9mm seating depth DOES matter...

[Shrinkmd]

I looked over my chrony data again and compared to the Lyman manual.

I made 2 batches of 124 LRN, some at 1.125, and shorter ones to fit a different gun at 1.097" The longer loaded ones were quite a bit below expected velocity, whereas the shorter ones were right on. These were Dardascast 124 LRN with CCI SP primers, CBC brass (Magtech)

Power Pistol 5.0 gr at 1.125" came out avg 1038 - Lyman says expect 1107 (quite a bit lower)

Bullseye 3.4 gr at 1.097" (min OAL was 1.065) avg 918 - Lyman says 939 (pretty close)

So, it looks like the 1.097" OAL gives the expected result from the manual (and functioned well in both pistols) 1.125 is too long, eh?

 

[mongoose33]

This is why you have to work up your own loads in your own gun.

 

[jmorris]

With such a tiny case volume why would you think it wouldn't have much of an effect? This is also why it is important to specify the exact bullet and OAL if you are giving out load information. 1.125" is not too long, most of my 9mm loads use a 1.160" oal you adjust powder charge accordingly.

 

[rjbishop]

Although I agree COL variations can introduce velocity differences, I don't know if they would vary that much (over 100 fps). Your post is somewhat apples & oranges, as you are using different powder for your two lengths. A more valid conclusion could be drawn had you used the same powder.

 

[LiveLife]

9mm OAL often depends on the bullet profile used. Most FMJ and plated bullet profiles I have used (more typical pointed noses) require 1.125" because the bullet begins to taper quickly and seating deeper than 1.12" results in loaded bullet with a diameter less than the inside case neck rim diameter.

Other bullet profiles, especially lead bullets, have more rounded nose and can be seated deeper without bullet diameter decreasing less than the inside case neck rim.

I especially like the Missouri Bullets 125gr small ball bullet profile because its profile allows you more OAL options. I have test loaded at 1.08", 1.09", 1.10", 1.11", 1.12" and they fed/chambered well while holding excellent neck tension. This allows me to adjust my powder type/charges for more tailored loads whereas more typical 9mm bullet profiles only allow longer 1.12", 1.125", 1.13" OAL which requires me to use limited range of powder charges (near max powder charges to properly cycle my stiff recoil spring pistols). 115gr bullets at longer OAL provides less bullets base to be seated for proper neck tension, so I limit my OAL to 1.125".

My recent range test produced the smallest shot groups with 1.08" OAL for MBC 125gr small ball for the same powder range load I was using. This indicates to me that this combination was getting consistent chamber ignition for consistent shots - for me, accuracy is everything if the load cycles my darn stiff recoil spring Gen3 Glocks (Gen4 Glocks are stiffer even). Besides, for those looking to minimize reloading cost, 1.08" OAL will also allow you to use the least amount of powder.

 

[Rule3]

Although I agree COL variations can introduce velocity differences, I don't know if they would vary that much (over 100 fps). Your post is somewhat apples & oranges, as you are using different powder for your two lengths. A more valid conclusion could be drawn had you used the same powder.

+1 Using the same powder would be a better comparison, but the general statement of seating depth is absolutely true especially in a high pressure round like the 9mm.

But does "size" really matter?

 

[ants]

Check the Lyman manual again, look at the description of barrel length and receiver. They generally use a 4" barrel on a Universal Receiver (it's not even a gun!) under lab conditions. Their twist rate, bore diameter, and groove diameter may vary from your own. And environmental conditions are controlled in the lab.

The conditions under which they test are not remotely like the conditions under which you shoot across your chronograph. It would be unusual to find that you get the same velocities regularly. By sheer accident, you will get their velocities occasionally. The point is that lab conditions and lab equipment vary the outcome compared to real-world experience.

The real lesson is this: If you are using a different gun and different bullet and different conditions than the lab, but you want your gun to duplicate the velocity in the published data, you generally have to make adjustments to get there. Sometimes OAL, sometimes powder charge, sometimes brass, or whatever.

 

[Shrinkmd]

Yes, I plan on making up some of the 1.097" rounds with the same brass (once they get out of the tumbler), bullets, primers, and powder, and just vary the OAL, and then shoot them out of the same gun (4" tube in a 5906TSW)

I will report back on what happens when I do so.

 

[Ol` Joe]

Seating depth in small high pressure cartridges definitely makes a difference, and can make a big one.
VihtaVuori warns of this in their manuals inre to the 9mm. The 3rd edition has it on pg 67 where in a graph they show pressure and velocity variation due to seating changes of 2mm total in 0.5mm increments. They found a 2mm change from 29.5mm to 30.5mm caused pressures to drop from ~320MPa to ~ 240 MPa about 40 MPa per mm, and velocity to go from 360m/s to 340m/s.
This translates to a change of 5800 psi/mm, or 11,600 psi and 394 fps for the total 2mm COAL change. Seeing that 1mm is 0.039" we are talking about a change in seating depth of less then 0.10" total.
If the data you are using has a COL listed it would be prudent IMO to follow it, and if you are using a different bullet then the data be sure to start low and keep a good on things.