5.56 barrel lengths vs. velocity.

[98Z5V]

I've talked about this before, here.  The data is here, where I've said it. 10.5" and the Mk18 10.3" barrels - there's a HUGE difference once you go to 11.5".  That's the minimum barrel length I'd ever recommend for a 5.56 gun, ever, based on years of data.  SOCOM recently (or not so recently) changed all their Mk18s to 11.5" barrels.  Because, math matters.

Check this vid out - I've preached it here before.

 

[Cunuckgaucho]

Video came at a good time, doing some research and now waffling between C7CT( Canadian version of MK12 1/0 but 20"barrel) and C8CT( Canadian version of MK12H)

[98Z5V]

1 hour ago, Cunuckgaucho said:

Video came at a good time, doing some research and now waffling between C7CT( Canadian version of MK12 1/0 but 20"barrel) and C8CT( Canadian version of MK12H)

I can tell you this much - I wouldn't shy away from a 16" barrel.  Lemme explain that a little bit.

#1.  I wouldn't do a 5.56 barrel shorter than 11.5", as a general truck-gun type of rifle, and I'd prefer a 12.5" - simply for the ballistics.  I have a 7.5", but that's truck-gun, or for running while you're inside a vehicle, and can't get out - yet.  It's hard moving a 10.3, 11.5, 12.5 inside a vehicle.  So, that short little 7.5 has one job.  The ballistics suck, as far as terminal velocity goes, but I only need it to get me out of a vehicle, or stuck in a vehicle - get the vehicle out of the area.  Off the X. As far as the Mk18 goes, the US Army SOF guys dropped the 10.3" barrels, and moved to 11.5" barrels for that platform.  Decided, done, no more 10.3s on Mk18s.  It's just ballistics. 

Now, 20" barrels are over rated.  I don't think any barrel should be longer than 18", for a general purpose, all around rifle.  Make an accurate load, build an accurate gun, and it's only a math problem after that, to the limits of your ballistics.  Need to go further than that?  Build a different caliber...  Every build has a different purpose...

#2.  18" 5.56 hand loads, my Mk262 clone load.  It's 2789.75 fps out of an 18" barrel, averaged over 30 shots.  The magic number that you need to hit in an 18" 5.56 gun running the heavies (75 or 77gr projos) is 2750 fps.  That's the magic number for making those projectiles perform at a minimum of 600 meters, but all the way, very effectively, out to 850 meters.  I've shot my handload out to 1100 yards on a fullsize IPSC target.  For some weird reason, I need that gun to make hits out to 1150 yards.  I just want that number.  I used meters in the first example because that's military measurement, and standards for the issued ammo.  I used yards after that, because I don't measure, estimate range, or shoot meters anymore.  I have no need for that now. 

Now, that same load through the 16" 5.56 gun.  It's had an avg of 2663 fps through a brand new barrel, and still wasn't broken in.  Those were just the first shots through it.  That's a 126 fps drop over the 18" gun, but also a brand new barrel, that needs shot more.  It'll pick up a little, somewhere in the 100~200 round count, and it'll smooth out.  If the load works, and is accurate, that'll be a badass (2" shorter) gas gun. 

Edited July 19, 2023 by 98Z5V

[98Z5V]

My 16" Grendel Mod H is the overall baddest gas gun I have.  It's the overall best gun I own.  That barrel length and caliber will do anything I could ever expect it to do, at whatever realistic distance I'd shoot it at, and it'll still smoke a Mk12 5.56 18" gun. 

Now, for 5.56, even though I stated that a 16" will get it done...   I'd do your gun in 18" barrel length.  Give the 5.56 cartridge every possible advantage that you can throw at it - make it an 18", and run the heavies for ammo.  It won't let you down. 

[AngelDeVille]

The video is interesting enough but won't change how I do any future builds.

I have a 10.5" 607 because it's badass, my go to favorite is the xm 177(e2) with 11.5" barrel

next most favoritest is a 20" A1

 

most are 1-12" and I have one 1-14" 

52-55 gr projectiles and I'm happy

 

some day I may pick up a crate of 62 gr, just to see how they perform but my platforms aren't built around a 1-7 or 1-8"

a 20" isn't too cumbersome for urban settings if velocity was remotely a concern, and 10.5-11.5" reaches out far nuff also

 

[98Z5V]

6 hours ago, AngelDeVille said:

most are 1-12" and I have one 1-14" 

52-55 gr projectiles and I'm happy

 

some day I may pick up a crate of 62 gr, just to see how they perform

Your guns will hate 62gr, if it's the M855 / SS109.  1:9" twist A1s hate that stuff.  That ammo was developed for the A2, 1:7" twist.  1:8" barrels don't have a problem with it at all.  Some 1:9" barrels like it, most don't.  Talking M855 / SS109 ammo here.