Yeah, I know. 7,62x39 wouldn't normally be my first choice, but the only rifle I have with magnified optics on it is my AK, I don't want to go the crappy scout/LER mount on my M39, and drilling/tapping it is also a no go. Also not interested in buying a new/used rifle and having to stock yet another caliber.

Besides, KS says you can shoot deer with any centerfire rifle now, we don't have a caliber limit anymore, and I'd prefer the x39 over a 5.56 anyway.

So, what I do have right now is about 150-ish rounds of the 125-grain Silver Bear SP ammo. Mostly I got this because it shoots pretty much exactly the same as the 124-grain GT FMJBTs that I normally shoot, and everything I've shot them into that I could recover them from had them expanding.  

Would I be better off with the 154's (necessitating a rezero), or will the 8M3 work okay? I'm curious as to if it will penetrate deep enough for this use.

I am also considering the Hornady SST's, but they are dang expensive; is it really worth 2x the price of the 123/154 SP's or 8M3? Especially in a non-reloadable case.

I have experience with other cartridges, but the SST seems to be ideal for white tails

X39 or 5.56 aren't optimal for several reasons, but some can be overcome.
SST gets good reviews. Tula 154 grain SP is a hard hitter with good terminals, and fan favorite of some other AK forums. I have used it at the range and it is a nice round. They seem consistent with the other ammo I use. 8M3 isn't what I would consider a good hunting round. Great man stopper, but not for game
Keep it under 150 yards. Practice, practice, and practice some more. X39 can be off more than acceptable amounts for me even in that range. Kansas deer are on average bigger than we see.
Do your part and both will work fine for you.

You'd be eating chunks of Russian lead with 8m3. Those are little frag grenades.

I prefer Tula 154gr.

8m3 fragments pretty hard. Would kill deer fine but just know what you're getting into and aim accordingly.

SST is good on deer. I didn't like its performance on pigs though

Best for hunting would be Fusion or TSX. Best from your list would be SST, but any 120ish gr soft point should do the job really well. Based on Buffman's test, I don't think 8m3 is a good choice at distance.

It’s made complicated that the 8m3 did not fragment in either an sbr or 16” rifle in his test.  It begs the question: is it a bad lot, as the 16” result was vastly different than yours?

My guess is that the clear gel is the culprit. Do you have any plans to test the 8m3 from an sbr into ballistic gel? I’m happy to provide the a,mo, but it may be ideal if buffman has some left, as this would Enable the most direct comparison.

I'd go for a TSX load as long as it shoots decent. It and the SST would be the two I'd prefer followed by a soft point. Pick whichever prints the best.

Shot an 8 point buck this year in a dense swamp using an Arsenal AK with an Aimpoint PRO @ 40 yards with SST.  It was walking toward me and I had a limited time clear shot.  Saw it kick after I sqeezed the trigger.  No blood trail or hair at impact site.  Found deer about 50 yards from impact.

Last year shot similar buck running across a field at about 125 yards with 308 American Whitetail Hunter.  Entry wound looked like an exit wound.  Crumpled on the spot dumping blood.

SST definitely worked but didn't leave a blood trail.  For similar bore there was a huge wound difference.

Any soft-point will work.

Accountant

I use the Golden Bear SP 7.62 X 39 with much success over the years. Dropped them hard , right now , no runaway !