Got to the range to test the new to me 300 blackout loads I finished.  All in all, fairly pleased with results, but a few questions as well.  Sorry in advance how long this post will be.  Wanted any input on the velocities in general, and if they look to be in the right range.

Rifle is 10.5 inch barrel with non-adjustable gas block, pistol length gas system.  Buffer is one of the H2 weight Armaspec Sound Mitigation buffers.  

Was using IMR4227 for the supers.  Hornady's book has a start load of 12.9gr, went up from there in 0.5gr increments.  Cut down LC brass, annealed then sized.  Trimmed to 1.367", used Hornady 150gr FMJ-BT and CCI400 primers.  COAL was determined using the .250 inch/mag rib trick, OAL ended up around 2.150.  All chambered well before shooting.  Shot and chronographed 10 rounds of each powder charge.  Results were:

12.9gr IMR4227  velocity = 1414fps, SD=42.7, ES=151

13.4gr IMR4227  velocity = 1506fps, SD=41.5, ES=135

13.9gr IMR4227  velocity = 1524fps, SD=28.9, ES=85

14.4gr IMR4227  velocity = 1587fps, SD=22.9, ES=78

All rounds locked the bolt back at the end, all rounds cycled fine.  Primers looked fine to me, no flattening.  Hoping the velocities look reasonable, that's mostly what I'd like to hear feedback on here.  They're slightly lower than what Hornady publishes, but I'm sure my barrel is shorter than their test barrel (I didn't see a length specified).  Since there was no flattening, I don't think I'm too hot either (I see other loads where folks are using up to 16gr 4227 with 150s).  Not sure if it's worth chasing more velocity?

What kind of groups are typically expected with 300 BO supers?  I was getting about 2/2.5 MOA (tested at only 25 yds, but 10 rds each charge), but vaguely remember reading other people getting slightly larger groups (just can't remember if that was only the case with subs).  Plan on shooting at farther distances next go-around.

Also played with some sub loads as well.  Casted the Lee 230 5R bullet, and powder coated with Eastwood powder, sized to .309, seated to OAL 2.180 (longest that chambered consistently in my rifle).  Never shot subs in anything before, so also bought some S&B 200gr (heaviest I could find) to compare with.  Box stated those would be around 960fps, I chrono'd them at an average of 961.  Tried IMR4227, LilGun and H4198.  A little long to detail all that stuff here, but was surprised that all my sub charges locked the bolt back.  Probably got about 4 MOA groups.  All loads except for the highest IMR4227 (9.6gr, around 1120 fps average) were subsonic.  Only had one round keyhole - in the LilGun.  Ejection was all at around 3-5 o'clock.

Thanks for the initial help on questions I asked before starting these loads - it helped a lot.  Gonna settle on a load from here, *maybe* chase a little more velocity for the supers, and then get the subs worked out for when I get the suppressor (looking at the YHM resonator K).

In the Hornady App they list the test barrel as 16", Smith & Wesson AR 15, 1 in 7.5 twist.  From what I've seen the app is the same as Volume 11.

150 gr FMJ isn't known to be the most accurate bullet. No FMJ is.

My 16" bolt action is capable of sub moa with 110 tac tx and 125gr smk.

125gr Sierra game kings usually shoot 1.5" groups.

Try a different bullet.

This is the group you should be seeing at 25 yards. Barnes bullet and H110 powder, meticulously loaded. This is also a 10 shot group, out of a CMMG 8.5" barrel. I don't have the exact recipe in front of me but it may still be in my data book. I have since switched bullets to the Nosler 110 grain Varmageddon. I get very similar results, but not quite as tight. Barnes were just getting too expensive for my shooting habits.

I really liked these bullets too. I kind of gave up on them after struggling to find them and learning they were discontinued...?

These bullets? https://www.midwayusa.com/product/2149663126
Midway has them $23.19 per 100

Dryflash is right, have had shockingly good results with them!

Thanks for the additional info everyone.  So it sounds like the larger groups is normal for the 150grs (especially jacketed).  Seeing as how the Hornady data is from a 16in, the velocities are not terrible either.  I guess they'll be suitable for plinking at short distances for now.

I had seen the 125gr Speers earlier, and was planning on trying them at some point.  Will probably keep playing with the 150s as well - I've got a decent supply of the FMJs, and also got the Lee 160gr mold that was mentioned in another thread on 300 BO (the "cheapest 300 BO load" thread), so looking forward to getting those rolling as well.  Poured a couple test bullets, then powder coated and got gas checks to seat on them well, so need to get a bunch made and test some loads with them as well.

Are the 160gr cast more accurate than the 150 FMJs?  Is it thought to be a jacketed bullet issue, or a bullet weight/powder issue that causes the inacccuracy?

Gah, you just cost me $231.90 dollars

Possibly. My cast bullet loads shoot well, but not nearly as well as match jacketed bullets.

Fwiw, my cast bullet loads shoot better than 4" at 100 yards and I do not use gas checks.

Cast bullet accuracy comes down to a combination of consistent casting, consistent sizing and loading. It can compound accuracy issues if your process is not correct.

Also, lead hardness...

Happy to help spend your money!!  (I may order some more too)