Pressure vs. tight cylinder throats

[Carsten]

Hi.

I am working up a load for my 6.5" S&W 629.
I am using the Lee 310gr. boolit with GC run through a .430 sizer and cast to 318gr.

The boolits come out as .431 but my cylinder throats are just at hair over .430 so the boolits will make a pretty tight fit in the throats.

I think I am close to the max load using 18gr. VV N110 and an COL of 1.1654.
This gives 1200 fps but without any apparent pressure signs.

The accuracy is pretty good so the question is about pressure vs. the tight throats.

I use the rear cannelure and the boolits are just touching the throats without any "free flight" what so ever.

Is this a bad thing pressure wise?
Like when loading jacketed rifle bullets touching the lands will raise pressure, will this be the case here as well?

I do not think so but would like your opinions about this matter.

[Tatume]

Hello Carsten,

It sounds to me like you have an excellent load. Having cast bullet diameters somewhat larger than the cylinder throats is fine. Optimal accuracy will occur when the range is between 0.0005" smaller and 0.0010" larger. Perfectly acceptable accuracy can be obtained with bullets larger still. Oversized cast bullets will be swaged to throat diameter, and undersized cast bullets that are not more that 0.0005" smaller will be "bumped up" to throat diameter.

The diameter of a cast bullet changes several times during the process of being fired. In addition to the transitions above, they will usually swell somewhat at the barrel/cylinder gap and will expand to fill the forcing cone. They will then be reduced to the diameter of the barrel. It is here that trouble occurs with extremely high pressure, as these transitions stress the breech of the barrel. The 454 Casull is a cartridge that can cause splitting of the barrel breech, and accelerated wear of the forcing cone if bullets that are too soft are used. This is also the reason S&W redesigned the L-frame revolver to create the M69 44 Magnum. They needed more steel in this critical transition area.

The point is, having your bullets sufficiently hard is more important than diameter, within the limits above. I like 2/3/95 tin/antimony/lead alloy for magnum revolver applications. My bullets have a hardness of 13 BHN when air cooled.

Take care, Tom

[leftiye]

First, what does your bore slug? That sounds like you may have undersized throats. My model 29 uses .432" boolits, and yes I did open them up. Boolits larger than your chamber throats will definitely increase pressure, but this is negated by you working up your loads But it will also negatively affect your accuracy. Size 'em down or open up the throats if they are not a thou. larger than bore already.

[Tatume]

P.s., there are different opinions on this matter, and also sources of potential bias. Use care.

[Carsten]

I have not slugged the bore yet but I will do so shortly.
I have used 3 different bullets in this gun:
Magtech 44 special CAS loads (just to get more brass).
Plated .429 240gr. swaged bullets.
Lee 310gr. cast boolits.
All have given excellent accuracy.

[RobS]

Compare your velocity that you are getting with your current max load of VV N110 behind a 325 grain cast bullet. If the 1200 fps is what you have personally chronographed and it's not over the max velocity published data for like powder/bullet combo then you are probably fine. If you haven't chronographed anything then??? we are just guesstimating and proceed with caution. If loads are worked up and velocities are charted then being a bit over the cylinder throat diameter can be ok; this has been done by many experienced handloaders before and is nothing new in reloading cast boolits.

As been said there are opinions both ways. I've loaded for both situations myself (sized to cylinder throat diameter and then .001 over) and simply verified velocities with my chronograph. Noting this though, to be on the safe side I stayed under max load velocities by 50 fps or more.

[RobS]

I seen from your post you are also using 44 special brass. If you are loading 44 mag middle to max it's probably wise to use 44 mag brass not the 44 special.

[Carsten]

I always chronograph when working up loads.
The 1200 fps is measured out of my gun.
I could not find any data for this boolit/powder combo but 1200 fps should be possible within SAAMI pressure with this boolit loaded long.
It also looks fine in Quickload.
The brass is CBC (Magtech) 44 Magnum and primers CCI 300.

[RobS]

I don't know a lot about N110 as I've not used it. There are others out there who have used it though and here is an article that speaks just a bit about the powder with a 295 grain Keith style boolit.
http://www.sixguns.com/tests/tt44mag.htm

As I look across some of the heavier cast boolit loads out on the net coupled along with comparing some of 300 grain jacketed max loads on the Vihta Vuori website, I think you are within reason and not over pressure. If accuracy is where you want it then it looks like you are good to go.

[Silver Jack Hammer]

Colt will not reduce the size of the their cylinder throats on the Model P single action army out of concern of over pressure. Even though consumers request Colt reduce the cylinder throats for accuracy. Brian Pierce, probably the most published single action writer currently has data which shows reduced cylinder throats do not increase chamber pressures and he has communicated this to Colt.

Interesting too, Brian Pierce has written that test results also jacketed boolits which are assumabley harder than lead alloy do not increase chamber pressures either.

[Piedmont]

Colt will not reduce the size of the their cylinder throats on the Model P single action army out of concern of over pressure. Even though consumers request Colt reduce the cylinder throats for accuracy.

Colt.......keeping us all safe from accuracy!

[Silver Jack Hammer]

Haha, that's great. I've got a buddy that keeps a little money coming in on the side re-cutting .38 cylinders to properly dimensioned .44 Specials. Wouldn't want to break his rice bowl.

[leftiye]

[QUOTE= Silver Jack Hammer Interesting too, Brian Pierce has written that test results also jacketed boolits which are assumably harder than lead alloy do not increase chamber pressures either.[/QUOTE]

It has been axiomatic forever that jacketed bullets produce higher pressures and lower velocities than equal weight boolits at the same charges. Anybody with a chronograph and a micrometer can verify this.

[Silver Jack Hammer]

Chronographs cannot be counted on as a method to measure pressure. Neither did the CUP system to measure pressure end up telling us the whole story.

[leftiye]

Nobody said anything about chronographs measuring pressure. But if you load the same load in two cases, one with cast, and one with jacketed, you should be able to see the VELOCITIES of the two and compare them.

Nor did I say anything about CUPs. Unless the cases were identical, you should be able to see if one showed more pressure sign than the other. Nope, I still didn't say you could measure pressure that way, only comparatively. If you know how.

[Silver Jack Hammer]

"It has been axiomatic forever that jacketed bullets produce higher pressures and lower velocities than equal weight boolits at the same charges. Anybody with a chronograph and a micrometer can verify this."

"Nobody said anything about chronographs measuring pressure"

????????

[Clark]

The big bullet in a small bore making no pressure increase is counter intuitive.
Likewise the pinched bullet in the chamber making a huge pressure spike is counter intuitive.
The way I reconcile it is to think in the time domain.
If the peak pressure of powder burn is not concurrent with the increase pressure to swage, the peak pressure does not change.
And if the pinched bullet delays the start of bullet acceleration, the increased pressure makes the powder burn faster which increases pressure which makes the powder burn faster ....

This phenomena of large bullets working in a small bore without pressure spikes is documented in P.O. Ackley 1966 "Handbook for Shooters and Reloaders Vol 2" chapter 7 "additional pressure tests":

"..30 cal barrel pressure barrel was fitted to the test gun, but the neck and throat was enlarged to accept the 8mm bullet, with the bore
remaining the standard 30 caliber. A Remington factory 30-06 cartridge with the 150 gr bullet had been tested and previously gave 57,300 psi, for a velocity of 3030 fps. The the bullets were pulled from two more Remington 150 grain cartridges and were replaced with 8mm 150 grain bullets. To everyone's surprise, although the velocity was rather erratic, these loads averaged 2901 fps, with a pressure of 40,700 psi."

[Silver Jack Hammer]

Clark: Very interesting post. Thank you for that.

Although not on point to this threat because it does not specifically address the pressure level at the cylinder throat, the NRA doing pressure tests made the following observation:

Several years ago the NRA fired a number of loads with Bullseye, Unique and 2400 with a half dozen cast boolit designs in .44 Special. A 429421 weighing 231.8 gr fired over 16.5 gr of 2400 chronographed at 1,148.1 fps from a 6.5 inch pressure barrel at 16,880 psi. The second load featured a Lyman 431215 gas check weighing 222.6 over 17 gr of 2400 that averaged 1,091.9 fps and 19,050 psi from the pressure test barrel. In reviewing the results, the NRA noted that the slightly lighter gas check boolit produced less velocity at a greater pressure than the plain base 429421 did over .5 gr less powder.

The NRA tested the Lyman 429422 hollow base SWC at 237 gr fired over 16.5 and 16.0 gr of 2400. Pressure test for the heavier powder charge was recorded at 21,370 psi and velocity was recorded at 1,197 fps. The lesser charge produced 16,930 psi and 1,130.5 fps. In this test the .5 gr increase in powder generated an additional 66.5 fps in velocity and 4,440 psi more pressure.

Clark, your post is more spot on than mine but I thought I'd share too.

[Carsten]

I did some more testing with the 1200 fps load today.
Many of the cases will fall free of the cylinder without using the ejecting rod.
Is this a false sense of security?

[Silver Jack Hammer]

Carsten: Yes. How easy the spent cases fall out is no indicator for pressure as a sign of over or under pressure.