Correct OBT Barrel Lenght *With* Muzzle Device?

[Grump]

When a rifle has a flash suppressor or muzzle brake installed, does one's search for an OBT load require that the calculations go to the end of the barrel length only, or to the end of the muzzle device?

And is using the OBT barrel time charts and QuickLOAD calculations/predictions even of any use if the rifle in question is a semiauto?

Yes, I have seen the time/bullet travel charts for a few of them and understand that the bullet really has passed the muzzle long before the bolt starts to unlock. But I do wonder if the impulses of gas tapping and starting to pressure up the gas system (either piston OR DI) during bullet travel overpowers the chances of successfully predicting an OBT load.

Actual experiences in predicting OBT, whether successful, inconclusive or failure, are preferred.

Alternatively, experimentally-proven OCW loads out of any semi-auto are welcome. I could then do some retrospective calculations.

Thanks!

 

[MALLARD]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

the gas blow back is irrelevant.

i would favor using bbl lengths that measured from the front of my bolt face to end the of the muzzle device.

 

[Grump]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

Beuhler???

It's been more than 12 hours. I'm getting impatient.

Thanks, Mallard.

 

[Temp9]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Temp9</div><div class="ubbcode-body">I didn't thoroughly read the paper, but I believe the answer is NO, it will not affect the OBT as described by Mr. Long. It may affect the bullet in other ways, such as the harmonics of the barrel. Consider this quote from the "Optimal Barrel Time Paper" by Chris Long:


"The pressure pulse from the gasses in the chamber<span style="font-weight: bold"> cause a traveling wave of stress</span> that bounces back and forth along the barrel between receiver and muzzle, slightly changing the bore diameter in the process. Minimum dispersion of the shots will result <span style="font-weight: bold">when the rate of change of the bore diameter is at a minimum</span>, and this dispersion will present the least sensitivity to load variations (charge, seating depth)."


It isn't the pressure wave itself which we are worried about, but rather the accompanying stresses in the barrel. Once the bullet exits the muzzle, these barrel stresses can no longer act on the bullet. The pressure wave will follow the bullet until it exits the muzzle device, but the stress wave stays in the barrel itself.

I will also note that I have been using the OBT method with QL with my AR, which is wearing a brake. My primary testing is done with an OCW test, and I verify my results with OBT/QL. Both my current loads (55gr, 77gr) landed dead nuts on a predicted OBT node.</div></div>

I quote myself here. Lame, I know, but this is my take on muzzle devices and OBT. The barrel, as far as Chris Long's barrel time theory is concerned, ends when the barrel no longer has mechanical contact on the bullet. The shockwave in question is not a pressure gas wave, but a mechanical wave through the barrel steel itself. Gas leaking through the gas port shouldn't affect the OBT, although the presence of a hole in the barrel might. I've not found this to be the case, though.

 

[MitchAlsup]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body">When a rifle has a flash suppressor or muzzle brake installed, does one's search for an OBT load require that the calculations go to the end of the barrel length only, or to the end of the muzzle device?

And is using the OBT barrel time charts and QuickLOAD calculations/predictions even of any use if the rifle in question is a semiauto?

Yes, I have seen the time/bullet travel charts for a few of them and understand that the bullet really has passed the muzzle long before the bolt starts to unlock. But I do wonder if the impulses of gas tapping and starting to pressure up the gas system (either piston OR DI) during bullet travel overpowers the chances of successfully predicting an OBT load.</div></div>

OBT theory is based on a vibration wave traveling up and down the barrel. There is a reflection at the muzzle end and another reflection at the action end.

The reflection at the muzzle end is well defined (sans supressor/brake). The reflection at the action end is not so well defined. By well defined i mean there the reflected wave looks just like the incident wave, in displacement, time and shape; just traveling in the opposite direction. This reflection occurs as pressure has no way to continue going forward and the springiness of the steel streches and then releases the wave back into the barrel.

The reflection at the action end is less well defined. Some of the wave energy reflects at the actioin. But some of the wave enters the action. There is an impedance mismatch at the joining point. The wave that does enter the action will eventually reach theback end of the action (tang) and reflect also. Some of this reflection gets into the barrel. So the shape of the reflected wave at the action end is "more complicated".

I suggest that with a supressor or brake on the muzzle end, the reflection becomes "more complicated".

A more complicated reflection at both ends does in no way obviate the OBT theory, it just makes the OBT theory useful as a starting point. After getting this starting pint, you then use OCW or other good load development techniques to "finish the job".

 

[Grump]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

Thanks all.

Thinking some of it through, the wave going forward will go farther than the muzzle before heading back.

And likely *may* pick up an early back-reflection from the barrel/device interface. Whether that most likely observable sub-wave would have any significant effect is another thing. I believe perhaps not, but I have not studied the amplitude of all this clanging around.

THEN the muzzle release of the bullet would happen "so many inches/mm" short of the end of the muzzle device, which would introduce a -X% element to the barrel time, precisely obtainable through some calculus but I'd rather just plot the distance/time curve for the load. Hmmm, if the FS is 3 inches longer and the barrel is 22 inches then the *distance* difference would be an even -12%. IF the speed at say 2600 fps were constant from 22 to 25 inches, then the OBT calculations for a 25-inch to accurately track the reflections would be a much smaller -9.615384615384615e-5, or .000096 of a second, or about 9.6 milliseconds sooner than for a 25-inch barrel. Better check my conversion from scientific notation and my math, that sounds LONG.

But since QuickLOAD ignores the muzzle device, I guess I could read the OBT chart for 25 inches, subtract the travel time difference from QL's 22- and 25-inch numbers, and use that as my target barrel time value for OBT.

I'm really curious about this because I tried to predict OBT a few years ago when exploring 155-gr bullets in the M1A, and had completely inconclusive results...mainly because I just couldn't reliably get the same accuracy that I was getting from the Hornady factory load.

Off to crunch numbers and check calibration tonight...

 

[Fred_C_Dobbs]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

I E-mailed Chris and asked him the same question, except regarding a suppressor. This is from his reply:

"...[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...."

 

[MALLARD]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

awhile back i asked chris long about tuners and OBT , this was his reply. ( hope he doesnt mind.)

"Thanks for the note. I have found that while the addition of a threaded (or clamped) mass on the end of a barrel (like a tuner or a muzzle brake) definitely changes the bending mode timing(s) in the barrel, it does not change the longitudinal wave timing. The reason for this is that a threaded or clamped connection simply does not allow much if any energy from the longitudinal wave to couple into the tuner/brake. This holds true at the receiver as well. The threaded joint is too compliant to pass much of the the shock wave. The result is that for this mechanical wave type, the effective barrel length is not affected by the addition of a tuner. So, you will find the minimum group dispersion when the exit time is aligned with one of the OBTs for that length barrel. However, the beam-bending modes are still there (al la Varmint Al), and the tuner definitely affects them. In fact, what you will see is that to get a really good tune, you have to meet both the OBT and the bending mode optimum exit times, and the tuner allows you to nail the bending mode timing. However, if you are not at an OBT, the tuner will not allow you to correct that exit time to shock wave alignment. I see most folks using the tuner to get out that last bit of vertical. What is happening is that they are at an OBT (especially in BR, most everyone runs about the same velocities and powders in the 6PPC, so they are all running an OBT load), but there might still exist a bit of mis-timing relative to the bending modes. The tuner allows them to correct this. A way that you can tell what mode is mis-timed is that shot dispersions caused by off-OBT conditions are usually round (random dispersion in the radial sense), but when off time for bending modes, the groups are often linear in some direction. A great rifle is one where the load that achieves and OBT also has optimum timing for the bending modes. I believe that this is why the BR folk use that 21 ¾” barrel length so much with the 6PPC."

 

[Fred_C_Dobbs]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MALLARD</div><div class="ubbcode-body">...A great rifle is one where the load that achieves and OBT also has optimum timing for the bending modes. I believe that this is why the BR folk use that 21 ¾” barrel length so much with the 6PPC." </div></div>
Speaking of which, I had another exchange with Chris concerning the story Secrets of the Houston Warehouse, which AFAIK is the original source of that 21 ¾” barrel length. I asked if he'd ever come across the SOTHW and, if so, could reconcile the "facts" of that story with the theory behind OBT. He said he had read it and he figured they essentially were doing OBT <span style="font-style: italic">backwards</span>, that they started with a fixed max load (or close to it) and then trimmed the barrel until its length produced an optimum barrel time.

 

[dbooksta]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

What about on the breech end? Should barrel extensions, which are usually heavy, torqued, and squared up to the barrel, be included or excluded from barrel length for OBT?

 

[Fred_C_Dobbs]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">What about on the breech end? Should barrel extensions, which are usually heavy, torqued, and squared up to the barrel, be included or excluded from barrel length for OBT? </div></div>
The application will answer your question if you left-click in the barrel length box.

 

[dbooksta]

Re: Correct OBT Barrel Lenght *With* Muzzle Device?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fred_C_Dobbs</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dbooksta</div><div class="ubbcode-body">What about on the breech end? Should barrel extensions, which are usually heavy, torqued, and squared up to the barrel, be included or excluded from barrel length for OBT? </div></div>
The application will answer your question if you left-click in the barrel length box. </div></div>

You mean QuickLOAD? This isn't about QuickLOAD, it's about OBT theory. The question is whether the barrel extension counts towards the length of the barrel for determining the "nodes," or optimal times after ignition for bullet to leave muzzle.

(I suppose it also complicates the problem: If the answer is that extensions should be included then the nodes would have to be recalculated since the impulse originates at a midpoint instead of an endpoint of the barrel.)