Bullets for a particular caliber are designed from the get-go to be stable during flight so conditions of a bullet being unstable during flight, yawing, or even tumbling before impact are rare. I have, however, experienced these conditions several times when testing bullets too long/heavy for the typical 1:11 twist in the 6.8mm and even the 1:10 twist for the .270. In these situations, I have found it difficult to even hit the target and if I do, the terminal trajectory is typically unpredictable.
Unstable conditions in flight can most likely happen in very cold weather as the air becomes a more dense medium for a bullet to "fly" through. As an example, a Hornady 130 GMX fired from a .270 Win with a typical 1:10 will likely lose stability if temperature drops below 15F. The denser the medium, the less stable a bullet becomes (the advantage of having a 1:10 twist in a 6.8mm if you hunt in cold weather with heavy/longer bullets). When transitioning from air to flesh, which is mostly water, the density of the medium increases significantly. For a bullet to remain stable after impact, it must immediately shorten and or increase in diameter to remain stable. If it doesn't, it will yaw, tumble, and often swap ends exiting tail first. I've seen this repeated time and time again. FMJs do not remain nose first. You are welcome to post links to videos that show otherwise. There may be a short .224 55gr FMJ that appears to remain stable on impact if shot from a 1:7 or faster twist, but it likely has already started to yawn as it exits the gel. Also, the majority of OTM do the same and, if they have thin enough copper jackets, will break in the middle and fragment. If their jacket is too thick and maintains structural integrity, they are no better than an FMJ, e.g., .277 Berger VLDs. The 115 SMK is one of the few 6.8mm OTM bullets that doesn't yaw on impact. The ogive of its copper jacket is thin enough to flatten before it can yaw then fragment.
If a bullet leaves the muzzle in a stable condition, its stability increases as it travels down-range because its velocity decays more rapidly than its spin/twist rate. The only issue I have encountered when doing terminal performance testing at long ranges/lower velocities is, if the bullet does not shorten or increase its diameter enough, it tumbles and over penetrates usually tail first. It does not act like a parachute. The mushroom part of the bullet is now in the wake of the temporary wound cavity created by the boat-tail not doing its job. The .224 Nosler 64gr BPB is a good example of this behavior if impacts are outside of 100 yards and shot from a 16" 1:8 twist or slower. I have not seen evidence of a bullet with a larger mushroom diverting off course. Because they have become shorter and larger in diameter, their stability increases. If they expand and shorten too much, they can become unstable due to the CG shift but very few bullets exhibit this amount of expansion performance. The bullets I have seen divert off course after impact are ones that yaw having not immediately expanded and shortened enough.
Regarding if there is a way to give up a little expansion and gain a little bit of penetration using the federal xm68gd ammunition, the xm68gd has excellent weight retention and shows good penetrates. For a 90 grain bullet, it accomplishes a lot for being on the smaller side of the family of 6.8mm bullets. It may be difficult to modify this bullet to do better.
When a bullet is on the verge of being so unstable that it is showing some of the shots keyholing on paper then it is much more likely to tumble and have penetration and direction of travel affected in tissue.
What I said was that SOME FMJ bullets enter and basically remain nose first.
When the word SOME is used, it is to imply that not ALL bullets enter nose first and BASICALLY remain nose first but SOME do.
The key word here is BASICALLY
There is obviously a difference in the AMOUNT of YAW experienced AND there is a Difference in YAWING versus TUMBLING.
I would be interested in links to videos that show FMJ Bullets ALWAYS EXIT base first.
Obviously if the bullet was about to keyhole upon striking the tissue the effects would be to have the penetration and direction of travel would be affected in an unpredictable manner.
AND this holds true whether the bullet is a FMJ or a conventional expanding bullet
Obviously no bullet is ever spinning perfectly around its axis whether in flight or in tissue.
As far as the Mushroom of the bullet not acting like a Parachute in effect because of the temporary wound cavity.
The bullet is creating the Temporary and permanent wound cavity as well.
ANY part of the bullet whether it be the sides of the bullet exposing more of the bullet to the tissue Because of Yawing OR Tumbling will slow the bullet down impending the penetration.
The same holds true with the Mushroom of the bullet as it slows the bullet impeding penetration.
And this holds true whether the Mushroom is being PUSHED or PULLED through the tissue.
Just because there is a temporary wound channel does not mean that there is NOTHING for the bullets mushroom to "CATCH"on and slow the bullet down.
A bullet does not have a perfect mushroom to bear with equal Pressure on tissue,there will be one side so to speak of the bullet that has exposed more or less of itself to the tissue which will slow the bullet and affect the Line of penetration.
Think about a Barnes X Bullet and its penetration
Less of the front of the bullet is exposed to tissue so GENERALLY it is slowed down less, tends to penetrate deeper and in more of a straight line.