can anyone tell me what the velocity difference is generally between a 12.5 inch barrel and the 13.7 inch barrel , 5.56mm ?? is velocity difference worth the longer barrel ? I notice the new UK ranger rifle (knight armament) is a 13.7 inch barrel.
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can anyone tell me what the velocity difference is generally between a 12.5 inch barrel and the 13.7 inch barrel , 5.56mm ?? is velocity difference worth the longer barrel ? I notice the new UK ranger rifle (knight armament) is a 13.7 inch barrel.
There's formulas for such things around here or online, as well as various YT vids of people testing such things. The bullet itself is an issue as to terminal ballistics, and what UK will be using. I think they use the L31A1 but not sure if that's still correct. Others here will know that info.
Plan on 40-80 fps, depending on ammo weight and pressure. Be aware that that’s within the normal variation for barrels that are the same lengths. The only two good reasons to choose 13.x is A)you want to use a 12.5” barrel, but have a 12” rail, which is also about 12.5”, or B)your only silencer is a Sandman S and you can’t/won’t SBR your gun.
As to the actual KAC gun, 13.7” is social-media popular lately, and that also happens to be the shortest barrel usable with their already existing 13” handguards. Its cutting it closely enough that it could interfere with silencer compatibility.
Rifleshooter.com has published a high-quality inch by inch test that you can reference. You’ll notice that lighter projectiles lose more velocity per inch than heavier ones. If you google “Labradar 1168”, you might find a thread here with some measured velocities in 12.5” and 14.5” guns.
Anything between 12.5” and 14.5” is pretty much the sweet spot for a general purpose carbine, with 16” being an outlying exception to that rule.
Trey Knight did an interview at SHOT where he said the reason was that they had a maximum length requirement with the silencer attached. He said that was the longest barrel that they could use with their can and be under the limit enforced by the MOD requirement.
I can't vouch for any of that, but it's what he said.
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would the extra barrel length from the 13.7 inch , be better for those longer range shots then the 12.5 inch ?
More velocity is always better but 50 fps really isn't going to make a huge difference. I am cutting mine down more for size convienence over terminal velocity. Bullets hurt whether they are going 2600 fps or 2700 fps. Shortened them up to make room for suppression.
It’s extremely minor, and like I said falls within variation of a given barrel length. There are 12.5” barrels that demonstrate velocities near or equal to 14.5” ones, and vice versa. Shooting appropriately zeroed 12.5” and 14.5” guns to the militarily practical limits for point target engagement, I don’t really notice the difference. It starts to become noticeable at 11.5”, for me. I’d divide practical barrel lengths into 3 groups, and within a group would see them as ballistically equal enough: 10.3”-12.5”, 12.5”-16”, and 16”-20”.
You’re simply not going to see a practical difference in bullet flight or impact between a 13.7” and a 12.5” or 14.5”. With a 50yd zero, its a 1/2moa difference at 500 yards, on a 2-3moa gun. The 50fps advantage is reduced to 35fps at that distance. Negligible.
I’d still personally choose a 14.5”-15” gun for a P/W, though 13.5” guns make great SBRs.
Eh, go with 13” and act like you have discovered the true best thing.
https://i.imgur.com/40gPNiA.jpeg
Decent discussion. SF group using 10.3" out to 600 yards. Starts at 2:45:
https://www.youtube.com/watch?v=WJNPY1dCges
Ammo selection is going to make a lot more difference than 1-2" of barrel IMO.
I'm not sure I completely get the 13.7 length. I had one for a while. And, assuming you aren't going to SBR it, by the time you P&W it's the same overall length as the 14.5 P&W. Which is what I eventually sold it for. I do want to build a 12.5 SBR next and play with it along side my 10.3 and 11.5. I think the 12.5 is at the length where a compact LPVO might make sense.
As has been mentioned, the UK KAC KS-1 contract had an overall length requirement including the suppressor. According to media, that barrel and suppressor combo meets the requirements.
As has also been mentioned, I believe, the 13.7/13.9” barrel moves a suppressor slightly to the rear. If pinned, the overall length doesn’t change unsuppressed but does suppressed. Additionally, I believe when I checked the weights on the KM6315 and KM6315-L, they were the same. So reduced barrel (and overall) weight by .4oz on a pencil barrel, reduced overall length suppressed by .8-.6” vs a 14.5 or 2.3-2.1” vs a 16, and mass shifted toward the pivot point (your body).
I probably wouldn’t get another 13.9”, personally, but neither would I get another 14.5”.
I’ve built a 12.5 mid length gas system and a 13.95 mid length. I can get you some fps with Winchester white box, pmc, or federal 223 and 5.56. It might be a while, but happy to run some numbers next time at the range with chrono.
Big fan of 12.5. If I could just have one, it would be a 12.5 carbine gas with a dot and magnifier.
Why car over Mid for the 12.5?
I go by 2% difference in MV for every inch of barrel length.
-TL
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This. Or a 1-4x/1-6x.
As for carbine vs mid, it just works. Has for a long time. I somehow have yet to test this with a MG lower and shot timer, but when switching from suppressed to unsuppressed and back, “short” gas systems seem to have less drama than longer ones. Since this is the opposite of what pretty much the entire internet feels, I’m open to being wrong. But the Crane study does seem to back this up by showing a greater RoF delta, IIRC. Either way, it’s the size of the hole, not the length of the gas tube that really matters, in my opinion as someone who isn’t an engineer.
“Same gas tube as legacy guns in current use by major militaries around the world” doesn’t market as well to tinkerers (like myself) as “we made this longer for optimized suppressed use”. Since few people own suppressors, and few people will experiment while keeping variables to a miniumum, the internet is largely how people get informed about this topic, for better or for worse.
On the flipside, I doubt anything is “wrong” with a well-tuned midlength 12.5” gun, especially if left in one configuration- suppressed or not. Though unsuppressed, it will need a blowhole to be reliable when cold and dirty.
My 12.5 mid-length runs like a top. Adding a griffin dual-lok 5 soon.
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I teach medical applications (officer down/rescue) class and it’s 2 or 3 days depending on the customer. Round count is usually 500-600 rounds per day, rifle and or pistol. Never had an issue. She gets pretty dirty, but is loved and clean regularly. I run a bcm mk2 buffer and h2 equivalent. When I get my suppressor I’ll tune her appropriately.
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https://faxonfirearms.com/faxon-duty...-ar-15-barrel/
I bought this back in 2014 but never installed until a few years ago. I’ve had no issues. It’s probably not the best barrel but I didn’t know what I know now. I had all the parts laying around so I threw it together.
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I have a 12.5 and 13.9 criterion core...I can get you velocities as soon as this lousy Michigan weather breaks.
American Eagle M193 average velocities
12.5 criterion core average 5 shots 2854
13.9 criterion core average 4 shots 3062
Chrono 15 feet downrange
That is pretty impressive difference for 1.4” spread.
The 13.9 only had 200 rounds down it when tested. Was a new builds. The 12.5 had about 500. 13.9 was shot suppressed yhm turbo t3. 12.5 shot cold u suppressed
13.9 handloaded 62gr gold dots 2.260 oal. Light crimp arplus 24.0 gr. 2766 fps avg
So, this is completely unscientific, but today my buddy chronod his 13" X95 SBR and averaged 2919 with American Eagle 55gr 60 degrees at around 400 feet elevation.
Same weather my 13.7" ballistic advantage did 2865 with M855.
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