I did some searching on the internet before posting and didn't find the answers I was hoping for.

I am thinking about ordering a 300 Blackout barrel. I saw some Wilson barrels that look nice. My question is: 16" vs 18" barrel? How much more velocity might I achieve with the 18" in a supersonic load with say 125 gr bullet than in the 16"? any estimates? Just asking out of curiosity

I know velocity questions and barrel length concerning 223 can be aggravitingly redundant......so please forgive me in advance for asking a velocity question....

PLUS ONE, but you could get a longer sight picture with irons on the 16 and 18 at least.

You must be talking only subsonic.

OP, I don't know the exact answer to your question since I don't have one in 18". With 125 gr supers, about 175 fps is gained from a 9" barrel to a 16" barrel. The extra 2" may give you a little more but may not be worth the extra length.

.300 blk vs. 5.56 barrel lengths

EDIT: ^ beat me to it...

At 300 meters, 300 BLK has 16.7% more energy than 7.62x39mm. Max effective range, using M4 military standards for hit probability, is 440 meters for a 9 inch barrel, and 460 meters for a 16 inch barrel. 300 BLK from a 9 inch barrel has the same energy at the muzzle as a 14.5 inch barrel M4, and about 5% more energy at 440 meters - even though the barrel is much shorter.

So, you are saying that both 5.56x45 from a 14.5" barrel, and 7.62x35from any barrel both suck at 440 meters.
I think that most people know this already.
I'm sure that the 7.62x35 is also much better than the 22LR at 500 meters.

A. 440 meters is over 4x the range we were discussing (100 yds)
B. Don't forget, most people are still discussing 5.56 in 55gr or 62gr ammo at 440 meters. I think I've only got 1 or 2 mags of that, everything else is 69gr or +....
C. It stands to reason that if the 300BLK (supersonic) from a 9" barrel has more energy at 440 meters than a 16" 5.56, it also has more energy at 100 meters.

Also, a lot of the data I'm seeing is comparing 62gr 5.56 energy vs 123gr 300BLK, I'd like to see the data when you move up to 75gr 5.56 or 77gr 5.56 (Mk262). And not just ballistics at range, but accuracy (precision) statistics.

There was a chart somewhere showing stuff like

16" Barrel comparison
@ Muzzle:
5.56mm M855 2938 ft/s, 1188 ft-lb
7.62x39mm Win, 123 gr SP 2245 ft/s, 1377 ft-lb
300 BLK, 125 gr OTM 2230 ft/s, 1380 ft-lb (16% over 5.56mm)

Accuracy From:
10" Barrel 5.56mm 3.9 MOA (8.1 in) @ 200 yds - 5 rds extreme spread.
9" Barrel 300 BLK 220gr OTM (with 762-SDN-6) 0.64-1.20 MOA @ 100m - Avg. Mean Radius.

Linky

Dont forget to take into account bias in methods of measurement (why the difference in measurement of group size?).

Damn, those numbers are quite impressive.

PLUS ONE

Why are subs not recommend for home defense?

Nevermind. I'm enjoying this too much!!

I know a lot of good folks on here are convinced that the shorter the barrel the more desirable the firearm. I say that is fine, after all to MOST here this is a just a hobby. Many enjoy the spirited debate also. The debates are great as it gets people to think more and dig into the technical stuff. Me, I just enjoy the hobby and personally I don't get involved in SBR's as for this guy they serve no purpose worth the expense and trouble. If I want a really short barrel I just shoot a pistol. I know some will swear you need all that power of the 300 to stop a bad guy. Maybe there is merit to that...maybe not. If I were a Special Operations professional in the field and my life was on the line.....my thoughts would probably be vastly different than what they are.

I build and shoot rifles for accuracy. I grew up on a farm and being able to hit a muskrat or ground hog at a few hundred yards on the first shot was important. SBR's weren't in on that mix! So, when I look at the 300 BLK I go for a 16" barrel. With the right stock it makes for a short and easy to carry rifle in the field. With a 16" barrel you can get great performance for hunting with good velocity and accuracy. For my personal uses a suppressor just isn't a priority. I need a fast moving round that will get some distance and stay accurate. I have seen some guys hunting with suppressors so for them it works. For me I am loading light bullets for more velocity and that sort of defeats the surpressor thing anyway. So, why would I spend hundreds of dollars on it? But, to each his own.

So, I have AR's with barrels from 14.5" up to 24", including an 18" one. The 14.5" is fast and moves well, the 16" is OK too but in second place to the 14.. After that they start getting long. The 18" and 24" heavy barrels I only use off a rest, but that is me. YMMV. After picking up a new RRA 16" "heavier but not bull" barrel rifle I think I have found the sweet spot of length, balance and overall feel. Don't get me wrong, I find the SBRs fun to play with, and they are cute! Is just not what I need for my uses.

Doubt it.