Range Report POI Shift with brake. Big difference.

[MDM]

Posting about a friend's rifle. He is shooting a 6.5-284 at a mild load with velocity right around 2800 fps, 142smk's. We were shooting at some steel at 785 yards. His JBM drop chart called for somewhere around 19 moa, but it took 25.5 moa to get on target. His drop chart was accurate last year and he had not shot it since then. He did just have a Harrel's brake installed.

What could be causing this? When he zeroed at 100, the difference with and without the brake was only .25". This has us stumped. I told him to check to see if the bullet could be contacting the brake on exit. That is the only thing I can come up with. Any ideas?

 

[KNIGHT11B4]

Re: POI Shift with brake. Big difference.

What optic and ammo?

Have you box tested to optic to ensure it is tracking properly? Did you shoot at any other distances?

That is much more elevation then it should take.

 

[MDM]

Re: POI Shift with brake. Big difference.

Handloads. 8-32 Burris Black Diamond. We shot at 380, 460 and 785. It took about 10.75 moa at 460, so yeah, atleast a couple minutes off at that distance. It should have been only taken 7-8 minutes at 460. I had to back the velocity on the program down to like 2460 for the drop chart to jive with what the gun was doing and there is no way in hell it is that slow. Chrony reads 2800 avg. Like I said, the only thing I can think of is the bullet is contacting the brake.

 

[KNIGHT11B4]

Re: POI Shift with brake. Big difference.

If the bullet were contacting the brake you should be able to tell.

I doubt that the flight of the projo would be repeatable enough to hit the target if it were contacting the brake. I would expect some other serious problems.

If it were me I would; Inspect the brake (but I doubt that is the issue), Box test the scope, and check the handloads.

If none of that is the problem then remove the brake and recheck.

 

[ggmanning]

Re: POI Shift with brake. Big difference.

I have brakes for 2 of my rifles. I take them on and off in favor of a threaded cap or a QD flash suppessor for a can. I see no POI change between the 3. I would also eliminate the scope repeatability and mount/rings screws. Did he shoot the rifle without the brake to compare?

 

[desertHK]

Re: POI Shift with brake. Big difference.

Is the brake properly timed?

 

[MDM]

Re: POI Shift with brake. Big difference.

I don't know. We cleaned the brake and barrel with Wipeout tonight. There was a little bit of copper in the barrel - light blue, but the brake - heavy dark blue came off the brake. I mean DARK blue and huge amounts.

 

[ryhag1]

Re: POI Shift with brake. Big difference.

Interested to see whats going on. I know the brake will change the harmonics but that much is surprising.

 

[former naval person]

Re: POI Shift with brake. Big difference.

Next time cleaning, clean with a carbon remover first, then inspect the inside of the brake for strike marks.

 

[winxp_man]

Re: POI Shift with brake. Big difference.

Ok how is he getting his velocities ? Is there a sure thing that he is at 2800 FPS ?

I have recently shot my .308 a 300 yards and I'm running 2600 FPS for 175 SMK in my rifle. The iPhone Ballistic FTE was right on not a drop less or more. I was shooting at clay pigeons all day and was dead on. I will say that if the brake is of quality its GTG. I personally have a SPS Tactical 20" barrel with a Roedale brake.


Aron-

 

[Greg Langelius *]

Re: POI Shift with brake. Big difference.

Since the bullet is out of the barrel by the time the brake begins to take effect, it's unlikely that the work the brake does would be affecting bullet flight.

If it is affecting POI, the more likely cause might be due to its added mass to the system.

The mass at the end of the barrel may be (and this is fairly likely) affecting harmonics.

Also the simple addition of the brake's mass to the system's total mass may be retarding recoil, slowing down muzzle flip, causing the bore orientation to be lower as the bullet exits.

What may seem like a big difference isn't really. If the difference in zero from 100yd to 1000yd is 30MOA, that's still only 1/2 of a degree.

But far more basically, it's a simple rule. If you alter the system; you should expect a corresponding alteration of POI.

Greg

 

[MDM]

Re: POI Shift with brake. Big difference.

Hi is going to try working up his load again. He is thinking the added weight may have messed with the barrel harmonics.

Is it possible for copper to build up in the brake even if the bullet is clearing the brake? I don't get any in mine is why I'm asking. His barrel is a factory Savage so it is probably pretty rough yet.

Velocity was verified over 2 seperate chronographs.

 

[427Cobra]

Re: POI Shift with brake. Big difference.

I bet the barrel is toast, 6.5-284 even with mild loads is a barrel burner, adding wieght is not going to jack the barrel harmonics up 6moa, have him(you) shoot it with a know good load thru a chronograph with and without the brake.

 

[MDM]

Re: POI Shift with brake. Big difference.

It only has 200 slow fire rounds thru it. It is not grouping at 100 like it was, before the brake, so he is going to work up the loads again.

 

[Greg Langelius *]

Re: POI Shift with brake. Big difference.

Before I did anything else, I'd fiddle with the load, to see if harmonics is an issue.

I agree that you're using a very light load; my 28" .260 gets 50FPS more with 142's. Under these circumstances, when barrel replacement time arrives, it might be a good move to do the replacement with a longish .260. L-W sells replacement-ready .260 and 6.5-284 barrels for the Savage as a regular stock order item. I have a pair of .260's of theirs, and they're superb.

Bullet strikes would probably leave a clearly defined copper streak somewhere inside the brake's exit opening.

Greg

 

[tomcatfan]

Re: POI Shift with brake. Big difference.

Hey man just a heads up, I recently went to the range with my Savage 10 fp with a brake on it. A buddy of mine was complaining about the noise he was getting next to me, so I pulled the brake. I got about a .5 inch poi change at 100 yds consistently without the brake. If you think about it, it makes sense. There have been a couple of people here that have hit on the harmonics of the barrel, so I won't beat a dead horse. It really should make a difference.

 

[Greg Langelius *]

Re: POI Shift with brake. Big difference.

It may or it may not be due to a harmonics shift. A properly tuned brake is adjusted, just like a harmnics tuner, until POI returns to the same location.

Any added mass on the muzzle will affect harmonics; but if it's added with proper consideration regarding harmonics, the differences (with a specific load) can be minimal.

I would consider this to be one of the better reasons why brakes should be installed by folks who understand such issues.

Greg

 

[tomcatfan]

Re: POI Shift with brake. Big difference.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">It may or it may not be due to a harmonics shift. A properly tuned brake is adjusted, just like a harmnics tuner, until POI returns to the same location.

Any added mass on the muzzle will affect harmonics; but if it's added with proper consideration regarding harmonics, the differences (with a specific load) can be minimal.

I would consider this to be one of the better reasons why brakes should be installed by folks who understand such issues.

Greg </div></div>

This is an interesting point, but could you expand on how you would "tune" the brake to have a minimal impact on harmonics. There are a lot of vibrational characteristics that go into barrels, actions, etc. How do you tune the dampener (brake), its just an added mass.

 

[Greg Langelius *]

Re: POI Shift with brake. Big difference.

Short answer; change the point along the threads where the brake resides. By moving the mass fore or aft, the barrel's harmonic frequency can be adjusted. The larger the mass, the less distance it has to move to make a significant difference. Most brakes are actually a fair bit more massive than any harmonic barrel tuners with which I've experimented. The mass is adjusted in or out, and a corresponding group size variation should follow. Doesn't take whole lot, far less than many might believe. You just adjust and shoot, adjust and shoot, until you see the best improvement. Unfortunately, it can also be a fairly critical adjustment and environmental swings can affect it. That's why I stopped playing around with my tuner experiment. Things would get amazingly good; then next time out, not so much. Too much time, ammo, and bore life being expended on 'retuning'.

Greg

 

[bigngreen]

Re: POI Shift with brake. Big difference.

There should be no copper on or in your brake, pull it of and shoot it!

 

[Greg Langelius *]

Re: POI Shift with brake. Big difference.

D'oh!

I completely missed the part about the copper fouling coming out of the brake during cleaning.

Sounds more now like there's an interference between the brake and the bullet.

Insert a snug cleaning rod (you can wrap it evenly with masking tape to increase its diameter) into the muzzle and check to see that the rod/brake clearance is even and obvious all around the rod. If not, there' a real possibility of strikes.

 

[Fred_C_Dobbs]

Re: POI Shift with brake. Big difference.

A while back, I exchanged E-mails with Chris Long, father of the Optimum Barrel Time method of load development, about whether a suppressor would affect barrel timing. His answer, verbatim, follows:

"...[A]ll my research and experience shows that the mass of a suppressor (or a muzzle brake) will change the bending modes, and therefore the point of impact (group center) somewhat, but the group size still follows the OBT for that specific barrel length, less the length of the suppressor. The reason for this is that the shock wave does not couple into the suppressor or brake through the threads much if at all, and therefore the reflection is still at the muzzle, not the end of the suppressor. I bet if you fire it without the suppressor, the group size will be about the same, but shifted on the target."

 

[Greg Langelius *]

Re: POI Shift with brake. Big difference.

Jibes.