Port pressure in a M1a

[fatelvis]

I’m trying to find the optimum load for my M1A using manuals and Quickload. Outside of trying the standby Imr and H4895 loads, I was curious what the port pressure should be, to avoid battering the rod and action. After plugging in the trusted 4895 loads Quickload is showing between 9,500 and 10,500psi at the port. Does this sound correct? Also, how about thegood ‘ol Garand in 30-06?Thanks guys.

 

[Bart B.]

1991 Specs for 7.62MM NATO Ball M80 are 12500 +/- 2000 using the copper crusher method. Measured by EPVAT the spec is 8702 (60 MPa) min to 12238 (85 MPa)max.

Actual measurements during LCAAP 2008 production of M118 LR was 11540. Spec was listed at 10200 +/- 2000.

For 1989 Match M852 the numbers were 13800 and 12500 +/-2000.

The difference in the LCAAP numbers is very likely due to powder. RelodeR 15 vs. IMR 4895 ? You decide.

 

[germanlongbow]

Goto M1A forums, everything you are looking for or NEED to know is there. Like this site people there are always willing to help.

 

[2506]

In either the M1 Garand or the M1A I see no need for anything but IMR 4895, and IMR 4064. Some use Varget and RL 15 in both also.

 

[riflewoman]

The 1965 spec

http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-C/download.php?spec=MIL-C-46934B.015852.PDF

 

[fatelvis]

The 1965 spec

http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-C/download.php?spec=MIL-C-46934B.015852.PDF

12,500, +/- 2,000psi. Exactly what I was looking for! Thank you everyone.

 

[Mad_Charlie]

Where in the bullet travel are you going to measure it?

 

[fatelvis]

Where in the bullet travel are you going to measure it?

I would think about 22.5" up the barrel. That's about where the port is.

 

[Doug Beach]

I would think about 22.5" up the barrel. That's about where the port is.

The barrel on an M1a is only 21.75” long.

 

[Bart B.]

I would think about 22.5" up the barrel. That's about where the port is.

It's about 13 inches. Barrel's 22 inches.

 

[fatelvis]

I would think about 22.5" up the barrel. That's about where the port is.

Sorry, I was thinking of the Garand when I posted this

 

[rammac]

All of my M1A rifles, including the SOCOM, have the port 14" from the bolt face.

Remember to set QuickLoad to display the pressure at the base of the bullet and not the pressure in the chamber.

 

[Bart B.]

I’m trying to find the optimum load for my M1A using manuals and Quickload. Outside of trying the standby Imr and H4895 loads, I was curious what the port pressure should be, to avoid battering the rod and action. After plugging in the trusted 4895 loads Quickload is showing between 9,500 and 10,500psi at the port. Does this sound correct? Also, how about thegood ‘ol Garand in 30-06?Thanks guys.

Here are match winning hand loads:

M1A, new cases:
44 gr imr4064, Sierra 168.
43 gr imr4064, Sierra 175 or 180.

M1, new cases:
50 gr imr4064, Sierra 168.
48 gr imr4064, Sierra 175 or 180.

If you use fired cases from these rifles to reload, they will typically shoot half MOA or more bigger groups. Bolt faces are not too square with chamber axis.

 

[fatelvis]

Remember to set QuickLoad to display the pressure at the base of the bullet and not the pressure in the chamber.

How do you change that to display base of bullet pressure?

 

[Mad_Charlie]

rammac, I too am curious about what you mean concerning "base of bullet pressure".

 

[rammac]

When you select the graphic that shows Velocity and Pressure vs. Barrel Travel you can select Graph Preferences (menu bar, Change Diagram, Optional Diagrams, Graph Preferences)


In the lower right corner you have the option to display the Projectile Base Pressure (psi) along the Y axis.

The software defaults to providing pressure information from the perspective of the chamber rather than the base of the bullet. Choosing this option will provide pressure at the base of the bullet and that will give you pressure at the port when the bullet has traveled down the bore to port's position (about 14" from the bolt face).

 

[riflewoman]

One tiny bit...chamber pressure at bullet travel X is not the same as port (Barrel) pressure. Port pressure is a bit lower than chamber pressure. The reasons are gas dynamics that I’m way too rusty on to explain.

 

[fatelvis]

One tiny bit...chamber pressure at bullet travel X is not the same as port (Barrel) pressure. Port pressure is a bit lower than chamber pressure. The reasons are gas dynamics that I’m way too rusty on to explain.

Ram, Thanks for the tutorial on Quickload.
Riflewoman, your tidbit of info makes me feel better, knowing that the erroneous port pressure I was observing, is in reality, lower than thought.

 

[Bart B.]

Port pressure is a bit lower than chamber pressure.

Peak chamber pressure is when bullets are a couple inches down the rifling. Port pressure is about 1/5th of peak pressure as there's a lot more space behind the bullet as it goes past the gas port. Easy to calculate the difference. 308 case volume is about .23 cubic inch, bore volume from chamber mouth to M1A gas port about .88 cubic inch.

Bullets are several feet out the barrel when the bolt unlocks then opens.

 

[riflewoman]

Ram, Thanks for the tutorial on Quickload.
Riflewoman, your tidbit of info makes me feel better, knowing that the erroneous port pressure I was observing, is in reality, lower than thought.

I wouldn’t bank on it being too much lower, certainly it has more variance from shot to shot than the reduction in pressure. Just be aware that when QL says chamber pressure is “X000” at the distance in question the actual port pressure will be lower.

Peak chamber pressure is when bullets are a couple inches down the rifling. Port pressure is about 1/5th of peak pressure as there's a lot more space behind the bullet as it goes past the gas port. Easy to calculate the difference. 308 case volume is about .23 cubic inch, bore volume from chamber mouth to M1A gas port about .88 cubic inch.

Bullets are several feet out the barrel when the bolt unlocks then opens.

Yes, but this isn’t the phenomenon. It has to do with the fact that the gas is flowing and the chamber pressure now has to accelerate the mass of the gas in the bore, etc. So when the bullet passes the gas port, the gas pressure at the gas port is a tad lower than the chamber pressure.