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Old March 26, 2013, 11:36 AM   #26
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,491
Txcider,

Welcome to the forum.

The cause of the confusion is differences in round nose shapes. Some molds cast more blunt hemispherical nose bullets that use shorter seating and some cast longer elliptical bullet nose profiles that more closely mimic the military ball bullet shape. Thus you can find 225-235 grain round nose bullets in lengths that vary from about .610" to about .680" inch. .45 Auto has a SAAMI COL of 1.175-1.275" to accommodate all bullet lengths from light target to heavy round nose shapes.

What actually matters is seating depth. That's because how far the bullet sits into the case determines how much space is underneath it for the powder to start building pressure in. Seating depth should be figured with respect to a maximum length case, regardless of your actual case lengths, to keep the powder space under the bullet constant.

Seating Depth = Case Length + Bullet Length - COL

So, for a given manual load in .45 Auto, since maximum case length is 0.898":

.45 Auto Seating Depth = 0.898" + Bullet Length - COL.

The military ammo is the .680" bullet, and it usually has a COL of about 1.260" to about 1.270" in what I've measured. Using the minimum end of that scale gets a maximum seating depth:

Maximum Seating Depth = 0.898" + 0.680" - 1.260" = 0.318"

That's for a military powder charge that develops about 400 ft-lbs of energy with a 235 grain RN bullet. That's a bit more than commercial hardball, which is usually a 230 grain RN bullet loaded to about 350 ft-lbs; about what the age-old standard load of 5.0 grains of Bullseye powder with a standard large pistol primer will get you. But let's keep that lower energy level as a extra safety margin. So, since that bullet is longest, consider 0.320" seating depth your lower limit.

To figure out if your bullet falls inside that range, calculate a minimum COL for it from that information by rearranging the first equation to:

COL = Case Length + Bullet Length -Seating Depth

If your RN bullet were 0.640" long (a common number), then:

Minimum COL = 0.898" + 0.640" - 0.318" = 1.220"

You can use that formula to get minimum COLs for different lengths and here's a table of the results if you do:

Code:
225-235 grain RN Bullet in .45 Auto 
5.0 grains of Bullseye
Standard LP primer.

 Bullet 
 Length	       Minimum COL

0.695 in	1.275 in
0.690 in	1.270 in
0.685 in	1.265 in
0.680 in	1.260 in
0.675 in	1.255 in
0.670 in	1.250 in
0.665 in	1.245 in
0.660 in	1.240 in
0.655 in	1.235 in
0.650 in	1.230 in
0.645 in	1.225 in
0.640 in	1.220 in
0.635 in	1.215 in
0.630 in	1.210 in
0.625 in	1.205 in
0.620 in	1.200 in
0.615 in	1.195 in
0.610 in	1.190 in
0.605 in	1.185 in
0.600 in	1.180 in
0.595 in	1.175 in
But there are several catches to the above. One is that you don't want to put the shoulder of the top end of the bullet bearing surface (full diameter cylindrical portion) below the case mouth. So, depending on the design, you may not want to go all the way to that minimum. Too long is OK. Too short is not.

Another catch is that you might have a design that has to go shorter to chamber. In that case, you want to reduce the powder charge and work it back up, watching for pressure signs just to be sure you are safe confining the powder more than originally intended.

Another factor is just the opposite of the above one, and that is that in .45 Auto, particularly with lubricated lead bullets, the small powder space allows a primer to produce enough pressure to unseat the bullet, often before the powder gets fully lit up. In that case, the seating depth changes in the chamber, lowering peak pressure. Again, load work-up is key.

A final consideration with lead bullets, specifically, has to do with how they align in the chamber. Their ability to shoot accurately and to reduce leading, it can make a significant difference to seat the bullet out to touch the lands. With some bullet designs this becomes too long to fit the magazine or to chamber properly. But if your bullet shape is short enough to allow it, then the gun will tend to shoot best when seated as follows:

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