The 240's will typically tumble, but not always. It you want to improve your chances for tumbling, drill out the HP a little. I shot a 'yote back in Feb with one and it did a lot of damage before exiting. Most was from tearing not expansion, but the effect was the same. You can also use a 220 RNSP, drill out a hole in the exposed lead and then score the jacket.
After years of tinkering, I came to the conclusion that worrying about tumbling and expansion with subsonics was not worth the effort. What your time is better spent on is developing a dope card for different temps and knowing how the bullet will react with a lubed/unlubed bore, cold bore, and warm bore. I've shot deer, hogs, 'yotes, rabbits, crows, and what ever else wandered within range with 190, 200, 220, and 240 SMK's; 180, 200, 220 RNSP's; and 150, 180, and 208 BT's. No matter what the bullet, the effect from a lung/heart shot was the same. It was far more important to know exactly where I was putting the bullet than what it was doing once inside the target. Unless you are taking pop shots and hoping for a kill, focus on the lungs. It doesn't take much to puncture the rib cage on any animal and pierce a lung. Indeally, you'll want an exit wound for blood trailing or a spine shot with a quick follow up in the vitals.
The temp sensitivity of these loads is significant due to the reduced amount of heat and pressure to overcome the friction of and temp of the bore. The difference in drop for my 308 between 80deg and 40deg is roughly 4". Same 240 in the 300 whisper with a shorter barrel has a difference of 2 1/2". The difference in drop from the first cold, unlubed bore in the 308 can vary between 2" and 13" from the second shot depending on weather conditions.
Tinkering with it can be fun, but I never found a magic bullet for subs that will blow through thick shoulder bones since that is a function of energy not bullet design. With some of the Hawk designs I've seen I may drop the cash to try them, but in the end even the cheapest pill through the lungs gets the job done.
Your results may vary...
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