OAL of 9mm

[pastordan]

Is the goal to make a round that is as long as possible for your gun and magazines? Then to figure the length with that particular bullet and then load the powder until you have the right velocity? Then do you change the length for accuracy? or the powder? I am seeing all the variables and trying to weed through them. Any help would be appreciated.

[Shoney]

Is the goal to make a round that is as long as possible for your gun and magazines?
RIFLE: Generally, yes. There are exceptions, but most rifle magazines are well short of the “sweet” area of 0.010” to 0.050” off the lands.
PISTOL: Generally yes, but the goal should be to function flawlessly.

Then to figure the length with that particular bullet and then load the powder until you have the right velocity?
RIFLE: No! If the magazine is the limiting factor short of the “sweet” area, you work up for accuracy, if you have a chronograph, you watch for the point that an increased load does not produce an increase velocity in proportion to the lower loads velocity increases.
PISTOL: Again load for accuracy, watching velocity carefully, especially for lead bullets.

Then do you change the length for accuracy? or the powder?
RIFLE: I only play with length after I achieve an accurate load with powder variation. Knowing that cartridges longer than the magazine will need to be hand fed.
PISTOL: NO! You only need to change powder. This is presuming the loads function properly.

[SL1]

For an autoloader like the 9mm, it is best to start with the cartridge length specified in your data. That is what was pressure-tested, and pressure can change a lot by changing the seating depth in small cases like the 9mm.

Typically, the thing that controls cartridge length in an autoloading pistol is the feed cycle, which is sensitive to cartridges that are too long or too short. So, starting with the length that the vendor uses to do pressure tests will get you close to what they think feeds right, at least in their test gun.

There is little advantage to loading bullets long to gain powder space in a 9mm. For constant chamber pressure, muzzle velocity increases by about the 4th root (i.e., the square root of the square root) of the space increase. So, if you got something like 5% more space by seating the bullet out further, you would get only about 1.2% more velocity. But, you can change pressure a lot and not have any way to see it by squinting at you fired brass.

For example, I played around with QuickLOAD to calculate the following. Making cartridge length longer by about 0.017" will increase powder space by about 5% and drop pressure by about 3,000 psi from a max load. If you add enough powder to get back to max pressure, you will gain only about 12 fps in a round that produces nominally 1270 fps with a 115 grain bullet. If, instead, you make your cartridge shorter by the same amount without changing the max charge, your pressure increases by about 3,500 psi, going over max, and you gain only about 27 fps.

Hopefully, this will show you that trying to juggle cartridge dimensions and then load powder to match velocities in the data can give you a wide range of potential pressures, because it will depend on (1) what powder space you end-up with, and (2) a lot of other variables including barrel length, chronograph accuracy, ambient temperature, power lot-to-lot burning rate and energy content variations, etc. etc. There are many ways to get way over the max pressure spec while getting only the velocity that is in the data.

And, for a 35,000 psi cartridge, you cannot reliably detect excess pressure from looking at fired cases until you are WAY over the SAAMI pressure specification.

So, start low and work-up enough of a charge to cycle your slide. Then, if necessary, juggle cartridge length to achieve reliable feeding. Finally, work-up to max (or less) charge weight to get the performance you want.

SL1

[pastordan]

I am humbled by the amount of knowledge out there on these forms and am grateful for the help. I have settled on 1.13 as my OAL for my 9mm (XD 5") and 3.8 grains of 231 is accurate in my gun with no fail to feed, fire or function (100 rounds). I am just getting started and I do not own a chronograph, yet, so I am shooting in the dark so to speak on my velocities at this point. Again thanks for the help.

[Unclenick]

'll add the caveat that with lead bullets and plated bullets, as opposed to jacketed bullets, it can help accuracy to seat a bullet out to be touching the throat when the case head just fits in flush with the back of the barrel where the breechface is. This is actually headspacing on the bullet rather than the case mouth (or the extractor, as can happen when cases are too short for the chamber). It also tends to reduce leading. A gun with a long chamber, though, may not let you seat a bullet that long and still feed properly. You have to test it.