Range Report OBT and different barrel lengths

[WDL]

I think some you here use QuickLoad and know about Chris Long's OBT theory. So herre is a topic I posted over at the WSSMzone forum.

Earlier today, I was researching what twist and length barrel I would like to go to if I re-barrel my 223 wssm to a 243 wssm.
So I worked up a load with QuickLoad to hit node 3 (OBT - Optimum Barrel Time) in a 24" barrel. Then I adjusted the charge to hit the same node in a 25" barrel, and then a 26" barrel. I had some interesting results.
As the barrel gets longer, naturally the barrel time gets longer as well as the node. But the charge gets reduced and the velocity reduces slightly to stay on the node. From 24" to 26", the velocity only slowed by 10 feet per second BUT pressure decreased by 4500 PSI.
I was initially going to go with a 24" barrel. QL says I would lose 62 fps with the charge remaining the same. But I believe I would gain accuracy. But after seeing the QL and OBT results, I will be going with a 26" barrel.

 

[kombayotch]

Re: OBT and different barrel lengths

I did a similar QL analysis for 308 Win. and found that in barrels between 22" and 28", there was very little velocity difference when loading to the OBT nodes for loads with Varget and bullets between 155 gr. and 190 gr. When I tested this in my 22" Broughton, 26" Bartlein and my buddy's 28" Kreiger, that is what I found. I only lost 20 fps going from 26" down to 22" and both my barrels were running faster than his 28" Kreiger at the nodes.

I measure the pressure of my loads using Pressure Trace. Look at a pressure curve:

The peak pressure is reached long before the bullet gets to the end of the barrel. It peaks around 1/4 of the way down the barrel. It then drops at an exponential rate from that point. The 26" barrel will have a lower pressure at the muzzle than the 24" barrel, but peak pressures will be the same (all other things being equal). This is meaningless even when considering the use of a suppressor because the suppressor will be designed with a safety factor several times higher than the muzzle pressures either of the lengths will generate, if that suppressor is rated for that caliber.

I would choose the shorter barrel, if it were me...

 

[WDL]

Re: OBT and different barrel lengths

Very interesting. And you are doing actual, where I am doing theoretical.
If I am looking at your chart correctly, you kept the charge constant, yet you had nearly a 4000 PSI difference between T1 and T4. I would also like to see results with a slower burning powder. Then I think your muzzle velocity would vary more.
In my work-up, I varied the charge to stay on the OBT node as I changed barrel length. And I found a reduction of 4500 PSI while only losing 10 fps with the 26" barrel. I was originally planning on going with a 24" barrel, but I really like that 4500 PSI reduction.

 

[MALLARD]

Re: OBT and different barrel lengths

nice post

 

[MitchAlsup]

Re: OBT and different barrel lengths

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kombayotch</div><div class="ubbcode-body">The peak pressure is reached long before the bullet gets to the end of the barrel. It peaks around 1/4 of the way down the barrel. It then drops at an exponential rate from that point. </div></div>

The rate of decrease in pressures is between 1/x**4 (quartic) and 1/x**5 (quintic) which is far from 1/e**x. That is it is not exponentially decreasing.

Rest of the post was fabulous.

 

[WDL]

Re: OBT and different barrel lengths

kombayotch, was the T4 trace the 28" Kreiger?

 

[kombayotch]

Re: OBT and different barrel lengths

This was all with the 26" barrel.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kombayotch</div><div class="ubbcode-body">The peak pressure is reached long before the bullet gets to the end of the barrel. It peaks around 1/4 of the way down the barrel. It then drops at an exponential rate from that point. </div></div>

The rate of decrease in pressures is between 1/x**4 (quartic) and 1/x**5 (quintic) which is far from 1/e**x. That is it is not exponentially decreasing.

Rest of the post was fabulous. </div></div>

Lol! Probably... I just meant that it was non-linear and that the rate of decay became less drastic as time progressed.

Exponential sounds so much more dramatic though...

 

[WDL]

Re: OBT and different barrel lengths

I just exchanged emails with Chris Long and he told me he has done similar stuff. He created a graph to determine what barrel length would put him on a node for a given charge with his 6mm Dasher.
He said he had discovered that shortening a barrel would require a slight increase in charge to remain on the node and velocity would increase slightly. Of course, the opposite is true when the barrel is lengthened.

 

[MitchAlsup]

Re: OBT and different barrel lengths

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kombayotch</div><div class="ubbcode-body">This was all with the 26" barrel.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kombayotch</div><div class="ubbcode-body">The peak pressure is reached long before the bullet gets to the end of the barrel. It peaks around 1/4 of the way down the barrel. It then drops at an exponential rate from that point. </div></div>

The rate of decrease in pressures is between 1/x**4 (quartic) and 1/x**5 (quintic) which is far from 1/e**x. That is it is not exponentially decreasing.

Rest of the post was fabulous. </div></div>

Lol! Probably... I just meant that it was non-linear and that the rate of decay became less drastic as time progressed.

Exponential sounds so much more dramatic though... </div></div>

Exponential does sound dramatic, and that's why the uninformed tend to use it in a hyperbolic (pun intended) sense.