12.5 inch barrel vs 13.7 inch barrel velocity

[SHARPSHOOTER282]

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.

[WillBrink]

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.

[1168]

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.

[mig1nc]

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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[SHARPSHOOTER282]

would the extra barrel length from the 13.7 inch , be better for those longer range shots then the 12.5 inch ?

[joedirt199]

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.

[1168]

would the extra barrel length from the 13.7 inch , be better for those longer range shots then the 12.5 inch ?

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.

[JediGuy]

Eh, go with 13” and act like you have discovered the true best thing.

[1168]

Eh, go with 13” and act like you have discovered the true best thing.

I’m putting a 13.5” in the handguard you loaned me.

[WillBrink]

Decent discussion. SF group using 10.3" out to 600 yards. Starts at 2:45: