Sherman Wildcats pressure limits?

[RevJim]

I used a 35 Whelen AI for many years. My favorite loads were worked up like I would any standard round. Back in '95 I added powder until the bolt was stiff or primers got flat then backed off to best accuracy. I always used new, fireformed brass for hunting. While I never had an issue with those loads, while checking the h2o case volume, powder charges, etc on Quickloads I found I was running them right at ( QL guesstimated) 72K! I have a repaired retina in my left eye, and I am very protective of my good shooting eye, I wear glasses, but all it takes is a blown case head/primer and I "risk" that good eye. Unacceptable, especially now that I "know" I was running that hot. I definitely liked how the rounds killed at those speeds ( 2970/Barnes 200X & 2600/250X, 250PT/2750) from a 22" barreled Mod 700 Classic Whelen) I made the decision and had that rifle converted/rebarreled to the 358 Norma Mag. I wanted the velocity but in a bigger, established pressure SAAMI round. It turned out swell but it got sold during a financial crisis ( Obammacare extortion/shakedown) I finally picked up a Mod 70/7x57 and an older Ruger 77/338WM. Both have established pressure limits/loads. Anyhow, if I have another rifle built, I am tempted to try the 280 Sherman ( "maybe" 6.5) but...I'm concerned about the pressure. I find the almost straight case/sharp shoulder cases don't show "typical" pressure signs; when they do they are "hot", ha. Anyone know just how to keep them loaded safe ( all the design will safely do is what I mean, not milktoast loads) or do most guys just run them hot? I'm curious...thanks guys. Jim

 

[MagnumManiac]

I run several Improved cartridges, both AI, and non AI Improved with 45 degree shoulders. I also run a 375 Weatherby that I need to fire form then run semi tested loads in.
I find that brass is brass, if you exceed safe pressures, you get cratered primers, ejector marks, loose primer pockets and other embossing on the case head.
My 375 Weatherby shows cratered primers, along with flattened primers, as soon as pressure exceeds max. The next step is always ejector marks and loose primer pockets.
My 338-416 Rigby Improved shows the same signs, albeit to a lesser extent in regard to cratering. The firing pin spring is quite stiff in that rifle.
The AI's still show pressure regardless of the fact that many believe the cases withstand higher pressure due to case shape, which has never been proven, nor has the theory that less body taper results in less bolt thrust and therefore less primer flattening.
I don't see how this can be true as the flash hole is still what applies the gas force to the primer.

Cheers.

 

[Rich Coyle]

If you get a half a dozen loads and to hold a primer, what is the concern?

 

[CliffM]

I will typically use 62,000 as the goal, using quickload to give an idea of powder and bullet combinations. I load 1/2gr increments till I get a litttle exractor mark or a change in bolt lift to find the max, then back down from that at least a full grain. My rifle will show nothing until it gets that close. Another rifle will begin to show changes in the primer at a fairly low pressure(52,000-58,000).

 

[RevJim]

Thanks guys. The way you guys describe load workup, the pressure signs you see, etc, is exactly what I've always done it. My first Ackley was back in '82 with a new Mod 700 classic .257 Roberts reamed out. It had a very long throat, so I just kept adding powder until I saw flat primers, then backed off. I also got the best accuracy at the top speeds. I figure I was indeed "safe as could be expected" from my old Whelen AI, I just got spooked, its tough being a "one eyed Preacher", ha. I also got to thinking that since the 280 AI has been SAAMIed that it would be a good guideline on top loads for the Sherman. I'm not going to sweat it, if I want one, I'll get it and just be cautious, "as usual". Like I said, I always "hedged my bet" by using only new fireformed brass for hunting. In fact, I never shot practice loads beyond 4-5 times then canned them. There was a factory rep for Weatherby told my smith that their factory loads are pretty much all going about 72-74K! They "want you" buy their ammo and not reload, ha.

 

[adam32]

An "improved" cartridge or factory should still not be run above the SAAMI limit of the parent case...I'm not sure if anyone has ever put a Pressure Trace on a "Sherman" cartridge but I have a feeling people would be surprised by the results.

Only two things make a bullet go faster...more powder or more pressure...

 

[bigngreen]

An "improved" cartridge or factory should still not be run above the SAAMI limit of the parent case...I'm not sure if anyone has ever put a Pressure Trace on a "Sherman" cartridge but I have a feeling people would be surprised by the results.

Only two things make a bullet go faster...more powder or more pressure...

We have ran a pressure trace, nothing abnormal, brass shows the same signs at the same pressures.

 

[Varmint Hunter]

I always work up loads using my Oehler 35P as a "guide" to pressure while observing SD. Any load that exceeds book max velocities is reduced. It's not exactly a secret that high velocity is usually associated with high pressure.

Watching the usual pressure indications on the brass and observing bolt lift resistance are both good but using a chronograph adds to the tools to monitor pressure.

 

[adam32]

We have ran a pressure trace, nothing abnormal, brass shows the same signs at the same pressures.

What were those pressures? Above SAAMI?

 

[stirner]

When I develop loads for a new-to-me cartridge, I perform whatever case prep is needed for that case. Then I load ladders, in 0.1% increments, using powders and primers that I think are best for that cartridge. I will load one cartridge of these at each increment, up to what I think will be max pressure. I usually losd sets of 5, as I use MTM boxes of 50. As I fire these, in ascending order, I check the case head for the usual signs of pressure. Cases are put back in the box in the order they were shot. If a case shows signs of pressure before shooting the top loads, I stop there, go home and pull the remaining bullets. I then go home and measure case diameter just in front of the extractor groove. This tells me what is the normal expansion for that case/chamber. If a test case shows pressure signs, this will also be shown in the case diameter, and will let me know when I'm approaching max pressure. Sometimes there will not be an indication on the case head, but is reflected in case diameter. If there is no indication of pressure on the case head or expansion, then I will load a couple more incremental loads to test. I also use a chrono to see if I'm getting the velocity I want, and any unexpected readings that are not commensurate with that particular load. These also indicate high pressures.

 

[bigngreen]

What were those pressures? Above SAAMI?

They were normal for normal signs, over for over pressure. Once you start pushing 65K you start seeing it. This was a Sherman, and with typical brass, now you go brass that'll hold the pressure better or double struck heads then you can run more pressure before seeing the typical signs.
I never ran the SS as the case is so short I could not get a sensor glued iver the chamber.

 

[bigngreen]

The Sammi pressure is a baseline number, beyond that it has no relevance, if your brass isn't showing signs at 70k it's as safe as brass that's at 60k showing the same signs.

 

[can1010]

They were normal for normal signs, over for over pressure. Once you start pushing 65K you start seeing it. This was a Sherman, and with typical brass, now you go brass that'll hold the pressure better or double struck heads then you can run more pressure before seeing the typical signs.
I never ran the SS as the case is so short I could not get a sensor glued iver the chamber.

do you think that there is any truth to the idea of straighter case body gripping the chamber better and reducing the bolt thrust or pressure on case head hiding pressure signs
is there even a way to test that with a pressure trace
just trying to understand if there is any truth to it

 

[bigngreen]

You can mask some pressure signs, the straighter design holds the chamber wall better, you can also make a lot of difference with chamber finish but the growth of the case head will still remain, ejector swip and even primer signs you can mask to some degree.