9mm bullet setback question

[tightgroup tiger]

I've read all the warning about bullet setback in 9mm and it causes high pressure spike that can damage your gun.

In looking at Lymans 49 edition it also has the same warning not to seat bullets at a shorter length than specified in the data. I've read a few explainations on thr about this but no one really explained why the pressure spikes from seating to deep

I also follow published data from the powder mfgs and cross reference to reloading manuals.

Lymans shows two different loads that are compressed loads for the 115gr jacketed data on page 341 of the 49th edition. So it obviously doesn't have to do with air space, or does it with faster burning powders?

What exactly happens when the bullet is seated to deep that makes the pressure go up higher than it should?

 

[Edarnold]

I've read all the warning about bullet setback in 9mm and it causes high pressure spike that can damage your gun.

In looking at Lymans 49 edition it also has the same warning not to seat bullets at a shorter length than specified in the data. I've read a few explainations on thr about this but no one really explained why the pressure spikes from seating to deep

I also follow published data from the powder mfgs and cross reference to reloading manuals.

Lymans shows two different loads that are compressed loads for the 115gr jacketed data on page 341 of the 49th edition. So it obviously doesn't have to do with air space, or does it with faster burning powders?

What exactly happens when the bullet is seated to deep that makes the pressure go up higher than it should?

Actually it's very much a matter of airspace in loads that don't fill the case. These are going to be the faster burning powders, which by their nature build pressure very quickly. The airspace provides a buffer for the pressure wave: without it, pressures spike beyond safe levels. Because the 9x19 operates at high pressures with maximum loads, and the case is so small, a few hundredths less airspace will result in significant increased pressure. The same change in seating depth in a .357 Magnum load would make less difference, because the case capacity is so much greater.

Seating depth is a major issue in the 9x19 because of drastic differences in the barrel leade between gun makers. Some use a very short throat optimized for the classic military FMJRN bullet contour, while others have a much longer throat. The problem arises when instead of the round-nose shape, we load HP bullets with a short truncated-cone nose like the Hornaday or Speer designs. In a pistol with a short throat, these bullets must be seated much deeper in the case than the round-nose just in order to chamber. If you take a maximum load for the round-nose bullet and substitute the deeper-seated XTP or GD, the airspace disappears and pressures skyrocket. Which is one reason why the 124gr NATO load can push close to 1300fps, while the commercial HP loads run 1150 fps. or less.
IMHO

 

[FROGO207]

The problem in simple terms is that with less space (volume) in the casing there is greater pressure developed before the bullet starts to move and this could exceed the ability of the firearm to contain it. If you increase the volume in the round then the pressure will be lower when the propellant burns. If the bullet is seated out slightly this happens in further and it goes up. Both the 9MM and 40S&W are higher pressure rounds and as a result small changes in case volume male a greater difference than in most other calibers. Make sure that your taper crimp/neck tension is good and there will be no problems anyway. I use the standard that if I can hold the round by the brass and push it against the bench and it gets uncomfortable to continue and the bullet does not push in at all as measured with the calipers my ammo is OK.

 

[LiveLife]

My QC for checking neck tension/bullet setback used to be pressing against the bench.

Now I measure the OAL before and after feeding/chambering a round from the magazine by manually releasing the slide.

I found the use of calipers much more precise in measuring decrease of a few thousandths than my eyeballs, especially when I am using shorter OAL than published load data.

 

[tightgroup tiger]

The reason I'm asking is I have 5 boxes of 115 gr XTPs that I got when I bought my LNL-AP and have always used 124 gr bullets since I have reloaded for 9mm.

I was looking in the Lymans 49 edition loading manual and according to it the jacketed hollow point 115 gr bullets (which one of them was xtps) are seated to 1.090 but the 124 gr in the same bullet config is seated to 1.075.

This didn't look right according to the way they listed all the other bullet weights in respect to OAL so I looked it up in my 47 edition and my 45 edition and they both list the 124 gr jHP at 1.120-or 1.125 which is what I thought it was.

That's .075" difference and I use Titegroup, Universal, and Power Pistol for loading these and list all three of these powders at the shorter seating depth.

Is this a typo or did I miss something?

 

[helotaxi]

The caution in the Lyman manual doesn't mean that you can't load a shorter OAL if you need to, but it does mean that their load data, particularly the max, is no longer valid.

The issue isn't entirely due to the pressure range that the 9mm operates at either. Much of it has to do with the very small case capacity compared to the bore diameter. A few hundreths of an inch of bullet seating depth equates to a significant percentage of the volume of the case. Pressure is inversely proportional to volume and burn rate (which increases gas volume and pressure) is proportional to pressure. What you run into is a self-feeding pressure spike when the case capacity is too small for the powder charge.

As a general rule of thumb, load semi-auto handgun cartridges to the longest OAL that will function in the weapon. Determine that length first using dummy rounds to check feeding reliability and that it won't jam a bullet into the lands. Once you've determined the length, you can begin load workup.

 

[tightgroup tiger]

The caution in the Lyman manual doesn't mean that you can't load a shorter OAL if you need to, but it does mean that their load data, particularly the max, is no longer valid.

I understand that part of it but with this being such a problem why are they saying to seat a 124gr bullet deeper than the same bullet in 115 gr when we all know that the 124 gr is longer?

I'm sticking to what I know is right and will continue to seat my 124gr bullets at 1.125 or longer, depending on which pistol I'm loading for. I load around maximum and never looked at the overall length of 124gr tc bullets in the 49 edition because all my other manuals always gave what I thought was the correct OAL.
.075 deeper might not be a deal breaker at mid range loads but I'm not taking any chances with Titegroup.

 

[918v]

There is a grenade threshold with every round.

With the 9 (low capacity high pressure round), the treshold is with a max charge of Clays (super fast burning powder) under a 147 (heavy bullet for the caliber) seated deep (1.050" or thereabouts). This is where you should be worrying about bullet setback. Don't play with Clays under 147s unless you have a death wish. Some people do it. It is doable, but you are walking the line.

For XTP's, use Hornady data because they use the shortest OAL. That way you can use longer OAL safely if you wish. Use medium burning rate powders. That way you have a cushion against setback. Even if the bullet sets back .050", the gun won't explode.

When you assemble your ammo take case to have good case tension, clean case innards(some folks using progressive devil machines put a bunch of brass in a bag sprayed with Hornady One Shot and shake them in there, but that gets lube inside the brass no matter what they think), and don't crimp the hell out of them on hopes of fixing prior negligence.

 

[helotaxi]

I understand that part of it but with this being such a problem why are they saying to seat a 124gr bullet deeper than the same bullet in 115 gr when we all know that the 124 gr is longer?

It all depends on the bullet shape and the chamber on their test barrel. The 115gn mil FMJ is a long skinny bullet for its weight. The 124's that I've loaded have more of a blunt shape and the bullet length is often shorter than some of the various 115gn. Factor in the TC shape of the 124gn and a shorter OAL might actually result in less bullet in the case. I don't have a large selection of 115gn 9mm bullets sitting around like I used to, but different bullets from different manufacturers varied significantly in length so you really wanted to find data for the particular bullet you were working with.

 

[tightgroup tiger]

my neck tension is extreemly tight, my dies may be alittle undersized. I never worried about not enough neck tension even with Federal brass.

I used to lube my shells with spray lube and tumble again, but I don't lube them at all any more.

I'll check out hornady's web sight to see what they have on line, or it I have to buy the book.

Thanks guys.

 

[rcmodel]

Hornady #6 shows:
115 XTP = 1.075".
124 XTP = 1.060".
147 XTP = 1.100".

Its probably all to do with the shape of the bullets & feeding.

The 115 has less nose then shank.
The 124 is shaped a little different.

rc

 

[tightgroup tiger]

Thank you RC. I loaded the 115 this morning at 1.095 so I will go back and reset them. I only loaded 25 of them.