COAL and Velocity?

[RC20]

Is there any known correlation between how far from the lands you are and how fast you are pushing a bullet?

Or, if you shoot at lower velocities (reduced loads) does the bullet tend to be happier further from the lands or closer?

Or is it barrel specific if there is any relationship

I am working with reduced loads as well as higher velocity in 308 and it seems the lower loads may be agnostic and higher wants more setback form the lands.

Still looking for one solid baseline load that I can shoot consistently so I can verify if the others are better or worse.

 

[T. O'Heir]

Lower loads may be what? They don't believe in a deity? snicker.
Anyway, 'off the lands' has nothing to do with velocity. It's a load tweaking technique that can be done after finding the most accurate load out of your rifle with a specific bullet weight.
Every rifle has a preferred distance for the bullet to move before entering the rifling. Isn't about velocity though. Every chamber is slightly different. There's no formula for determining what the distance should be either. Strictly a trial and error thing.
Never seen much difference myself. Mind you, the only rifle I have that would possibly show any difference is my '03A4 and it shoots one hole groups anyway. Never bothered tweaking the load for it.

 

[RC20]

Badly lstaate4d.

I am working with both slow loads and faster loads.

Ergo curious if anyone had noticed a correlation that if a barrel had a tendency to prefer closer to lands with slow vs hot?

Just conjecture but some indications the 308 either likes them close with slower loads or does not care a lot, still wrinkling that part out.


I know hot loads vary and need to be played with.

If I was getting on hole groups at 100 yds I would not mess with the loads either!

Best I did was .2 at 110 and only once sadly.

 

[Rimfire5]

RC20

Sorry for the long post, but you are raising a complex issue and I think you might be mixing two concepts.

Seating a bullet out further to increase the COAL does change velocity with a given powder load and trim length for a given bullet because it increases the space available for the gases to expand in the cartridge before the bullet moves out of the neck. As you increase the COAL the velocity drops a bit.

Some rifles get best accuracy with the bullet seated close to the rifling. Most recommend 0.020 to allow some jump. Others claim that their rifles prefer the bullet to be seated right into the rifling to get the best accuracy.

However, when you seat a bullet into the rifling, the instantaneous pressure increases significantly because the bullet has to overcome the resistance of the rifling before it can move and release the pressure there is a spike in PMax. If you are already loading at the max pressure, that might get dangerous.

Using QuickLoad software, I checked the change for an increment increases of COAL for a .308 for different loads of Varget (it is a popular .308 powder choice) with two different bullets in a 24 inch barrel from a relatively light load at around 80% of PMax at SAAMI COAL of 2.800 to just over 100% of PMax at the same COAL.

Interestingly for the Hornady 168 grain AMAX and the Sierra 168 grain SMK, the percentage change per 0.020 increment of COAL was around 0.33% drop in velocity for each increment and the PMax also drops between 1.5 to 1.76% for the Hornady AMAX and 1.47 to 1.68% for the Sierra SMK.

Also, because the bullets are different lengths and shapes, for the same seating depth of 2.800, the SAAMI recommended COAL in all the manuals, the Hornady PMAX was 45.2 grains of Varget at 100.3% of PMax and the Sierra SMK was 46.1 grains of Varget at 100.1% of PMax.

The choice of bullet does make a difference so you really have to check for every bullet and powder combination you use.

Before you do any testing with extended COAL seating, I would recommend that you check your chamber depth. I have two rifles that are great shooters that came from the factory with very tight chambers and a SAAMI COAL was almost touching the rifling. I have others that have a 0.090 jump if the bullet is seated at SAMMI COAL. However, all of my rifles seem to shoot best with at least a 0.020 jump. Some of them shoot as accurately with a 0.080 jump as with a 0.020 jump. The velocity and the powder as well as the bullet weight and bullet shape make much more of a difference than COAL. I only start to play with COAL after I have found what bullet weight and shape shoots best in a particular rifle. The length of the bullet body touching the rifling seems to make an enormous difference in accuracy for some of my rifles. For example, I have one .308 Savage that shoots heavier bullets most accurately. It loves 175 SMKs and TMKs, as well as 190 and 200 SMKs. It hates 180 SMKs. The reason is the bullet shape of the 180 has less bullet touching the rifling and the rifle doesn't shoot that particular bullet well.

I have another Savage .308 that loves 150 SMKs, 155 TMKs and 168 SMKs and TMKs more than 175 SMKs and TMKs. It is the same length barrel and is supposedly an identical barrel. The particular barrel does make a difference so I would recommend that you check your rifle's preferences before you waste your time with COAL variations. That is the tuning step after you get into the right combination of powder, bullet weight, bullet shape and velocity for your particular barrel.

 

[RC20]

Rimfire5:

Thank you. While I do understand the relationship on COAL and pressure it is a good one to make sure not only I do but anyone re-loading does. I come from a Survyeing background ogirinaly and cross checks in that field are mandadory. All ifnormaion is repeated and the lowliest guyon the crew (usualy me) was required to question anyting uncetian.

Mostly I am working with low loads so the pressure should not be an issue and any high ones the COAL if close to the lands will be approached carefully.

There looks to be a node just short of the lands that does well and another one .030 off the lands.

There is a lot of good information in the rest of your post a swell.

So far my 308 (Savage Varmint barrel) has not expressed a preference, with the right loads its shooting most of it sub 3/4. I have yet to try the 175 Sierra and only the 180 SST Hornady with a non listed powder (R17, I have reference its just not a go to powder for 308 but seems to do better at higher speeds and heavier bullets ).

One odd one I did come across was 747 in low loads (magnum primer) that exhibited signs of over pressure (sticking bolt and flatten primers). 42 and 44 grains

Possibly low powder load and magnum primer lighting things off too fast, I will try regular primers.

 

[Unclenick]

This is a plot made from strain gauge data collected in the mid-1960's. What you see to the right of the minimum value is what QuickLOAD calculates. What you see to the left of the minimum is due to the taper of the bullet ogive gradually closing off the gas bypass path that is normally present after the case neck separates from the bullet but before it gets moving. That bypass affects how fast the peak pressure builds. A bullet with a very gradual taper along the sides, like the round nose bullet used in collecting the data, has to move further out to close the path off than a stubby ogive bullet like a benchrest bullet does. So that quarter inch of difference you see between contacting the throat (far left) and the minimum is due to that gradual taper, while a short radius ogive spire point might be there just 0.050" off the throat.

 

[RC20]

That's an amazing graph