkward position on rough ground, so I keep the gun in my hand when I'm doing anything but eating or going to the bathroom.
You have to be a good hunter and a good shot to kill blacktails in the brush with a handgun. Lots of things will work but I prefer a 240-grain cast SWC at about 1,000 fps from a tuned 44 Magnum or 45 Colt revolver with adjustable sights and a good trigger. The 45 ACP just isn't quite enough of the things I need for the way that I hunt.
Let me know if you have questions.
Okie John