Explain How 7 PRC Has Tighter Tolerances VS 7 MM Rem Mag, without bashing any company

[Barehandlineman11]

There is nothing wrong with the chamber, the virgin brass is short.

"Peterson makes the traditional SAAMI spec'd version that headspaces off the belt. But it also makes a version that headspaces off the shoulder. Peterson calls that version the 7Rem Mag "Long". This listing is the LONG spec version."

Derek Peterson, president of Peterson Cartridge explains, "Traditional .300 Win Mag casings tend to show signs of case head separation after only 5 to 7 firings.

"Here's why. The SAAMI specs for any caliber, list a minimum and a maximum tolerance for each dimension on the casing and the rifle's chamber. With most calibers, the max dimension of the casing, for length-to-shoulder (L-T-S), has the shoulder of the casing right up against the chamber wall. With .300 Win Mag, unlike most other calibers, if your rifle's chamber is cut to the SAAMI minimum for L-T-S, and your casing is at the max length-to-shoulder dimension, the casing shoulder is still .0095" away from the chamber wall. With the same minimum rifle chamber, a casing at nominal L-T-S is .012" away from the chamber wall. In a worst-case scenario, if the casing was produced at the SAAMI minimum L-T-S, and the chamber was cut at the maximum L-T-S dimension, the casing shoulder could be up to .026" away from the chamber wall. That might not sound like a lot, but it is.

"That large gap is what causes the casing to stretch so much when it is fired. It's that stretching that causes premature case head separation."

Peterson continued, "With Peterson .300 Win Mag- Long casings, we make them with a longer L-T-S dimension. All the other dimensions are SAAMI spec. But the longer L-T-S prevents the casing from stretching excessively on the 1st firing. The casing has more support, which translates to less stretching, which translates to longer case life."

I know what it is. And the Peterson long cases are few thousands longer for better fit in Saami chamber. That much virgin brass being short is strange id believe one chamber was set too deep before i believed that much brass was that short. But this i why i chamber off the brass i intend too use and not a go guage.

 

[VinceMule]

WapitiBob, thanks for teaching that!

 

[cajun]

Stretching 20 though in length? Sounds like over chambering.

It's an issue with all belted mags. If you run them off the belt they wont last long.

 

[VinceMule]

I have run loads with pressure off my 7 STW, 7 Mag, and 300WM. Primer pockets give up the first in Rem brass, but it is hard to switch when the groups are so tiny. So, I have an Ammo can full of new brass.

When a guy has his reamer ground by JGS or Manson, it would be a great time to allow for this issue on case length. Honestly, till this discussion, I just figured the growth was normal, and since my High-pressure loads do not last long(5-6 firings), I just figured it was par for the course, these are hunting rifles, not target rifles.

I have a 7 Rem Mag "match" reamer and will look at the print to see if it is shorter, I doubt it.

I have been shooting the 7 Mag since 1978, I may have seen 3 Remington cases develop a line about .300-.400 in front of the Web which indicated a case head separation was about to happen, and I have had one case head separation, easily removed with a 45 caliber pistol brush.

There is another real can of worms we deal with, and that is the difference between web dia in the chamber, the web dia the die sizes to and spring back. Often, you have to push the shoulder back much further than you like to size the web dia to where the bolt is closing easily(0.0005 very often).

Reloaders are getting advanced enough to deal with these two issues separately with different dies made for each(ring die and shoulder bumper). I hope, this is one area in which we may see some corrections being made in the PRC line of cases.

 

[E4aker]

I know what it is. And the Peterson long cases are few thousands longer for better fit in Saami chamber. That much virgin brass being short is strange id believe one chamber was set too deep before i believed that much brass was that short. But this i why i chamber off the brass i intend too use and not a go guage.

A belted case headspace's off the belt. The shoulders on virgin brass are typically .015 or so short from the shoulder of the chamber when chambered to proper headspace.

The Peterson long brass I had in the shop was around .012 longer (base to shoulder datum) then the Norma brass in 300 win mag

 

[VinceMule]

This is why my 264WM is far superior in every way over the PRC, no matter which one you use as the parent…29.5" 8" twist barrel, custom throat/leade dimension and no taper. The taper is important on SAAMI chambers because originally, the cartridge was designed to run 'bore rider' bullets to reduce pressure…many have forgotten this point.

Cheers.

Magnum, would you mind explaining this issue? I use a bore rider reamer for my 6 PPC.

 

[Mike Matteson]

The release of new cartridges seems to be what the "originals" should have been. Just add faster twist, more freebore, longer heavier higher BC bullets to oldie but goody and guess what you end up with? Plus not having to beg for brass.

I saw that a long time ago in new rifle cartridges coming out over a long period of years. I really feel it's just to sell rifles to people. "Get the new one it's better than the old cartridge in that caliber. There a saying, Fool rush in, where wise men don't go. I am not saying there isn't new cartridges that are coming out that do make a big differnce. Improvement over old stile cases. Just like the 22/250 that came out a great many years ago. It wasn't a barrel burner, like the 220 swift. Not much difference and a little slower too. What's being built out there now, most are barrel burners too. I can as a kid of 14yr looking at bullet for my 300 H & H mag. 220gr were round nose bullets. They are still out there, but have the bullets changed over the years. Some of the biggest changes have been over the years are, How and the way bullets are being built. Cases being made, Powders being made, & The practices or steps that have changes over the great many years. Barrels twist rates are increasing do to how bullets are being built presently, and looks like a winner too. Even farther back as a very young kid, that one old guy at the time in Fallbrook Cal that worked over rifles and loaded for them. Just how many all lead bullets he had. That doesn't fit now days in the rifles and some pistol being built. Even in the inline black powder rifles. The bullets are being pushed beyond what they will handle.
I know that any new rifle that I am having built in the last 2 years are in the area of 7-1 rate, not the 10, 11 or slower rates of the days of old. There are some rifles that I won't change the twist rate like in my 220 swift at 14-1, but I use 55gr bullets in it only.

 

[Buckys]

alignment would be more of a shoulder angle and body taper issue wouldn't it?
maybe it was poorly explained.
as I don't want to repeat a poor attempt maybe a link with some ideas

Modern Cartridge Design: Why New Rifle Cartridges Are Superior to the Classics

Modern Cartridge Design has revolutionized how we build cartridges, leading to hunting and competition rounds that outperform the classics.

www.outdoorlife.com

This is off topic since OP specifically asked about 7prc vs. 7rm (so I got his ok to post here).

To the "modern cartridge design" article from John Snow talks about, the 7wsm (and saum) seems to me a "semi-modern" cartridge and many of the key attributes like case taper, shoulder angle, etc. are found in the 7wsm (and 7saum).

The shoulder angle is 35' in the wsm vs. 30' in mrc/prc.

The neck length is the one thing with the wsm design that does not lend itself well to running e.g., 175-190 grain bullets in the 7wsm after a barrel replacement to faster twist.

 

[coop2564]

Hornady is king of hype!

 

[Dirtrax]

Have y'all ever heard of "clickers?" The entire line of PRC cartridges suffers from them. ITS BECAUSE THE TOLERANCES ARE TOO TIGHT!
The good news is Alex Wheeler has reamers to solve the problem.
My old school 7mm Practical can beat up your 7-PRC! I'll meet you behind the bleachers.

 

[E4aker]

Have y'all ever heard of "clickers?" The entire line of PRC cartridges suffers from them. ITS BECAUSE THE TOLERANCES ARE TOO TIGHT!
The good news is Alex Wheeler has reamers to solve the problem.
My old school 7mm Practical can beat up your 7-PRC! I'll meet you behind the bleachers.

A "TOLERANCE" is the total amount a dimension may vary from an upper and lower limit. That has nothing to do with what you are referencing. Tight tolerances are desired.

Not enough clearance can be a problem though.

 

[73driver]

Have y'all ever heard of "clickers?" The entire line of PRC cartridges suffers from them. ITS BECAUSE THE TOLERANCES ARE TOO TIGHT!
The good news is Alex Wheeler has reamers to solve the problem.
My old school 7mm Practical can beat up your 7-PRC! I'll meet you behind the bleachers.

Awesome !!! Who makes rifles and ammo for this great cartridge ???. In my personal view a cartridge that launches a long for caliber bullet say 160 for .277, 190 for .308 or 170 for 7mm at about 2950-3000 fps MV is IDEAL. I say ideal because with this class of bullets at this velocity you get plenty of velocity, decent energy to 500-600 yards, accuracy and lower end of recoil and better barrel life than the more overbore offerings. If you like more speed then the PRC is not for you. If you want a 7mm that is accurate, efficient and delivers energy out there (maybe not to over the horizon) then the PRC is a legit option and I like that Hornady is coming up with well designed cartridges that make sense.

 

[gregsjt]

Have y'all ever heard of "clickers?" The entire line of PRC cartridges suffers from them. ITS BECAUSE THE TOLERANCES ARE TOO TIGHT!
The good news is Alex Wheeler has reamers to solve the problem.
My old school 7mm Practical can beat up your 7-PRC! I'll meet you behind the bleachers.

Look at the reamer and cartridge prints of the SAUM, Ultra Mag, WSM cartridges. They all have the same tolerances between case and chamber. Those cases also suffer from the clicker problem from time to time. I personally think the issue has more to do with worn reamers than with anything inherit in the cartridge designs.

 

[VinceMule]

Reamers are ground with */- 0.0005 Tolerance per prinit specifications. They can be a tad over or undersized. Various brass makers also vary on web dia.

You need .003 at a minimum clearance in the web on brand-new brass, and many custom gun makers go larger, often much larger.

You are indeed in trouble if you only have .002 clearance on the new brass vs chamber dimension, which would be considered a "match" dimension of old-school thinking. I have been busted by this on several occasions. Issues with European brass vs American brass in dimensions can be a real head-banger.

Some gunsmiths can Lap out a chamber by another .0015-.002, don't expect this to be cheap.