Why is my 7mmWSM faster than 7mmPRC

[NZ_Fclass]

The interweb says that the PRC was developed by Hornaday in response to a SOCOM request for a cartridge to made 2000yd hits on a man sized target.

Then they pretty well failed. A 175g ELD-x at 2000 yds is subsonic and has only 355ft/lbs energy. Even if you get the wind right...

We could always have a discussion about the 7mm Rem mag versus the 7x61 Sharp and Hart Super?

 

[ebb]

NZ that was kinda my point, Hornaday has been reinventing the wheel, and hyping everything they come up with as the next great thing. While reality is not the same as the hype.

 

[FrankG]

Some barrels are faster than others

but do we always know why?

Unless you measure the bore and groove to the 4th decimal place... a tighter or looser bore will effect the pressures and velocities differently.

Also what was the chamber cut to? Custom? Saami min spec or production spec.? Production chambers I've seen drawings where some are as much as +.002" bigger on diameters.

Production barrels are typically bigger and I've seen some way undersize and you think they wouldn't have left the plant.

Same with custom barrels... I hate it when someone compares this brand or that brand to another brand and say... "oh... so and so's barrels are faster than the other but people don't have a clue as to the bore and groove sizes or even what chamber reamer spec cut each barrel.

I'm helping a guy on the hide right now. Box 300PRC ammo and he's blowing primers. Right away someone said the ammo is bad and send it back to Hornady. I chimed in and said hold the phone for a second. So we got into the barrels and chambers... the barrel he has on the gun is from a custom maker.

Sure enough they had to drop to the smallest pilot bushing they had which was .2995" to chamber the barrel and that was still tight. So bore is already undersize. Now you have to ask yourself what is the groove size at? The place that chambered it upon chambering and extracting a live round... you could see all the marks from the throat and rifling on the bullet. So now the chamber is undersize as well. A close guess from me is his pressure is up around +16k psi. Add that to ammo that is running about 65k and now your up around 80k psi or even more. That's why he's blowing primers.

Later, Frank
Bartlein Barrels

 

[deadidarren]

In my experience, it's not hard to outrun a 7PRC. I have found it to be one of the most underwhelming cartridges made. Gun writers sure love it, but I haven't seen it do anything special at all.

Yeah it reminds me of the 6.5 creedmoor

 

[Orr89rocz]

but do we always no why?

Unless you measure the bore and groove to the 4th decimal place... a tighter or looser bore will effect the pressures and velocities differently.

Also what was the chamber cut to? Custom? Saami min spec or production spec.? Production chambers I've seen drawings where some are as much as +.002" bigger on diameters.

Production barrels are typically bigger and I've seen some way undersize and you think they wouldn't have let it leave the plant.

Same with custom barrels... I hate it when someone compares this brand or that brand to another brand and say... "oh... so and so's barrels are faster than the other but people don't have a clue as to the bore and groove sizes or even what chamber reamer spec cut each barrel.

I'm helping a guy on the hide right now. Box 300PRC ammo and he's blowing primers. Right away someone said the ammo is bad and send it back to Hornady. I chimed in and said hold the phone for a second. So we got into the barrels and chambers... the barrel he has on the gun is from a custom maker.

Sure enough they had to drop to the smallest pilot bushing they had which was .2995" to chamber the barrel and that was still tight. So bore is already undersize. Now you have to ask yourself what is the groove size at? The place that chambered it upon chambering and extracting a live round... you could see all the marks from the throat and rifling on the bullet. So now the chamber is undersize as well. A close guess from me is his pressure is up around +16k psi. Add that to ammo that is running about 65k and now your up around 80k psi or even more. That's why he's blowing primers.

Later, Frank
Bartlein Barrels

yup. Dimensional tolerances certainly a big factor. And also throat design. I’ve seen/heard of +P throats/chambers that can take a cartridge to abit higher velocity without pressure signs for a given powder charge. I would be curious if rifling type would make a difference, number of grooves, shape of the cut, etc

 

[FURMAN]

As others have stated there are numerous factors that can affect velocity but the 7WSM brass I have is nearly identical to my 7prc brass in case capacity. I would consider these cases ballistic twins with the PRC being much better designed in the saami throat.

 

[MTlager]

My 7 Saums and 280 ai’s do the the same thing as the 7 prcs will do with 4 grains less powder ! But will get better life out of the prc brass .

 

[FrankG]

yup. Dimensional tolerances certainly a big factor. And also throat design. I’ve seen/heard of +P throats/chambers that can take a cartridge to abit higher velocity without pressure signs for a given powder charge. I would be curious if rifling type would make a difference, number of grooves, shape of the cut, etc

The trade off on the +P throats is your making the free bore longer and bigger in diameter. What it is for each chamber/cartridge I don't know but typically doing things like that will work against you barrel life wise.

Style of rifling and number of grooves doesn't change a thing/effect pressures and velocities. Same goes with the twist. Your back to the total surface area of the bore

The 5R rifling does help fight bullet failure and help the BC and the flight of the bullet. That's been proven now 3x in the last few years. Last test was run just shy of 2 years ago now.

Later, Frank
Bartlein Barrels

 

[Tangent]

I've been surprised that the actual bore/groove diameter isn't more of a thing in rifles. When I was shooting revolvers that was the first thing you'd check, makes a big difference with cast bullets.

I have run a 155gr Scenar-L and a 155.5gr Berger with the same loads and rifle and interestingly the Lapua run at 100fps faster. I'm presuming bullet diameter, bearing surface, hardness of jacket all play into that.