View Full Version : 300 BLK vs 5.56mm for barrel length efficiency
These graphs illustrate the diminishing returns of velocity gain as a function of barrel length. We know that for 5.56mm, a 24 inch barrel is a really efficient one. The slope of the velocity curve at 24 inches is equal to the slope of the velocity curve for 6.8 at 19 inches and 300 BLK at 16 inches. 20 inches in 5.56mm matches about 13.5 inches in 300 BLK. 16 inches in 5.56mm is like a 13 inch 6.8 and 10.5 inch 300 BLK in this regard. A 14.5 inch 5.56mm barrel matches a 10 inch 300 BLK barrel as far as slope of velocity gain. An 11 inch 5.56mm barrel is about like a 300 BLK at 7.5 inches. 20 inches in 300 BLK? Might as well be a 29.5 inch 5.56mm barrel in terms of how much left there is to get out of it by going longer - so I would consider that that longest anyone should want to do and almost everyone should stay with 16 inches.
http://img692.imageshack.us/img692/3653/screenshot20120210at814.png
Any chance of modifying that chart to also have a graph with the energy, and/or impulse?
I am adding energy to it now. It may make more sense to look at the slope based on diminishing returns of energy rather than velocity.
Scrap all this. I am redoing it with a different method.
Very interested in this. I'm bouncing between an 8", 9" and 10.5" build. So many functional variables...
I am still looking for a trajectory chart for the 300 blk, 7.62x39, 6.8, 5.56, and 7.62.
I know, lots of variables bullet size and weight etc. I am really only wondering about high velocity rounds. Anyone have one handy?
MeanStreaker
02-27-12, 18:07
I am still looking for a trajectory chart for the 300 blk, ...
+1 to this.
+2 I'd like as much info as you'd be willing to provide Mr. Silvers.
Wow, Robert:D was just discussing this tonight as I will be getting a 300BLK upper soon and our 4th AAC suppressor for it and my brother and I were trying to work out which barrel length was going to suit our needs. I really look forward to you putting this together for us to help all at M4C make better decisions on your new cartridge! Thanks:D
RogerinTPA
02-28-12, 06:24
What would be the optimum zero for the flattest trajectory and longest engagement range, without hold over, for the 300BLK, similar to the 50/200M zero for the 5.56?
Yeah I used to really want a 14.5 inch AR until I started hearing how the round was designed to be shot out of a 20 inch barrel.
300 BLK is best with about a 200 yard zero for normal ammo, and a 100 yard zero for subsonic.
blacklabel66
03-09-12, 18:47
Does anyone have a chart showing the drop using the 200yrd zero and supersonics from 0 to say 300-400?
Scrap all this. I am redoing it with a different method.
Looking forward to this;)
I just bought a Ti-Rant 9, Ti-Rant 45, M4-2000 a couple of weeks ago and I would buy a 762 suppressor and a 300BLK upper tomorrow if I could get a little more educated on the cartridge exterior ballistics.
If I was 25yr old I would be out getting an AAC Tattoo:p
No ink for me I think I'll spend my $ on guns, gear and ammo:p
As if we're not asking enough... suppresed graphs with barrel length and trajectory, etc, etc, blah, blah...
Seriously, any info would be much appreciated, especially suppresed.
Thanks for the work you've done and the work you will do in the future.
Mr.Silvers any news on your testing?
Silvers,
let me give you an easy one. I scrapped my 6.8 SBR. But I am still interested in an SPR build for a 6.8. What would be the ideal length and where is the point of diminishing returns with regard to barrel length on this cartridge. Or is a 6.8 SPR just a bad idea?
Thank you
PB
With 300 BLK you get 8 inches of barrel for free compared to 5.56mm, so I would do a 300 BLK SPR as a 16 inch. For 6.8 and barrel efficiency, it would have about the same rate of diminishing returns as a 20 inch.
http://img72.imageshack.us/img72/7813/screenshot20120318at110.png
To be clear, you would do a 20 inch 6.8 SPR?
Thank you for your reply.
PB
What would the 300 BLk vs 5.56 trajectory chart look like with a 16 inch barrel.
With a focus on 300 Blackout:
What barrel length is best for sub sonic ammo?
Sub sonic Suppressed?
A mix of sub sonic and supersonic ammo suppressed and unsuppressed?
Quiet Riot
03-21-12, 09:19
With a focus on 300 Blackout:
What barrel length is best for sub sonic ammo?
Sub sonic Suppressed?
A mix of sub sonic and supersonic ammo suppressed and unsuppressed?
Barrel length doesn't have the impact on subsonic ammo that it does on supersonic because you can load your rounds to be right at 1050fps regardless of the length you choose. Getting the action to cycle is then a matter of tuning your port size.
AAC sells a 9" barrel that is designed to cycle subs unsuppressed through supers suppressed. You could go shorter if, again, you tuned the port size to allow cycling subs unsuppressed.
The same question is always worth asking, though- why would you want to shoot any real quantity of expensive, underpowered subsonic ammo without a suppressor?
It's also worth noting that Noveske specifically tunes their 300 BLK barrels so that they DON'T usually cycle subs unsuppressed because they feel their port size makes supers suppressed more reliable. AAC might disagree, but I literally put my money on Noveske when I bought their 8" barrel.
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