308 reloading conundrum

[kendoka]

I have recently been working with a pretty typical 308 load: 41.5 grains of 4064 with Sierra 175 MK using Federal Gold Medal brass. The load groups well, .25 - .30 inches at 100 yards. Sub MOA at 900 yards. I have shot the 168 grain bullet for High Power and other pursuits, but have been shooting a long range competition that reaches out to 1000 yards, and reached the limit of what the 168 grain bullet can accomplish. Hence to change to the 175 grainer.

The surprise is that I am getting a velocity of 2680 fps for this load, and am seeing some signs of overpressure even though 41.5 grains of 4064 is the minimum published load data. That data cites ~2500 fps for the minimum load. I am exceeding that velocity by a fair amount. I am confident that the velocity data is accurate. I use a Oehler 35p chrono and have also run the load through a friend's Oehler just to be sure.

The rifle is a custom build on a Remington 700 with a 22 inch Hart barrel with a 1:12 RH twist.

Despite the mininum load, I get some overpressure signs: a hard bolt lift and a little primer flattening. I don't feel any unusual or excessive recoil, but reading the brass makes me think there is some overpressure.

Anybody have some ideas on this?

 

[ClarkEMyers]

That data cites ~2500 fps for the minimum load. I am exceeding that velocity by a fair amount.

If your velocity exceeds the published data than your pressure exceeds the published pressure.

"Muzzle velocity for a selected load is a function of the mean effective pressure and the barrel length."
- Lloyd E. Brownell, Ph.D. - Firearms Pressure Factors, Wolfe Publishing Co.

"Because velocity is the by-product of pressure, a chronograph can, albeit indirectly, give you an insight into pressure being developed by a given load. While a chronograph is no substitute for a pressure gun, your velocity data can alert you to potentially dangerous pressures before you're stuck trying to super glue your receiver back together."
- Barnes Bullets #2, Ronin Colman, "What Do You Do With A Chronograph?"

"The best predictor of pressure is a chronograph. If you're getting muzzle velocities noticeably higher than is common for that cartridge, powder and bullet, then your pressures are higher than they should be. It's that simple. If you work up loads using that rule, then you don't need to look at fired cases, because the pressure will never get high enough to form ejector-hole marks on the case heads or blow primers.
...next to real pressure equipment, a chronograph is the best guide to excessive pressure for the home handloader. There are no “magic barrels” that allow another 100 to 200 fps"!
- Handloader, June 2004, “More Pressure Experiments”, John Barsness.

"A chronograph does many things for the handloader. Obviously it indicates just how fast the bullet is going, but it also tells us how consistently the bullet leaves the muzzle. This may or may not have any bearing on accuracy, at least at "normal" ranges, and holes in the target are the ultimate measure of consistency. But when things are perking right, the chronograph is one more indicator of consistency.
More importantly -
Think of the firearm as a single-stroke internal combustion engine with an expendable piston, the bullet. Everything else being equal, a certain average pressure will always produce the same velocity. This means that, everything else being equal, if the velocity changes so did the pressure..."
- Handloader editor Dave Scovill.

"One of the most valuable applications for a chronograph is in keeping loads safer. Loading manuals provide a guide, generally suggesting a starting and maximum load. These load recipes are determined by careful loading and pressure testing. Maximum loads are based upon industry standard chamber pressure criteria. Generally speaking, with a given set of components, the higher the velocity, the higher the chamber pressure that is required to produce the velocity. If one of your loads produces velocity in excess of what a loading manual indicates, you can also assume that the pressure is in excess. If the velocity of one of your loads equals the maximum listed velocity in the loading manual (with a comparable barrel length), it’s prudent not to exceed the powder charge level you’re using, even though the quantity of powder you’re using might be less than what is suggested as the maximum quantity in the manual."
- Accurate Arms Loading Guide #1 - Chronographing Metallic Ammunition - Rick Jamison.

How much is an open question. It's not a linear relationship therefore reading backwards doesn't does not give any idea of peak pressure. Therefore I consider it unwise to shoot a load that exceeds published velocity - taking into account barrel length, elevation and temperature and such of course.

There's a pretty good, though not perfect, correlation between pressure and speed. The correlation gets better with a better definition of pressure. Pressure can be peak pressure or area under the curve or other things (CUP, PSIA, SAAMI what have you.)

 

[FLIGHT762]

I shoot a 700 308 Win. with a 24" 1/12" Hart barrel. The barrel shoots a bit fast with my loads. With 175's I'm using 41.0 grs. of AR Comp and get 2650 FPS. The only 4064 load I use is 43.0 grs. with a 168 Nosler CC and I get 2765 FPS.

If you are getting the velocity you say, I'd back off your load .5-1 grain and see what you're getting. If you get 2550-2600 FPS, I'd try that and see how your brass looks.

If you're 2550+ you should get to 1000 and stay supersonic. No sense beating your Rifle and your brass.

 

[Grumulkin]

Despite the mininum load, I get some overpressure signs: a hard bolt lift and a little primer flattening. I don't feel any unusual or excessive recoil, but reading the brass makes me think there is some overpressure.

Anybody have some ideas on this?

1. The hard bolt lift all by itself means there is overpressure.

2. Primers in a rifle will always be flattened to some exent. If the flattening goes clear out to the edge of the primer pocket in your 308 Winchester, it may mean excessive pressure. With the 223 Remington cartridge, even with excessive pressure the primer will not be flattened clear out to the edge.

3. You shouldn't expect to feel "unusual or excessive recoil" with an overpressure load.

4. Velocity is not a direct sign of pressure. A hard bolt lift and primer flattening and other signs are direct signs of pressure. In fact, velocity can even decrease as a load is incresed to excessive levels. When I work up a load, velocity is NEVER a criteria I use to determine whether or not the pressure is OK.

In fact, one guy who runs a gunsmithing operation tested, as I recall, 10 supposedly identical rifles. There was about a 200 fps difference between the low and top end of velocities with some identical loads.

 

[Okcafe86]

My Hornady 9th lists 178-180 gr pills with a max load of 41.3 gr of imr 4064.

 

[LNK]

What is your OAL? If the bullet is engaging the rifling, that may cause the pressure increase. I run 42 grains of 4064 with the same 175 SMK and do not have those issues. Though the Vanguard has more freebore than the Remington. Mark up a loaded round and eject and look for land contact.

LNK

 

[kendoka]

LNK,

You may have a point. I am right on the lans. I think I will reseat a shade deeper and see what happens. It will be one of my experiments on this issue. The other will be a reduced load. I am also going to try the same load with different brass.

Rather than being annoyed or troubled by this kind of thing, I suppose we should relish the learning opportunity.