OBT method, barrel length vs velocity

[Tripodmvr7]

Herewith a QL process in helping someone with load development with a Grendel and 108 Lapua Scenar bullets. Propellant is our local S321 from Somchem.

I first ran a calc to get on a node with a safe charge and gave him 27gr as an intermediate start load. He then shot a mini ladder at 200m. The ladder was his choice. Chrony at 5m so I have added 10fps to the readings.

I then calculated Node 6 at a charge of 27,2gr and advised him to check loads from 27,2/8 to see which performs best. This advice stems from combining his ladder data where two charges gave identical speeds. This is an indication of a node. The QL prediction is very close, but the practical might show a slight difference.

 

[Mikecr]

Ok, there is a vibration born from max pressure peak that travels back and forth at the speed of sound in barrel steel. Theoretically, a major element of this opens the muzzle, while there, and this would be detrimental to consistent bullet release. Considering the research by Harold Vaughn, as published in Rifle Accuracy Facts, It is reasonable to avoid muzzle release at one of the peaks pictured. That is, better to be somewhere between them.

As to predicting troughs and peaks(supposed nodes) in between the major peaks,, that I gotta call boarsheet on.
There are all kinds of vibrations(including other modes, and sources) reflecting around, adding and subtracting with interference. All this is abstract and unknown.
With this, I'm telling you, that you are going to have to powder develop for best results -regardless of fun with math over barrel lengths.
POWDER ADJUSTMENT IS TUNING.

If you install a tuner on your gun, you can shape grouping a bit, but you will still have to powder develop for best results. So this is just what we do -without fretting over OBT.

 

[asd9055]

Mike,
Has anyone ever suggested that, no matter which method you follow, (OCW, Ladder, Satterlee, Ladder with MV, Modified ladder, combination of any, QL, and many I have never heard off) there is a way not to do a "powder development" and fine tune? I have never seen one. I don't remember anyone suggesting one. The only difference with ALL the methods, is where you start and how much powder you burn getting there.
I appears you believe in barrel harmonics, but you don't believe they can be predicted. No problem.

 

[Mikecr]

appears you believe in barrel harmonics, but you don't believe they can be predicted.

That is correct. Just like actual tuning results cannot be predicted.
Press a thumb against the action tang amidst group shooting, and tell me what that did to the OBT. Nothing right? No entry for that in the calcs.
Develop a load off bags filled with sand. Then shoot off bags filled with kitty litter. Why the change in results with the same load, same barrel, and same speed of sound in that barrel?
When you adjust bullet seating, the single biggest affect to precision, and impossible to predict, why the results while it's not changing barrel time?

A good shooting load can be simplified or complicated.
But, it always ends with load development under use conditions, to find the results you're after -wherever they happen to be.

 

[asd9055]

That is correct. Just like actual tuning results cannot be predicted.
Press a thumb against the action tang amidst group shooting, and tell me what that did to the OBT. Nothing right? No entry for that in the calcs.
Develop a load off bags filled with sand. Then shoot off bags filled with kitty litter. Why the change in results with the same load, same barrel, and same speed of sound in that barrel?
When you adjust bullet seating, the single biggest affect to precision, and impossible to predict, why the results while it's not changing barrel time?

A good shooting load can be simplified or complicated.
But, it always ends with load development under use conditions, to find the results you're after -wherever they happen to be.

Not wasting my time with you anymore. Wind changes, bullet hits elsewhere, what happened to barrel time...that is your logic. There are many things that affect where the bullet hits, interior ballistics and exterior ballistics...you just trying to find excuses to support your fantasy...good luck

 

[Stammster]

TripodMVR7 - as requested

Gun:	Browning BAR
Caliber:	270 Win
Barrel Length:	20.5" (as measured)
Brass:	Hornady
Case Capacity:	70.95 gr (avg) @ 2.532"
Bullet:	Hornady 130gr SST (27302)
Primer:	Win WLR (Lot# PGL687G)
Powder:	Alliant RL26 (Lot# 0819Z013120)
COAL:	3.230" (seated at canalure)
CBTO:	2.631" (0.080" jump)
Velocity Recording:	Magnetospeed Sportster
Temp:	75F
Charge Weight	Velocity (3 shot avg - fps)	Group Size (3 shot - inches) @ 100 yds	OBT Time (ms calc)
59.5	2996	0.70	1.089
60	3024	1.20	1.074
60.5	3045	0.91	1.060
61	3070	1.98	1.046
61.5	3093	0.99	1.030
62	3125	1.28	1.016
62.5	3148	1.98	1.002
63	3179	0.80	0.988
63.5	3214	1.87	0.975
QL Inputs
Weighting factor:	0.456​
RL 26 Ba (calculated)	0.365​

The above actual data seems to match QL pretty well. This lot of powder has also given the same burn rate in other loads (130 NBT, 145 ELD-X, etc.).

By interpolating between the 20" and 21" bbl, I come up with an ideal OBT time of 1.051 ms (node 5) at 3060 fps. 61 gr gave 3070 fps @ 1.046 ms. As you can see, that gave a terrible group size amongst the surrounding charges. 63.5 had a called flyer, otherwise it would have been smaller. All groups were typically triangular shape.

Case capacity as measured, 70.95 gr, with a spread between 70.8 and 71.2 gr over qty 10 twice fired (non-resized) brass.

All shots were off a rock solid bench, using a led sled. Wind was +/- 10 mph.
Note: 63.5 charge is just over than SAAMI max 65K psi, but I didn't see any pressure signs. At 63.8 gr I saw some minor signs after the first shot at 3245 fps, so I stopped and pulled those loads. Calculated pressure 68Kpsi.

 

[Tripodmvr7]

1. You will see that lowering the Weighting factor gives you lower pressure. I do not touch that figure unless I have accurate pressure data from a reliable source and advise as to not play with that.
2. The case capacity is quite larger than the default 67gr.
3. The safest Node is No. 5 and 57K psi will ensure good combustion and case sealing.
4. Node 5 at 1,051ms relates to 60,6gr and speed of 3050fps.

 

[Tripodmvr7]

Quick Load is not a wonder child that gives you accurate loads when you enter figures, but herewith the points where it does shine.

1. It can predict speeds from charges and the known parameters of the rifle to within say 50fps. This enables you to skip starting loads and use an intermediate load with safe pressures. From there you can calculate to within less than 20fps with the info gathered from shooting. Shooting confirms the calculated results and now you can give a better prediction of the nodes and see if that is true by shooting again.
2. Deciding on barrel length is easier if you know what speed you want to achieve and using QL. Although it predicts pressure this is not 100% accurate and erring on the safe side is advised.
3. Deciding on propellants is also easier as pressure and speed are supplied. It also gives case fill and amount of propellant burnt.
4. As a tool to analyze different components it is very good if sensible decisions are taken. Loading to max pressure is not my idea of a sound decision and I try to stay in the 50-55K psi range.
5. It saves on components and range time. I have gotten a friend of mine's 375 onto an accurate node with 8 shots. Good enough to go hunting. One day was taken up with this.

GIGO is very true with QL and sound judgement is necessary. Actual shooting confirms your predictions.

 

[Stammster]

Thanks.
Questions to ponder
1.). So why the bad group so close to the ideal?
2.). 3050 fps +/- 10 fps seems to be the recommended "ideal" OBT based muzzle velocity for this node. My original argument / question was around this being the same for multiple barrel lengths. So why not just simplify it to that. Let people strap on a chronograph and aim for a charge weight that gets them there.

Why not simply just give the following:
Node 4: 3270fps
Node 5: 3060fps
Node 6: 2930 fps
Etc.
Am I trying to oversimplify things? Perhaps, but IMO so are OBT times...but in reality it's just appears to boil down to the best velocity to target - essentially independent of barrel length. So no need to even involve time - as it seems to cancel out.

I'm also going to make some runs with other calibers and bullets weights to see if this crosses over to other calibers. Initially my 25-06 data is showing it's pretty close with node 2 at 3790 fps and node 3 at 3390-3460 fps and node 4 at 3220-3270 fps, with 85-100 gr bullets.

 

[Stammster]

50-55k is ok if your punching paper, and perhaps for a cartridge that's only good for 60k. With the 270 Win, I see some clear external ballistic advantages, especially energy and wind dope at long range by trying to accurately push into the 63-65k range (65k SAAMI rated).

FYI, This gun and load is not my long range setup, and in fact I've settled on 59.5 gr for Texas deer and hogs. It shoots continuously less than 1 MOA at 3000 fps. The OBT node recommended at 60.5-61 gr didn't. I also have a good load with 60 gr H4831SC that shoots well at 2970 - 2990 fps.

Plus at my normal ranges of 50-200 yds, I didn't want to push these cup and core bullets at too hard of an impact velocity.

 

[rammac]

None of you guys seem to know how to use Quickload to it's best advantage, you don't just look at barrel time values.

I compare;
The pressure curve graph data
Fill percentage
Chamber pressure
Barrel Time
Bullet Travel at Pmax
Ballistic Efficiency

At a minimum I look at the pressure graph, chamber pressure, and the barrel time.

Getting the bullet to exit the muzzle at the right time isn't the entire picture, getting it to exit at the right time along with the proper burn characteristics is. The Z1 and Pmax values must be as close together as possible and they must coincide with the proper barrel time.

The Z1 line represents when the powder's burn characteristic changes from being progressive to being degressive, in other words, it's when the powder's burn rate can no longer keep producing enough gas to overcome the increasing bore volume as the bullet moves along the length of the barrel. Pmax is obvious. When you get Pmax and Z1 to match you get the least amount of gas pressure disturbance, it's like having the proper follow through when you throw a ball, the smoother the release the more accurate you will be.

 

[asd9055]

50-55k is ok if your punching paper, and perhaps for a cartridge that's only good for 60k. With the 270 Win, I see some clear external ballistic advantages, especially energy and wind dope at long range by trying to accurately push into the 63-65k range (65k SAAMI rated).

FYI, This gun and load is not my long range setup, and in fact I've settled on 59.5 gr for Texas deer and hogs. It shoots continuously less than 1 MOA at 3000 fps. The OBT node recommended at 60.5-61 gr didn't. I also have a good load with 60 gr H4831SC that shoots well at 2970 - 2990 fps.

Plus at my normal ranges of 50-200 yds, I didn't want to push these cup and core bullets at too hard of an impact velocity.

I too load to SAAMI typically. It is interesting what you are finding. In my mind there are a couple of things.

1. I use QL with every load. IMHO, QL can not account for the different barrel contours, which would give different harmonics. So I take what QL suggests and work around that load to fine tune. Tune it the barrel/rifle/bullet/powder!
If you look at FEA (finite element analysis), you will see how barrel harmonics can be different, and in fact that is why people used various methods of barrel tuning, way back it was just a rubber ring on the barrel, the the Browning BOSS, and now many others on the market. A retired Lawrence Livermore National Laboratory (engineer?) has some fine examples on line.
2. On the same line IMHO opinion is OBT. I believe in OBT. When Christopher Long came out with it, I am not sure if he considered all the different barrel contours and weights. It does make a difference. So I treat is as an approximate location for a node.
3. Bullet stability and external ballistics

I wills till use QL. With some of my rifles is right on. But some times I have to work more than others. Still it is better than the alternative.

 

[asd9055]

None of you guys seem to know how to use Quickload to it's best advantage, you don't just look at barrel time values.

I compare;
The pressure curve graph data
Fill percentage
Chamber pressure
Barrel Time
Bullet Travel at Pmax
Ballistic Efficiency

At a minimum I look at the pressure graph, chamber pressure, and the barrel time.

Getting the bullet to exit the muzzle at the right time isn't the entire picture, getting it to exit at the right time along with the proper burn characteristics is. The Z1 and Pmax values must be as close together as possible and they must coincide with the proper barrel time.
View attachment 219235

The Z1 line represents when the powder's burn characteristic changes from being progressive to being degressive, in other words, it's when the powder's burn rate can no longer keep producing enough gas to overcome the increasing bore volume as the bullet moves along the length of the barrel. Pmax is obvious. When you get Pmax and Z1 to match you get the least amount of gas pressure disturbance, it's like having the proper follow through when you throw a ball, the smoother the release the more accurate you will be.

I am still learning...I know I have ways to go.

 

[Tripodmvr7]

50-55k is ok if your punching paper, and perhaps for a cartridge that's only good for 60k. With the 270 Win, I see some clear external ballistic advantages, especially energy and wind dope at long range by trying to accurately push into the 63-65k range (65k SAAMI rated).

FYI, This gun and load is not my long range setup, and in fact I've settled on 59.5 gr for Texas deer and hogs. It shoots continuously less than 1 MOA at 3000 fps. The OBT node recommended at 60.5-61 gr didn't. I also have a good load with 60 gr H4831SC that shoots well at 2970 - 2990 fps.

Plus at my normal ranges of 50-200 yds, I didn't want to push these cup and core bullets at too hard of an impact velocity.

You have a 20.5 inch barrel. A speed of 3000+ is really good out of such a barrel. Think about driving with your car. You don't keep the revs at maximum when driving as it saves on wear and tear. Running at 65K psi puts extra stress on your lugs and cases. Rather use a longer barrel to get extra pace.