Home Forums Hunting & Shooting Ask The Gunwriters OAL vs Pressure?

I have read that reducing OAL in a bottle necked rifle round can increase pressure. I have also read that it can reduce pressure. I read a post here at the CF awhile back by John Barsness about OAL and pressure, but I can't find it. I also found this article by Barnes on OAL.
http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

So, I thought I would come here and ask the Gunwriters, does reducing OAL in a bottle-necked rifle round increase pressure, decrease pressure or have little or no affect?

It 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.

Handgun powders are much faster-burning, and even regressive, meaning pressure peaks when they're first ignited. Thus seating them deeper allows more time for initial pressure to build.

This effect is slightly exaggerated in revolvers. The initial, quick pressure rise is relieved somewhat when the bullet passes the cylinder gap, allowing some gas to escape. When bullets are seated deeper it takes them longer to pass the cylinder gap, giving pressure more time to rise.

This is an excellent question, one which I have asked myself in the past, and John's reply makes absolute sense, except in one regard (that I may well be wrong about):

Increasing overall length, (i.e. seating the bullet less deep) I would believe, would reduce pressure with consideration to such procedure increasing the case capacity (given the same powder charge)until of course one got very close to the lands, resulting in the scenario John describes.

I think the enlightenment I gained here is in regard to John's description of progressive powder behavior, and when peak pressure is reached.

There's a lot of handloaders out there who believe they can gain a reduction in pressure by seating their bullets so as not to take up so much room in the neck/case.

Perhaps this needs some re-thinking. Or, perhaps there is a balancing act going on, and there is a point where this "gain" of seating bullets long(actually a perceived reduction of pressure)is negated by the bullet being closer to the lands.

John's contention certainly fits with Weatherby's technique of "free-bore" toward pressure reduction.

Tahnka,

Seating rifle bullets further out to "increase powder capacity" doesn't work the way you suggest, for the reason I previously stated: The faster a rifle bullet is traveling before encountering the rifling, the lower the peak pressure, because the bullet engraves more easily when traveling faster.

This has been demonstrated over and over again in pressure laboratories. You can demonstrate it yourself by shooting 5 rounds of a given load in one of your rifles, with the bullets seated out as close as possible to the lands--or even touching the lands. Next shoot 5 rounds with the same load with the bullet seated one turn of the seating-die spindle deeper, and repeat with another 5 rounds with the spindle another turn deeper. The average velocity will be slightly less with each batch of deeper-seated loads. (That is, it will be if your chronograph is accurate enough and light conditions are consistent.)

Muzzle velocity is one of the direct by-products of pressure, and the one most easily (and accurately) measured by home handloaders.

Myth buster.

I don't know the answer, I always went with decreasing OAL decreasing psi. Then I got Quickload and it indicates much higher peak pressure with shorter OALs (all else being equal). That does not make sense to me though. Having a greater volume when the bullet hits the rifleing would seem to decrease peak pressure to me.

Just curious (never used Quickload), does Quickload "know" the distance to the rifling? Or is assuming some constant distance? If the latter is the case, it would make sense that it predicts higher pressure with decreasing OAL.

Haven't used it in awhile, but I don't think it knows. I'm certainly not a QL expert though. What you are suggesting makes sense though, because you would have a smaller volume.

Don't think velocity is necessarily a good indicator of peak pressure. Total pressure while the bullet is in the barrel results in velocity. You can have a very high, but short, peak pressure and have a lower velocity than another powder that has a lower peak pressure, but broader pressure curve. That's how I understand it anyway.

Well I guess I should hold this one but ........


How does crimping affect all this????????

prm,

I suspect we are talking about two different things:

Velocity does NOT indicate anything about peak pressure in DIFFERENT LOADS.

It is, however, a very good indicator of peak pressure when the same load is fired in the same rifle. If a different seating depth shows higher or lower velocity, that does indeed mean that pressure is higher or lower.

Major edit... Yes I agree with that.

Ackley did some testing on long freebore systems, and found that it doesn't yield higher MV.

I think you really have two effects working in opposition to each other. Smaller initial case volume increases pressure, while freebore reduces it.

The bullet get most of its velocity while pressure is near its peak. So, while velocity is determined by the total area under the curve, for any given bullet and powder combination, MV is highly correlated with peak pressure.

You certainly have two opposing forces here:

- smaller expansion chamber = higher pressure (shorter OAL)

- more bullet momentum at time of engraving = less inertia to overcome = less frictional force = less pressure (shorter OAL)

It is my suspicion (based on the experimentation that John mentioned) that the first factor is the minor one in the overall pressure equation, with the latter being the major player here. So while reducing the combustion chamber does, indeed, increase pressure, the corresponding increase in bullet momentum when it engraves in the rifling reduces the pressure, which far outweighs the slight pressure increase caused by the smaller case capacity.

QL does not consider bullet jump or crimp. It assumes the bullet doesn't move at all until a specified bullet engraving pressure is exceeded. Even the Army has only recently begun trying to model bullet engraving for small arms in its software.

Thanks! Had a friendly disagreement on this same subject a couple of months back with a guy at the range. He supported his claim that pressure is increased with deeper seating by quoting QL calc's - I assumed at that point that QL did not consider bullet jump/distance to lands, but simply looked at case volume dependent on seating depth.

I can see where modeling bullet engraving would be a chore and include many variables!

It has to be more than a suspicion when you consider that the relative change in the volume of the powder chamber is but a small percentage of the capacity and volume, particularly in the larger cases.

Thanks guys, that's what I needed to know.

No offense JB, I almost always agree with you and I greatly respect your experience and knowledge, but I do believe you have missed this greatest contributor to pressure in many modern powders and rifles. Case capacity has a great and direct correlation to pressure, and seating depth directly impacts this affect. I have a Model 70 bolt action 284 Winchester and I can easily exceed most listed loads with zero pressure signs due to the fact that those listed loads minimize the oal to 2.800". I would challenge your conclusion based on this one cartridge, not to mention experience I have had with both a standard box Rem-700 7mm-08 and a Wyatt's box fitted 7mm-08. No comparison in what compressed loads can be used without any pressure signs. So in all due respect, I believe the gun that allows a longer Oal can always be loaded to a higher powder capacity and higher velocity. And if you review your answers to many similar questions, I believe you will find that your conclusions match this one. I have validated your very own 4 to 1 powder to velocity calcs with my Wyatt's 7-08, my 7-08ai, and my 284 win.
Respectfully,
bludog

bludog, if I understand you right, you are saying a rifle with a longer than standard throat can be loaded with add'l powder and achieve higher velocity? If so, I totally agree, but I think you're talking about something slightly different than this topic is discussing. We're not comparing one rifle with shorter throat to another with a longer throat which I believe is the example you're using, we're discussing same rifle, just seating deeper or further out. If I misunderstood you, I apologize.