4000 fps in a 7mm STW w/140 Bullet?

Bart B said:
The highest max average pressure for a 7mm cartridge is probably the Weatherby Magnum version at 65,000 psi using SAAMI transducer measuring systems.

If that 'smith was near me, I'd consider betting him a bushel basket full of hundred dollar bills that his 7 STW would not push a 140 out over 3600 fps average from a 26" barrel with a max average pressure at 65,000 psi.

I had a 'smith back in the '60's tell me his .264 Win Mag would push out 140's at 3500 fps at normal safe pressures. After I offered a similar bet with him, he refused saying he didn't want to take my money.

I'd stay far away from that guy.
Click to expand...

The 7 roy and 7rum both "top out" at 65 KPSI.

I had a strain guage on my sendero 7stw (26" bbl.); I'll try to dig up a few of the graphs and post them. They were a bit of an eye opener; easy to extract stuff was sometimes putting up over 70KPSI peaks. I never got balsy enough to push it 'till she got sticky though.
 
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These are the four most pertinent ones-- the other one had little labeling and was likely just shot to confirm. Remember, I had no reference ammo, so it was basically just a comparison between loads. The two that say winchester are the 150 grain win factory load that was availability for a time. Notice it actually has the highest peak pressure.

RL 25 and 7828 were putting up within 25 fps of each other.

the first ans last trace were rl25, btw...

notice the small secondary peak just after 1 millisecond; it is a mild tight spot in the barrel that was noticeable with a patch or brush.
 
Last edited:
That load of 76 grains of 7828 has a peak pressure spread of about 6,000 psi. I didn't see what the muzzle velocity spread was about the 3375 fps average. Such spreads with slow powders are very normal and usually cause greater muzzle velocity spreads than a faster powder at the same average peak pressure.
 
78 grains 7828 with the 140 sierra was my standard load for quite a few years until rl25 came along. I switched because rl25 gave me the same basic speed (3350 fps- vs 3375 fps for 7828) with significantly less pressure peak and a bit better uniformity. It is also just as capable accuracy wise; both handloads were 1/2 moa average.
Some of the peak pressure spread on the handloads was likely mixed trim lengths. I was using borrowed equipment at the time so I was more concerned with getting the ammo loaded than it being perfect. As long as the brass was usable and the charges/ bullet seating were correct it went home.
Part of the reason I asked my brother in law to help put a gauge on my rifle was the fact that there was NO data for rl25 at the time as it was a new introduction. I was very concerned that I was getting the charge weight to correct levels.
 
I think the ultra-slow powders are producing more repeatable pressure curves these days. That's good for accuracy.
 
gilmillan1 said:
Update: 90 grains of powder and a 35 inch barrel. Possible?
Click to expand...
You will likely hit the point of diminishing returns around 30 inches on barrel; just guessing here though. I'd query some of the threads on the bug 338's for more info on that. They've been playing with long pipes for as while.

I'd guess 3700- 3800 fps top even pushing to 65,000 Kpsi+ peak and using slow enough powders for compressed charges.
 
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