Internal ballistics and shockwave theory.

[Robojaz]

Good morning. I am new to this forum this being my first post and would like to thank all in advance for their advice.

I recently read a paper on internal ballistics and shockwave theory by Christopher Long and have a couple of questions that no doubt have been asked many times before.

I have yet to test the theory on the range on one of my own rifles but have gone as far as calibrating Quickload to a powder/projectile combination for a Rem 700 SPS tactical chambered in .223. From this using the guidance from the paper I have determine an optimal charge that I have yet to test.

If I am successful I want to apply the theory to a 7mm Rem Mag that I use for hunting.

My question is about what to do in terms of barrel length If the rifle is suppressed with an overbarrel suppressor? (Note. I am from New Zealand and supressors are readily available and common on hunting rifles here)

I am guessing that, once the projectile has left the muzzle, but is still travelling through the suppressor, it is no longer under the effect of barrel harmonics, aside from the small change in Velocity that occurs when using a suppressor. However, wouldn't the shock wave continue to the end of the suppressor before returning back up the barrel?

My dilemma is what to do to in order to determine what node/barrel length and ultimately the correct Optimal Barrel Time to use, if for example the bullet leaves the muzzle at 24" but the shockwave continues to the end of the suppressor, an additional 8"?

Also, do I need to make any adjustments to account for Velocity changes that result from suppressor use?

Any assistance would be much appreciated and thank you in advance.

 

[Rust]

The probable largest factor will be the added weight of the suppressor on the end of the barrel which will change the characteristic movement of the barrel in the same manner a barrel tuner does.

Once the bullet exits the muzzle barrel, it has no further influence on the bullet. You likely will have to develop a load for use with a suppressor for best accuracy.

You might check this for some analysis, http://varmintal.com/aeste.htm

 

[Ned Ludd]

Ideally, you'd do the load development without the suppressor attached, then tweak it after putting it back on using seating depth. But you can probably do the whole thing with it attached if necessary. It's OBT theory, so you can forget about the tuner-like effect of the suppressor, that's not what OBT theory is really aimed at optimizing. It's all about the longitudinal shockwaves, which I strongly doubt will be propagated through the suppressor in a way that is meaningful to tuning the load. Certainly the bore of the suppressor is large enough so as not to impede bullet movement after it has cleared the muzzle. Use your actual barrel length as measured from muzzle to boldface, and whatever velocities you obtain (with the suppressor attached, if you choose to do it that way).

In my hands, once I have "calibrated" QuickLoad with a known charge weight from a specific Lot of powder/bullet/primer/brass, the final OBT loads typically end up very close to the prediction OBT loads (i.e. usually within 0.1 gr or so). I find the actual barrel time of optimized loads normally runs just a tick slower than the values in the table. For example, I might see 1.371 to 1.376 ms for a load corresponding to Node 4 for a 30" barrel (predicted OBT = 1.3684 ms). However, at that point, the target and MV data are going to be the readouts, not the predicted barrel time from QuickLoad. Low ES/SD usually correspond to the best groups in my hands, but if not, the groups will be tuned using seating depth after the optimal charge weight has been identified. So find the charge weight first, then do the final tuning with seating depth. Obviously in your case that will need to be done with the suppressor attached. There is no doubt that the suppressor will have significant effects on barrel harmonics, but the initial part of the OBT methods is aimed at barrel timing and the shockwave, as well as optimizing charge weight for those processes, not "classical" barrel harmonics. You should be able to tune for those effects using setting depth.

 

[Robojaz]

Thanks and Great, advice. I think I will run the initial calculations without suppressor and see what groupings I get. As you say it's about finding OBT. Once I have that I will put on suppressor and see what difference it makes and fine tune with seating depth as you recommend.

I am still interested to see if longitudinal shockwaves transfer from barrel to suppressor, in effect acting mathematically like a longer barrel and changing timing, but that is only to satisfy my own curiosity.

Fired the .223 loads yesterday and all produced sub 1/4 MOA groups with some being nice tight 5 shot cloverleaf groups. Very keen to apply to my other rifles, takes majority of guess work out.

 

[Homerange]

Thanks and Great, advice. I think I will run the initial calculations without suppressor and see what groupings I get. As you say it's about finding OBT. Once I have that I will put on suppressor and see what difference it makes and fine tune with seating depth as you recommend.

I am still interested to see if longitudinal shockwaves transfer from barrel to suppressor, in effect acting mathematically like a longer barrel and changing timing, but that is only to satisfy my own curiosity.

Fired the .223 loads yesterday and all produced sub 1/4 MOA groups with some being nice tight 5 shot cloverleaf groups. Very keen to apply to my other rifles, takes majority of guess work out.

If you do workups without the can they won't necessarily be applicable to when it's on......think 2 rifles, one with the can and the other without. If you intend to use the rifle 100% with the can on, do your workups to suit or you'll end up chasing your tail.
Every rifle I own that has a can obtains better grouping with it on.

 

[mikecr]

Keep in mind that optimum barrel time has nothing to do with load tune.
OBT is merely identifying timing points of undesired muzzle expansion(mechanical).
Then any timing steering clear of that is described as 'optimum'.

But standard load development result choices should steer you away from bad timing, including timings other than OBT.

 

[Canadian bushman]

Keep in mind that optimum barrel time has nothing to do with load tune.
OBT is merely identifying timing points of undesired muzzle expansion(mechanical).
Then any timing steering clear of that is described as 'optimum'.

But standard load development result choices should steer you away from bad timing, including timings other than OBT.

+1

Regardless of how accurate any individual's OBT or OCW predictions can be, certain aspects of load development must be learned from the target.

 

[Robojaz]

Completely agree. However, if a few 'simple' calcs can get you to a good starting point, through comparing different loads, then why not.