Brake’s effect on velocity readings ?

[Topwater]

I’m ladder testing a new rifle. During testing with the Magneto V3 chronograph. I have experienced muzzle velocities that appear to be quite inconsistent in relation to varied powder weights. A buddy suggested that I remove the brake and test again. I only have 75 rounds down the tube. My question is if there’s any validity to the idea of the brake affecting testing. Thanks, folks.

 

[DaveTooley]

muzzle brakes can affect velocity. Those with small bores, just over bullet diameter, can add 20 or so FPS. Those with an expansion chamber still increase velocity but a smaller increase. But any increase is consistent. Velocity compared to charge weight is not always linear. Testing just a few rounds doesn't always give you a true number.
In 40+ years of shooting I have never once used a chrono to tune a rifle. The target tells all.

 

[MSS]

The brake might be affecting the accurate reading of velocity. I use a labradar and I get some crazy readings on my braked .300 wm some times. It’s pretty fussy about placement.

 

[Ned Ludd]

In my hands, velocity with or without a brake on one particular rifle didn't change noticeably per the MagnetoSpeed. What did change noticeably was the POI shift observed with the bayonet attached. Because that shift in POI is caused by muzzle blast off of the sensor deck, putting the brake on, which diverted some of the muzzle blast off to the sides and away from the sensor deck lessened the POI shift.

In your specific example, why not just remove the brake and see if it makes a difference? It would not even require a full test series to determine whether the brake itself was the culprit, just a few loaded rounds at two charge weights sufficiently different that you should observe a change in velocity. I would guess the brake was not the problem, but a simple test would tell you with certainty. If the brake is the problem, then you would know. If the brake is not the problem, you can focus your efforts elsewhere.

 

[ronemus]

There should be no shot-to-shot variance in velocity with a properly installed brake (emphasis on properly). You should check that it is tight and square, and that the brake is colinear with the bore. Also inspect the lips of the baffles to check for strikes - there should be no shiny spots or copper deposition. There isn't much difference in the effectiveness of a brake up to ~0.030" over bore diameter, so I don't invite trouble by making the clearance too close.

 

[HandgunHTR]

A couple things.

First, when you say “inconsistent“, what do you actually mean? Really high ES? Big jumps between different powder charges? It is a bit hard to give a specific answer without knowing the details.

Second, you mentioned that you only have 75 rounds down the barrel. In my experience, it takes about 100-150 rounds before a barrel “settles down”. During those first rounds, it will tend to speed up as the throat is fire-polished and all the little imperfections are smoothed out or filled in. If you have a hand-lapped barrel it may take fewer rounds.