Quickload Users, chime in here.

[steve4102]

It is well known around here that in a bottle necked rifle round the longer the OAL the higher the pressure. Our Moderators, Unclenick and Brian have posted this along with charts and graphs on more than one occasion.

Here is one such graph from Unclenick.




According to QL, increasing OAL increases case capacity and reduces pressure, The deeper the bullet s is seated the higher the pressure.

This is contradictory to OAL vs Pressure discussed here often.

I doubt that this is a flaw in QL software, so it must mean there is a "tweek" or and adjustment in "start" pressure or something to adjust for "Jump to the Lands".

What do you QL users do to adjust for this "quirk" in QL?

Thanks.

[jwrowland77]

I don't have QL, but I have found and verified with chronograph, that as my OAL got longer, my velocity decreased, when I went from .125" off lands to .01" off lands.

This would tell me that my pressure decreased as well.

[Brian Pfleuger]

Originally Posted by steve4102

I doubt that this is a flaw in QL software, so it must mean there is a "tweek" or and adjustment in "start" pressure or something to adjust for "Jump to the Lands".

What do you QL users do to adjust for this "quirk" in QL?

Yes, the trouble lies in the "all or nothing" nature of QuickLoad's suggested adjustment.

It suggests adding 7,200psi when loaded to touch the rifling but makes no recommendation for loads OALs that get closer and closer.

In reality, there should be a small increase every time the OAL is made longer. As you can see from UncleNick's chart, the pressure increase is fairly linear betweem 0.200 and 0.050, so I would surmise that the Start Pressure should be adjusted linearly between those values (or at least that's close enough) as the OAL is made longer, with the minimum value being somewhere near 0.200 jump, at least in .30-06.

Originally Posted by jwrowland77

I don't have QL, but I have found and verified with chronograph, that as my OAL got longer, my velocity decreased, when I went from .125" off lands to .01" off lands.

This would tell me that my pressure decreased as well.

Well, it tells you that the AVERAGE pressure over the bullets entire ride decreased (actually, the area under the pressure curve) but it doesn't tell you anything about peak pressure.

It is possible though, it depends on how long your bullet is and how much it's length is intruding on powder burn space. It's a general rule, not an absolute. QuickLoad adjusts for changing case capacity automatically, so the only unknown is the Start Pressure. It is possible that the increased start pressure masked by a decrease pressure from increased case capacity. That's exactly what happens in small cases (especially handguns).

[Dave P]

"It is well known around here that in a bottle necked rifle round the longer the OAL the higher the pressure."

That is an incorrect, incomplete statement that can cause confusion.

If one increases OAL only, the internal pressure goes down, not up.

If one loads long, so that the bullet is in the rifling, that will increase pressure.

[Brian Pfleuger]

Originally Posted by Dave P

That is an incorrect, incomplete statement that can cause confusion.

No, it is not. See the chart in the OP. That is the example often posted by UncleNick.

It is generally true that increasing the OAL of a bottleneck rifle cartridge INCREASES the pressure while shortening it DECREASES pressure unless/until the bullet begins to significantly encroach on powder burn space. The larger the case (or more over-bore the cartridge) and longer the neck, the less likely that pressure will increase with shortened OAL because the percentage of case capacity taken up by the bullet is less, relative to the capacity of the case.

[steve4102]

Sorry Brian, I didn't mean to start this debate all over again. I have QL and was was just curious how experts like yourself and Unclenick, tweak QL to accommodate the increased pressures associated with distance to the lands.

[iraiam]

I don't have QL, but my experience falls in line with the chart in the OP. moving the bullet closer to the rifling (increasing OAL) increases pressure. I always figured this was due to the bullet having less distance to travel before encountering resistance from the rifling, this means that the pressure would start building sooner and get to a higher peak with the same powder charge.

I don't have equipment to actually measure pressures to compare the curve shown in the graphic, but I do see an increase in OAL cause an increase in pressure and velocity.

[Dave P]

"It is generally true that increasing the OAL of a bottleneck rifle cartridge INCREASES the pressure while shortening it DECREASES pressure ...."

That makes no sense to me. Totally illogical. Please explain why.

[steve4102]

"It is generally true that increasing the OAL of a bottleneck rifle cartridge INCREASES the pressure while shortening it DECREASES pressure ...."

That makes no sense to me. Totally illogical. Please explain why.

I will quote others that are more knowledgeable than I.

From Unclenick our Moderator.

The existence of the minimum pressure point was explained in the 1960's by physicist Dr. Lloyd Brownell (no relation to the gun parts company, AFAIK). When a cartridge fires, the brass expands at the neck and releases the bullet. It has to be accelerated forward to meet the throat, which takes time, during which the lower mass gas accelerates more easily and bypasses the bullet and starts leaking into the bore. (In super slow motion films you can see this gas precede the bullet exit at the muzzle, usually kicking some unburned powder particles along with it.) That gas bypass stalls the rise in pressure in the case just as bullet movement causes expansion of the powder burning space to commence. The more gas bypass there is, due to deeper seating, the later and lower the peak pressure in the chamber will be, due to that stall in pressure rise giving a head start to expansion. But only until that pressure minimum point in seating depth is reached.

The reason for the minimum is that there is a counter influence at work: as you seat deeper the powder starting burn volume is decreasing. More gas packed into a tighter space means higher pressure, and that increases burn rate and that causes faster rate of rise in pressure. As a result, if you seat deeply enough, that faster rise in pressure while the bullet is still in the case overwhelms the effect of the stall in pressure rise caused by the gas bypass effect. In other words, either side of the minimum pressure bullet seating depth, the shrinking powder space effect and the gas bypass effect trade off which one is the dominant term in determining peak pressure.

From John Barsness of Handloader/Rifle Magazines.

Reduced OAL decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.


That's all I got. This forum has some moderators and some members that are experts in this, hopefully they will chime in and help you out.

[jwrowland77]

Originally Posted by Dave P

"It is generally true that increasing the OAL of a bottleneck rifle cartridge INCREASES the pressure while shortening it DECREASES pressure ...."



That makes no sense to me. Totally illogical. Please explain why.

I tested this not to long ago.

Loading .308, OAL was 2.800. .125" off the lands. Had a MV of 2650.

As I moved closer to the lands in .02" increments, MV decreased. I went all the way to where I was .02" off the lands. 5 set of 3 rounds each and each set had a lower velocity than the other.

I'm sure though, had I gone further to where I was maybe .005" of the lands, then I would have seen an increase in MV, signifying an increase in pressure.

Had I started at .02" off lands and worked my way shorter, I would have seen the reverse and seen MV increase, signifying and increase in pressure.