All I can attest to is my experiences. The heavier bullets tend to deflect less (maybe they deform less?), they seem to keep there integrity better if they hit something, (Quote "Heavier bullets in a given caliber tend to be designed with thicker jackets for deeper penetration on larger game. Deer don't need them"). And they often seem to expand less/slower than lighter bullets. This isn't all bullets, just the ones I've tried. And also some bullets designed for quick expansion seem to come apart easier if they hit something between the rifle and the target.

It also depends on what you hit, I don't shoot if it is a impossible shot, but a little tall grass, a tall green weed or a twig is enough sometimes.

The larger mid velocity calibers most often have heavier bullets, the weight seems to help them deflect less.

I have caught many Deer over the years using the tall grass, that the mower misses, next to a barbed wire fence. They seem to like using it as cover to sneak across open areas. I've shot through a wisp of grass or tall weeds and had some bullets come apart, the Deer ended up looking like I'd used a shotgun. I had one Deer that looked like road kill. It doesn't happen often, but often enough to be a consideration.

It has happened too many times to be fiction. I've also had numerous hole in hole out with no expansion, the majority with heavier bullets. But I sure haven't tried every possible bullet available. I Mostly shoot Core Lokt now, a reasonable compromise IMO.

It is often a compromise and no one bullet is going to perform best in all situations.

Another consideration is what your rifle prefers. Mine prefers a 165- 167 gr pointed boat tail. Yours may like something else.

Don't know about the "grass/brush bucking" stuff but I do know that the 7mmRM with just your standard cup and core bullet can and often do blow up pretty badly on close range shoulder hits on whitetail. Just seen the results all too often to deny that it does happen. But such is the problem when you over gun yourself for your quarry. Unless you plan on extended, over 400 yard, ranges a magnum cartridge rifle is just overkill for whitetail. You will experience bullet failure much to often at close range. It's just the nature of the beast. Are premium bullets necessary? Not really, as long as you don't mind tons of meat damage from bullets blowing all to hell at close range.

As far as the grass and brush bucking, big and slow bullets like a 30-30 or a .45-70 have a tendency to overcome grass and twigs a bit better than your fast movers. But that is NOT the reason they are called "brush guns". The term brush gun comes from people using a rifle that is shorter, more maneuverable, and you aren't afraid to get it a bit roughed up from traveling through the heavy stuff. It has absolutely nothing to do with the caliber although brush guns are most generally thought of as 30-30, 444, .45-70, 35rem and mostly they are lever actions. Mostly a cheaper cost rifle for use out to 150 or so yards.

There you have it! Your description is exactly why they are called brush guns, short and quick handling, not because they can plow through brush and still be on the mark. I saw a demonstration once on TV where they showed that a lighter faster bullet actually did better shooting through brush than a 30-30 and a .35 Rem. In fact, I would not have shot at all in the situations shown in the demo. I do not believe in shoot and hope or a magic bullet.

I had a little 2 inch sapling that a buck was standing right behind (by right behind I mean like an inch or 2) that my crosshair covered. I shot dead center through that sapling with a 240gr corelockt bullet. (was my .444 marlin) Buck fell over one way, sapling fell over the other :D Blew through the sapling, still hit right where I was aiming (high shoulder) and took out the spine. Was right at 50 yards. Needless to say I was surprised.

If I had to guess Oldtimr, Id have to say the lighter faster bullet did better because of flatter trajectory so probably flew the path with less contact in the flight path than a rainbow big and slow slug. But if a little one does make contact, especially at the beginning of the speed curve, they have much more chance of going off course or even blowing up. But I agree that choosing a clear shot lane make much more sense that trying to "thread the needle".

I had a twig smaller than a pencil deflect a 300 gr bullet from my 375 H&H mag in Cameroon once. Cost me a huge Western Hartebeest bull. If an elephant gun can be deflected then anything can. After the shot at that bull we saw the twig fall, cut cleanly by the bullet. I knew exactly where I was holding and that bullet never touched a hair on that bull.

My 7MM RM is a Sako A7 Stainless

I use Hornady 139gr. SST's

I have a 7wsm. My son shot a small buck at about 15 yards with a 150gr fusion bullet. You have made a good choice.

Therein lies the key super_hunt! (by right behind I mean like an inch or 2)
ANY bullet, no matter the caliber or weight WILL BE DEFLECTED!
If you are familiar with angles and trajectory, you should know that at 2 inches, the deflection will be at a minimum, and still well within the kill zone.
Add a foot or more to that deflection and you have at best a "clean miss," or worse, a wounded animal to track!
No caliber or weight can defy physics! Waiting for a better shot opportunity is the ONLY choice! "It was the opportunity of a lifetime!" Is still a piz-poor excuse for taking an obviously risky shot!

Reading is fundamental Jag. Did you fail to see that I wrote "2" sapling that my crosshair covered up". I didn't see the thing! I know ballistics and how bullets travel pretty well and no if I had known that sapling was there I wouldn't have taken that shot. There isn't a bowhunter or rifle hunter out there that has spent any time in the woods, especially as long as my well over 60 years in them, that hasn't hit a little twig or heavy grass that they just flat out couldn't see. Now if it's a physics lesson you seek, big and slow bullets can and will tend to continue on their trajectory after contact with a piece of grass or a small dry twig more so than a small and fast bullet. The reason is MOMENTUM. They may get knocked off their axis and begin tumbling but they will continue on a straighter path than a light fast mover simply because of the weight. Just like if you hit a large animal in the head with a big slug verses a small fast mover. The small fast mover may deflect because of the angle of impact whereas if you hit at the same angle with a slow and heavy, MOMENTUM will allow for less deflection caused by angle. Take a hogs scull for instance. Some are under the false impression that a hog skull is thick and tough when in reality it is quite thin. The reason frontal head shots fail so often on hogs, especially from fast moving light bullets, is because a hogs scull is at a very steep angle. I've seen a lot of hogs "creased" by .223's as well as a few from my own 6.8spc. But my .458 SOCOM has yet to "crease". It does the Hulk smash :D Now, I'm not saying one should try to shoot through heavy cover with ANY caliber/cartridge but when it comes to the physics of a bullet, your heavier bullets stand more of a fighting chance than a light bullet if they do contact that unseen twig.

Chill a little Super! :s2:
The last part of my post was not directed at you personally! Looking at how I wrote it, I can see the implication though, sorry, my bad!
Also, I am not trying to judge anyone for unforeseen accidents, chet happens!
I once accidentally killed 2 deer with my 7MM RM. I waited until the deer in the back was well clear of the front deer. At 150 yards and no downward angle, I shot, killed and recovered the first deer, not realizing that I hit the second one too! A few days later I found the second deer while investigating Buzzards! To this day, I am at a loss to explain exactly how the bullet deflected through deer #1 at such an angle to take out deer #2!
I do despise the intentional taking of risky shots though!