9mm Pistol Seating Depth Question(s)

[1SOW]

This is a theoretical question.

I'm loading a fairly new bullet, Berry's 124gr HBRN .356" dia.. This is basically a 135gr bullet that has had the base hollowed out so it's lightened to 124gr. It's a long-sided 124gr bullet with a somewhat pointed round nose.

Using a medium fast powder (Vit n320),
The bullet can be loaded at 1.132" with a .234" seating depth to get 1050'/min with good accuracy.
It can also be loaded as short as 1.125" at a .241" seating depth with slightly less powder to get the same 1050'/min also with good accuracy.

Both loads feed and 'feel' fine. There are no overpressure indications in this "light load". The base of the bullet is well above the powder level.

My question (finally): How does 'deeper seating' a bullet like this change pressure, case expansion and recoil? Are there drawbacks or gains from loading it shorter or longer?

 

[Blue68f100]

Normally as a rule when you seat 0.050" shorter you need to back off 0.1gr. So if your in the lower end of the chart you will be fine. It's due to less volume in the case for gas expansion.

 

[cfullgraf]

Using a medium fast powder (Vit n320),
The bullet can be loaded at 1.132" with a .234" seating depth to get 1050'/min with good accuracy.
It can also be loaded as short as 1.125" at a .241" seating depth with slightly less powder to get the same 1050'/min also with good accuracy.

You answered your own question. A fine example of a shorter cartridge-over-all length raising pressures.

Your example shows the same velocity with less powder for the shorter cartridge. The lower powder charge at the shorter length raised the pressure to about equal to the longer cartridge with the larger powder charge.

If you load the lower powder charge at the longer length, the velocity should be less.

 

[Walkalong]

I have been loading that bullet recently at 1.140 to 1.145 using N320. My guns have really shot it well. I have not played with the OAL to see if shorter shoots any better. Since I am not at max, I would not worry about loading it shorter until it got really short, and or my velocities told me I needed to back off the charge a little. Seating a bullet shorter, or simply increasing the charge, in little cases can sure make a big difference in pressures.

I have some bulged .32 ACP, .32 Mag, .38 S&W, and 9MM cases to prove it. *whoops*

 

[rfwobbly]

My question: How does 'deeper seating' a bullet like this change pressure, case expansion and recoil?

• As has been explained above, and as you know, there are numerous contributors to chamber pressure, but the main 2 are case volume under the bullet and amount of powder. What's happening is easiest to understand with a simplistic mathematical model. Using a seating depth of "2" times a powder load of "3" equals a pressure/bullet speed of "6". (2x3 =6) Increasing the seating depth to "3" and reducing the load to "2" still yields a pressure/bullet speed of "6". Follow? You have merely swapped the multipliers to achieve the same product.

Now, this is the most simplistic example, but it does convey the point. This "model" mainly holds together because in this low-to-mid range load there is still a 1:1 relationship between chamber pressure and bullet speed. At very high loads, this is definitely not the case, but for minor PF loads, it works.

• It then follows that since the relationship between pressure and bullet speed is 1:1, the chamber pressure is the same in both your loads. Thus we can assume that, since it's pressure that's deforming the cases, the case deformation will be the same in both your loads.

• Recoil has nothing to do with the boom!, and everything to do with the bullet's weight and movement. Since both bullets weigh the same, and both bullets accelerate to the same speed, we can therefore assume that recoil will also be very similar, if not exactly the same.

Are there drawbacks or gains from loading it shorter or longer?

Like Mr. Along, I also load this bullet at 1.140" with great results. It has taken me a long time to understand, but you want the bullet seated deep enough into the auto cartridge case to be fully supported so that it doesn't get knocked off axis when it's slammed into the feed ramp. But on the other hand, you want enough of the bullet exposed where the case crimps on a section of the bullet with a full diameter. (Not because the crimp holds the bullet, but because [again] you want that maximum length of case support for the bullet.)

The ogive on the Berry is so graceful that finding where the .356 diameter ends and the ogive begins can be difficult, but 1.140" delivers. That is not to say that I'd use the same OAL on another brand of bullet, but for the Berry 124gr RN and 124gr RNHB it works.

Hope this helps!

 

[john16443]

Jay from Berry's has also indicated in the past that these 124 grain HBRN bullets could/should be seated to the same depth as his other RN (noin HBRN) 124gr product. I also load mine to 1.14", and there's plenty of bullet body in the case for good neck tension.

 

[1SOW]

Thanks guys. I basically understand and agree with what's been said.

I still wonder about "case expansion". Maybe this is just splitting hairs, but as soon as the bullet moves, the pressure starts to drop. Does seating the bullet deeper (with the same pressure) "change" case expansion by exerting said pressure at a different point (deeper) in the case?

rfwobbly, your ratio example for 'linear pressure changes' is excellent.

 

[AABEN]

Try 1.095 that is what my load book calls for 130gr . That is with #2 #5 #7 accurate max is 1.169

 

[1SOW]

AABEN:

Try 1.095 that is what my load book calls for 130gr

Sorry, your using the wrong load data. A Berry's 130gr "RN" would have the case mouth up on the ogive at that length. "Published" data gives 1.140" My bullet in question is the 124HBRN.

I have a perfectly good oal that shoots well. I'm just asking a theoretical question about seating depth effects.

 

[tightgroup tiger]

Like Mr. Along, I also load this bullet at 1.140" with great results.

OK, I have read all the posts in the thread very carefully, and it throws up a red flag for me.
My question in trying to understand this is
My sr9 has a longer distence from the where the case headspaces to where the bullet lands start then my pf9 has. noticebly longer. The sr9 will accept the seating distances you discribed here. With my pf9 I have to deap seat the same bullet to 1.070 to keep from hitting the lands.

I'm using the same published data for both pistol loads. 4.5gr of univesal clays and the same bullet the OP is using.
Does this mean I have to reduce the powder charge in the pf9 to stay within the same pressure rating as the sr9? It sounds like it.
The recoil is what I would consider normal in the sr9 but the pf9 about breaks my hand. I assumed it was because of the pistol being so much smaller and lighter.
I recently bought some Hornady Critical Defence loads and shot through my pf9 and the recoil was half what my handloads were. Yikes!!!
This explains some things.

 

[Walkalong]

The sr9 will accept the seating distances you discribed here. With my pf9 I have to deap seat the same bullet to 1.070 to keep from hitting the lands.

I'm using the same published data for both pistol loads. 4.5gr of univesal clays and the same bullet the OP is using.
Does this mean I have to reduce the powder charge in the pf9 to stay within the same pressure rating as the sr9? It sounds like it.

A seating depth .070 deeper is most certainly making a difference in pressure. I would be inclined to go to 4.4 Grs, or maybe even 4.3 Grs, with the shorter OAL.

A chrono would tell you in a hurry.

Mixed brass, 4.4 Grs of N320, and the Berrys 124 Gr HBRN loaded at 1.140/5 with a WSP primer gave me an Avg 1024 FPS from a 3" EMP, an Avg 1137FPS from a 5" 1911, and an Avg 1138 FPS from a 16" AR.

This tells me that the pressures are fairly mild, and that the powder is burning up quickly and not taking advantage of the longer 16" barrel. This is the first load, out of many, that has given no more velocity in the longer barrel. Interesting.

Start low, work up, use at your own risk.

 

[tightgroup tiger]

Walkalong,

thank you

TT

 

[1SOW]

Some of my loads didn't reach the pressures/speeds I expected. One very important thing I was overlooking, and that rfwobbly reminded me of; was the 'Extra air-space in the HB bullet--duh.

I tested several oals and "light" powder loads to gain some experience and to see and feel the changes. Each load, I went to the same speed to see the powder and oal relationships. There was actually very little change in accuracy and feel with different oals AT the same speed.

Interesting. I wasn't smart enough to measure the spent cases to see if the cases expanded differently--wish I had. It was worth the 60 rds spent.

124 hbrn @1.135" over 4.2grs n320 gives around 1050'/sec out of a CZ 4.72" bbl. CE Chrono. This is a good load for ME. [Actually the LEE disc hole drops 4.15grs ]
Stay safe. Start low and work up--I did.

Thanks for your inputs.