Load Data - Seating depth

[Gregory Gauvin]

In the manuals I have a load is given for a particular bullet (or kind of bullet) with one seating depth. For different bullets, different seating depths are given (obviously).

Are there manuals which show various seating depths for one particular load, say, should you want to seat the bullet longer/deeper (but not exceed the minimum)? Do such manuals provide velocity/pressure differences between the two seating depths?



Suppose you are working a load up for .45 ACP. The load manual calls for an OAL of 1.090", but you seat the bullet to factory ball ammo specs at 1.270". Now let's say the load is listed for 830 ft/sec. Will you still be around that velocity or will lengthening OAL that much drop your velocity and pressures way down?

[Unclenick]

In general, no. And no.

They don't list multiple seating depths because pressure changes with seating depth. If they have a maximum load, you should not normally seat deeper, as that raises pressure. Conversely, if you seat out, that generally lowers pressure. The exception is seating out very close to or touching the lands of the rifling with the bullet, which raises pressure again. What seating depth that will be depends on the depth of the throat or the length of freebore cut into your rifle. These vary with chamber reamer design and sharpness, so an accurate blanket answer can't be given.

The above are all reasons you use a starting load and work up, looking for pressure signs in your gun. The manual's peak pressure may or may not be safe in your gun, even though the seating depth is the same.

[Gregory Gauvin]

If I began working up a load, but used a max OAL versus a shorter COAL then listed in the manual, and reached the max published charge data, would it be safe to say that because I used a longer OAL and dropped pressures and velocity, I am still not at unsafe pressures (no signs of pressure) and that I could, very carefully and slowly, increase powder charge until there is a sign of pressure?

I know you're not suppose to A.) Use a shorter OAL then the listed minimum and B.) Never exceed maximum loads.

However, a "maximum load" is a listed OAL and a powder charge. By "exceeding" you are increasing pressure which would mean 1.) decreasing OAL from published minimum OAL at maximum charge weight or 2.) Increasing powder charge when utilizing minimum OAL from published maximum load.

So by adjusting OAL (longer) is it safe to say you are decreasing the load? If increasing OAL lowers pressures and velocity, it's almost like saying you are decreasing the charge. So by making an adjustment to one, it effects the other?

What I am experiencing is that I like to seat my bullets to MAX OAL, and as I work up my loads, which data is listed as seated much deeper, I reach the max charge before I reach listed velocity. If I seat the bullet deeper (as per manual), I attain velocity. But to get said velocity with longer OAL I must exceed the listed powder charge.

Am I breaking the rules?

[Unclenick]

What you are saying is basically correct. The size of the volume the powder burns in changes the peak pressure you get. Bigger volume, lower pressure. You might get that bigger volume from seating your bullets further out, or you may get it from choosing a brand of case that is bigger on the inside (weight is often an indicator, but exterior dimensions can vary, too, so that's not guaranteed; only measuring case water capacity makes a certain determination). Either way, the maximum powder charge that is safe in your gun is likely to be individual, and with that extra volume, it may well need more powder to make it to the same pressure the manual authors got.

If you get the bullet out so far that it touches the lands, the pressure can go back up again due to the static coefficient of friction the bullet has at contact being higher than the kinetic coefficient of friction the bullet has when it hits the lands running. It can also go up due to there being less opportunity for a portion of the propellant gas to bypass the bullet before the bullet fully plugs the bore. This makes a bigger difference in a rifle than in most pistols, but keep watching for pressure signs as you move the bullet out. The easiest is an observed increase in velocity without the powder charge being increased.

If, as you noted, the velocity is declining as you move the bullet forward, you are not seating far enough out to see either of those effects. Basically, with a fixed powder charge, if velocity is decreasing from shallower seating, then so is pressure. Bringing the charge up to match the velocity you got from loading to the manual specification will get you close to, but not quite up to the same peak pressure you had with the manual load. That is because part of the return in velocity is coming from the greater gas volume the greater charge provides post-peak.

This is one of the few instances in which using a chronograph to adjust a load is a good idea. Simply using it to match manual velocities is unsafe, since your gun may not get the same velocity from their loads. But matching what your particular gun got from a manual-spec load is OK, since that is an apples to apples comparison (provided you use the machine in the same weather conditions in both cases; some chronographs don't handle light changes well).

The shorter-than-maximum COL's in manuals are intended to optimize the chance of trouble-free feeding in the largest variety of guns.

By the way, in case it is causing confusion, many decades ago the American Rifleman began using the term Cartridge Over-all Length, which they abbreviated to Cartridge OAL. Others dropped the word "Cartridge", leaving just OAL. Still others exchanged that word for its initial,"C", to come up with COAL. The funny thing is, the word "overall" has been a compound word since Chaucer's time. It doesn't need a hyphen and it doesn't need more than the letter "O" for its initial. For that reason I, and some other folks and some manuals, just use COL, for Cartridge Overall Length. That's the way it really should have been written originally. The bottom line is that COL, COAL, and OAL all mean exactly the same thing: the length of the finished cartridge from stem to stern.

COL is also over-relied upon, IMHO. What affects pressure is the the space the powder is burning in. Since bullets of the same weight can have different lengths (not counting dealing with boattails) the thing that will make those different lengths have the same powder space under them is giving them the same seating depth. Some of the older manuals, like the Hornady #2, give seating depth rather than COL. Giving two same-weight bullets the same seating depth lets the same charge give them the same peak pressure. Modern manuals may give different COL's for their bullets, but usually list only one set of charges for the weight. That means they likely worked that charge up for the deepest seated among the bullets, and just let the pressure and velocity of the rest drop a bit. It leads people to the illusion that only bullet weight matters when choosing a charge.

Seating depth is determined by the location of the bullet base in the case. To figure seating depth, just add the lengths of the bullet and case together, then subtract the COL. To figure COL for a given seating depth, just add the case and bullet lengths and subtract that seating depth.

[Gregory Gauvin]

I think I'll print this and post it next to my powder burn rate chart on my wall.