Muzzle brakes and velocity

[BammaYankee]

I am prepping for a NRA 1000 yard F-TR match in about a month, and found myself in a pinch in that the offical rules will not allow the use of a muzzle brake. Since I had never shot my rifle (.308 Rem 700 PSS) without one, I had no idea how it would perform. So, off to the range I went today and was a bit surprised to find my shots falling about 2.5" low at 100 yards. Velocity was averaging ~ 2360fps from a 21" barrel. I was using factory Black Hills 175's for this 'experiment' that were taken from the same lot I had used to shoot a 182-2x at a recent 1000 yard club F-TR match.

I plan on using handloads this time to make up for the loss in the velocity... I am just curious as to why the difference between having the brake installed versus not. The brake itself has an expansion chamber at the muzzle and holes the entire length (I wish I had a picture). Otherwise it is pretty 'plain jane' (non adjustable etc.).

Anyway, I realize there are a myriad of designs out there, but do muzzle brakes in general usually add to velocity?

 

[General Geoff]

If anything, I'd think muzzle brakes would reduce velocity...

 

[Mainsail]

I'm beyond pleased that you spelled brake correctly. There have been many topic titles and posts thereto where it’s misspelled ‘muzzle break’.

 

[BammaYankee]

General,
I would have thought so too... But it consistenly shot low with other loads as well. Maybe it's something particular to my brake.... I was not the original owner so I do not know the mfg.


Oh well... Maybe it's just 'one of those things'....

 

[General Geoff]

Were you measuring velocity with a chronometer?


If you're just calculating velocity based on bullet drop, you may be off. The brake may have just affected the bullet's trajectory for some other reason, such as uneven gas port distribution.

 

[BammaYankee]

Mainsail,
You're welcome!

 

[rcmodel]

The brake may have just affected the bullet's trajectory for some other reason

+1

Change in barrel harmonics could easily move the POI that much.

Unless you can measure an increase or decrease with a crony, I would have a very hard time thinking any muzzle attachment would add enough velocity to change the trajectory 2 1/2" at 100 yards.

That would take a major change in velocity!

rcmodel

 

[BammaYankee]

Yes... I was using a chronograph. Accuracy with and without the brake was the same (sub .5 MOA). I shot about a dozen groups without the brake... All nice and tight, just low.... Wierd!

 

[DaleA]

Did you check the velocity with and without the muzzle brake that day? I'd be interested to know what the numbers were.

 

[BammaYankee]

Dale,

I did not check the velocity with the brake on today, but in the past the same BH 175 loads with brake have consistently run ~2600 fps.

Actually, they are so consistent that using the ballistic calculator, my cold bore shot was only about 9" off the X ring the last time I shot at 1000 yds.

 

[TexasRifleman]

In my playing around I've not seen brakes to change velocity very much but they can change POI to a GREAT extent.

I would think that a change in POI with/without a brake is more about barrel harmonics than velocity.

My Browning BAR hunting rifle with the BOSS brake has taught me a lot about harmonics. It matters much more than I ever thought.

 

[DaleA]

Thanks for the numbers.

I'm puzzled about what's going on here too but that's what makes this site interesting for me.

I tend to like the explaination some of the others have put forward of barrel harmonics coming into play here to change the point of impact.

At any rate good luck in your match. Anybody hitting a target 1000 yards away gets a lot of respect in my book.

 

[Zak Smith]

You shouldn't see any actual velocity change going from a brake to no brake. The 2360 fps was probably a chronograph error due to more "blast" being directed forward. I have seen chronographs do this when the muzzle device is changed. The POI difference is almost certainly due to changing the muzzle device (harmonics, droop, etc) and NOT due to velocity.

 

[kingjoey]

A muzzlebrake won't affect velocity. It can, however, cause issues with barrel harmonics and can affect the POI dramatically. We specifically designed our KA-1830 brake to be lighter than a similar length of barrel to reduce its effect on the harmonics (we haven't seen any change in accuracy or POI). Heavy brakes will often dampen barrel harmonics and will cause a wide change in POI when removed. A lot of times they will get a large change in POI just from switching to a different ammo load. I don't recommend bulky brakes for anything that has to shoot accurately.

 

[SP Shop Foreman]

Maybe I can shed some light on this from our manufacturing standpoint.
We have many prototypes on the shop shelfs, lots of stages of success and failures. Without getting into all of it I can give you the basics the left us with the two types we now manufature.
This is the end result of a long series of clamp-on type muzzle brakes.

The bore inside diameter is what it now is to keep maximum braking effect and not change POI. Some of the predecessors had smaller I/D on the bore and did change POI, some very adversely. This one is an extremely effective brake.

This threaded muzzle brake is the epitome of an effective brake for the k31. Its k31 specific using GP11 and like cartridges.

This model must be installed on a threaded muzzle. A k31 specific muzzle threader is available for this process.
Its virtually fool proof and the boss's wife did the original demo. She had never threaded anything other than a needle in her life, but did a perfect job first time.
The threaded brake has a much smaller I/D and has no impact on POI whatsoever. Because the I/D is smaller the brake has the maximum braking action possible.

The barrel damper is k31 specific and has shown itself to cut groups by a nominal 35 to 45%. It is also threaded and performance and weight are based on the GP11 and similarly loaded cartridges. This one took a lot of prototypes made in 1/4 ounce increments before the damper was fine tuned.

This is a fully dressed k31 with the threaded damper isntalled. It also has the Type P Diopter, Bipod Adaptor and Buttplate Adaptor.

All of these k31 accessories shown represent a very large number of prototypes and development for each. Most of them require no alteration to the rifle at all, and two of them do require threading. I hope this does help somewhat.
Thanks

Allen
SP

 

[taliv]

what zak said...

and something else to consider if you're not convinced: use a ballistics calculator to see, all other things equal, how much change in MV it takes to change POI 2.5" at 100 yrds. I'm going to guess somewhere around 800+ fps. pretty well outside the reasonable range that muzzle devices like suppressors can effect velocity

 

[jerkface11]

Umm I don't see where you listed the velocity WITH the brake installed.

 

[Ol` Joe]

Muzzle brakes act as "tuners" changing the barrels vibration, as did the old Browning "Boss" system, and will affect accuracy and POI. Actually any wgt added or subtracted from the barrel will change the harmonic, even cutting a 1/2" off to recrown.
Here is a good artical on tuners.
http://www.6mmbr.com/gunweek080.html

Velocity shouldn`t be affected at all. The brake is vented and no extra "push" from powder gas should occure when the bullet passes throught it. No velocity gain or loss was seen when I put the brake I have on my 708. That`s only one instance, but I pretty sure it is the norm. I`ve talked to guys with the Boss on their rifles and all claim their is a difference in impact when the wgt is removed from the barrel. They don`t however seem to agree on on much change occures. None have mentioned velocity change.

 

[BammaYankee]

Great feedback from everyone... I really appreciate it! What yous guys said (hey... I may live in Bama but I'm still a Yankee!) about harmonics & POI makes sense.

Also, I do think I set the chrony too close. I remember getting several error messages.

I'll be headed back soon and I'll be sure to set it back an extra 10 feet or so. This time I'll compare velocity with and without the brake.

Again... Thanks for the feedback!

 

[45crittergitter]

+1 on the kudos for spelling "brake" correctly. (Am I really cheering for correctly spelling a 5-letter word??? :banghead For the unclear: Think "brake" as in "slow down," or "brake" your car, not "break" as in "broken."

 

[BammaYankee]

Follow Up.....

Well, after moving the chrony back it seems that velocity with factory (BH 175, .308) and handloads did not change significantly with or without the brake. I can only conclude that the POI shift was due to harmonics, as several folks have correctly pointed out.

Thanks again!