Home Forums Hunting & Shooting Ask The Gunwriters Cartridge pressures

In reading reload charts it has always caused me to wonder why the different powders and max loads for the same cartridge never achieve the same psi. Is there a reason that if the max psi is 63,000 for a given bullet that you should be able to reach that pressure safely for other bullets in that cartridge. There must be a reason that I am missing. I know you never start at max load and should work your way up but it is still a puzzle to me.

One reason is the density of the max load with different powders. In the same cartridge, a bulky extruded powder may completely fill the case or even be compressed and only get to 61ksi. A much denser spherical powder load get may get all the way to 63ksi with room left in the case. Think Retumbo (bulky sticks) vs Ramshot Magnum (dense ball)


The other reason to consider is the the max load pressure number we see in the loading manuals is a maximum AVERAGE pressure. We are not seeing the standard deviation of pressure produced by that maximum load with that exact bullet, powder, and pressure test barrel used to develop the data. If a particular max load in testing has a larger deviation and extreme spread, the average pressure over a given quantity of reference loads fired may be seem a bit low but the shots that read high may be flirting with the ceiling. In this case the publisher of the data is does the responsible thing and holds the max load to a level where the probability of a shot every breaking through the max allowable pressure ceiling is very low.

Very good explanation by Tennessee.

I will add however that while the publishers of the reloading manuals have to abide by SAAMI and CIP MAP you as a handloader do not.

In a rifle chambered for a 65K cartridge like the .270 Win. if it occasionally sees 67K it isn't going to explode on you.

So, is it reasonable to say that a .270 Win. shooting a 140 gr. Accubond can achieve 3200fps from a 24" barrel with the right powder and just slightly exceeding 67K?

Barely and I wouldn't feed it a steady diet of these loads but the occasional excursion into the high 60K's isn't likely to be catastrophic.

QL shows two powders which can push that bullet to 3200 at just under 67K.

Great to hear that. Thank you. I've only been using factory 140 gr ammo and they chrony out at a max of about 3025fps; the guys at the range said I should be able to beat that by at least 100-150fps handloading while watching for pressure signs.

You might be able to beat it by a lot more, since traditional "pressure signs" often don't show up until 70,000+ psi. But that doesn't mean the load would be safe.

Using the SAAMI MAP of 65K for that cartridge you should easily get into the mid to high 3200's.

I'd recommend four powders to try.

Accurate Magpro
Vihtavuori N560
Norma MRP
IMR 7828ssc.

I prefer Magpro as it meters so well and gives fantastic speeds.

I've never tried it as it isn't available locally but Ramshot Magnum is one I'd really like to try. It's a ball powder like Magpro but reports are that it's more temp resistant than Magpro.

Actually, the methods used by the powder companies with their elaborate pressure testing equipment are just a rough approximation of a cartridges safe pressure and velocity.
To get a true max load, you need to depend on the guy that shoots three loads with case of an unknown history over a $100 chronograph and uses precise pressure testing methods like bolt lift and the "thunk" of the bullet hitting a white rock. The ballistic labs confuse the issue by considering too many things like ambient temperature, altitude, and mean, average and maximum pressures, what the hell ever those are.
If you want to get real technical, you can even get a ruler and measure case head expansion.

Just when I thought I knew something about the hobby, you come along with all these advanced methods....

I've not tried all powders known to man that would make sense in a .270 but to date I've found the 3K range for a 140 to be fairly much the end of the line. Personally I feel that if you've a good load that's running 3025 I'd load up more (or buy more) and get to filling arks. IMO and IME trying to beat that number by 100-150 fps isn't something I'd be spending my time on.

Dober

The older I get, the more conservative I get, in many ways. Personally I would shy away from shooting a .270 that was getting 3200 fps with a 140. I have backed away from loading that fast even with a 130 these days. 3100 with a 130 or 3050 with a 140 will do everything that can be done with a 24" .270 and you can relax while pulling the trigger.

"Pressure signs" are what happen just before things get really silly, and the guys at the range probably won't pay for a nice new glass eye.

I guess I am turning into a fuddy duddy.

But we have people who've posted in this very thread say numerous times that the .280 Rem with it's 60K MAP can virtually reach the speeds offered by the 65K .280AI. Same thing with the 61K 7RM when compared to the .280AI.
They always say "at equal pressure". I always say once they exceed book max with these cartridges they really have very little idea what pressures they are actually running.
Advocates of the 7RM have said more than once that exceeding SAAMI MAP is common and easily done. They've advocated using 7Roy loads in the 7RM as well.

I personally have zero problem running the .270 Win, .280AI and other 65K cartridges at a 65K MAP. If one cartridge pushes 67K and the next pushes 63K my MAP is 65K. That 67K load doesn't scare me any more than the 63K load.
I don't even have a problem running the 60 and 61K .280 Rem and 7RM up at 65K because the cases have been proven in other cartridges as being capable of handling that pressure but I know once I exceed the pressure tested book max I'm in uncharted waters regarding actual pressure.

nsaqam ?
How do you know that one shell is at 63K, the next at 65K , and another at 67K? Please submit details.

I don't anymore than any handloader without pressure measuring equipment (nearly all of them) knows what pressure they're running.

Read the book pressures all you like and you still don't know what that combo runs in YOUR rifles.

I use the exact same indicators everyone else does sans pressure measuring equipment.

I measure the speed, compare it to QL, and make a guess. If both of those match I'M confident my loads are safe in MY rifle.
When I get a bunch of reloads on each case without primer pockets enlarging and no signs of incipient casehead separation my confidence in that load is bolstered.

PERSONALLY I'm not concerned if I'm pushing 68K as long as my brass tells me it's fine.

Whether anyone else is comfortable with MY loads for MY rifles is immaterial.

I will get the Pressure Trace system someday however.

If you are running that close to the max and exceeding the max pressure " occasionally ".........even if you did not have a catastrophic failure......would you not eventually create an excessive headspace problem in your rifle ?

It seems to me if you want to run your .270 that fast.......maybe you should get a .270 Wea or a 7 mag.

nsa - I bought a PT stystem last fall (?). First of all my computer is Windows 7 and is/was not compatabile with it. I've been working 2 jobs (1 part time) and got involved with remodeling where I'm fixin to move.

Soooo I haven't had time to check into any changes with PT OR to atempt to use it. Using it is HIGH on my short list AFTER moving and getting settled.

Good luck getting a pressure measuring system.

20 plus years running one of my .270's at max pressure and the headspace on this rifle (a Savage 111) hasn't changed one iota.

The cartridge is SAAMI'd at 65K and a rifle factory chambered for that cartridge should be able to handle that pressure in perpetuity.

That'd kill me Jwall!

I'll have to check into the compatibility issue.

Thanks for bringing it to my attention.

BTW, when I began reading your post I was hoping you wanted to sell it to me REALLY cheap!!

No such luck!