300BlkTalk

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.

Hi Robert,

Any idea how come I can never see the pictures you post on here? All I see is a pic of a frog in a block of ice with the words 'Domain Unregistered. to view, register at: bit.ly/imagshack-domain' ... I'm assuming it must be some trouble on my side as everyone else seems to see em.

I do know what this picture is though as I saw it on the facebook page earlier. I assumed all the others were old and had been deleted, but this I know is new.

Cheers,
Gunna.

Sounds like ImageShack does not serve to your country unless you register.

Very informative data.

For those of us planning for SBRs that still want to maintain good terminal ballistics from super sonic rounds, it sounds like barrels between 9" and 11" or so might be the sweet spot in terms of compromising weapon length with bullet velocity. The key variable in good performance there would be enough velocity for reliable expansion of soft point or BT ammo.

A very cool follow on study would be the effects of barrel length on terminal velocity at certain ranges for some populer .300BLK bullets. For example...If I want 1800FPS at 200yds from Nosler 125BTBT's (Noslers minimum speed for expansion is listed as 1800FPS), I neeed a muzzle velocity of YYYY. To get muzzle velocity of YYYY I need a barrel legth of at least XX inches.

I might start to tinker with the ballistics software I have at home and see what I can come up with.

I am redoing all this. I came up with a better way to do it.

Will the updated version give a recommendation on an SBR barrel length? What I mean by this is similar to what jblom said, where the sweet spot is.

I am thinking the sweet spot is 16 inches - equal to the efficiency of a 20 inch 5.56mm and yet has 100 more ft-lbs of energy than a 20 inch 5.56mm.

I would say most people should want a 16 inch if they think 5.56mm should be 20 inches.

A 9 inch 300 BLK matches an M4 (14.5 inch).

So basically it's not worth SBR'ing

It is worth SBRing because the cartridge is effective from even 6 inch barrels. You may have seen my 9 inch Barnes bullet expanded at 300 yards. Well, it would look the same from a 6 inch barrel at 200 yards. That is a very useful range.



But 16 inches makes a lot of sense - as it surpasses a 20 inch 5.56mm - which we all know is really what a 5.56mm should have for a barrel.

loki.fish wrote:So basically it's not worth SBR'ing

I'd say out of the 3 calibers in the comparison it's the one most worth SBR'ing.

Using a short barrel involves obvious trade-offs in terms of ballistics. The numbers show that the 300 BLK gives up very little ballistics-wise, certainly much less than the other two calibers.

In fact, given that 16 inches is the point of diminishing returns, I think you're missing much of the utility (and fun) of the 300 BLK if you don't SBR it.

I think it's awesome that you can use a barrel length that is considered absurdly short in 5.56 and be just fine in terms of velocity (and terminal ballistics now that there are properly designed bullets), operation of an AR, and silencer wear and tear.

With an 8" or 9" AR upper in 300 BLK, you can now have a silenced weapon that's as short as an unsilenced M4, yet it can hit just as hard, or harder.

That very small loss of velocity for such a large reduction in barrel length is a much better trade than you get with the 5.56 or the 6.8, especially considering that the velocity in the short 300 BLK is well within the design parameters of the 300 BLK's bullets.

HTH