- July 5, 2016 at 12:05 am #28757Sgt. MikeParticipant
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The 7mm Remington Magnum is a great caliber but not without it’s pitfalls for Reloader or Handloaders. Which include the following:
1. Case head separation and lower case life why you ask
a. Well the cartridge has two call outs for head-spacing the primary is the Belt which is
Chamber reamer is .220″ to .227″
the cartridge is .212″ to.220″
that leaves a slop .007″ to .015″” at the belt , this means that unless the shoulder contacts with the chamber the firing pin then moves the case forward prior to crushing the primer igniting the powder before pushing the case back to the bolt.
Majority of the manufacturers usually cut the chamber at the max (if not a little bit deeper for functioning)
The ammunition Manufacturers swage the brass belt at guess where yup you guessed it toward the Min side for again reliability
b. Now the other issue is the relationship of the shoulder (datum dia is .420″)
Chamber is 2.1253″ plus .0100″ that puts the dimension at 2.1353″
Cartridge is 2.1162″ minus .0070″ that is 2.1092″
That means that there is .0091″ to .0261″ gap between chamber and brass
The RELOADER whom follows his die manufacturers directions (full length dies that is) will have the same problem as the factory ammo shooter, this slamming back and forth will cause a case separation at some point.
There should be some differences in chamber to cartridge for feeding and the ability for the brass to spring back but these differences are a bit excessive and leads to short case life
The HANDLOADER however will notice this in quick order his solution is to set the die to head space on the shoulder and not the belt this will greatly reduce the stretching of the case and ensure a little bit longer life of his cases. A headspace measuring kit or tooling such as the Stony point which was bought out by Hornady it’s usually about $35.00 is cheap insurance. Another solution is to use neck size dies this will probably be great advice for the novice.
The gunsmith can also help this by instead of headspacing at .220″ to .227″use .215″ max instead, this will reduce the slop by .005″ or so.
2. Barrel life
The 3 degree Leade I’m quite certain does not help matter as usually slower powders is selected which actually burns hotter. This has been tested by various government arsenal and they found that slower burning powder with certain coating will cause erosion of the bore, much like the corrosive priming of years ago. I tend to think that a selection of half that value for the prints to say a1 degree 30 minute leade would help as this would not delay the bullet as much as 3 degree would. Now admittedly this time is probably in the trillions of a nano second but it does add up in repeated firing. (the 300 Win Mag uses 1degee 26 minutes 37 seconds, versus the 7mm Rem Mag’s 3 degree)
One solution use a faster powder selection from published loads, or use the bottom end of the published load data for informal shooting and saving full house loads for hunting or matches. However caution is warranted as some faster powders are not suitable because of the overbore conditions are a prime combination for SEE.
While I wrote this predominantly on the 7mm Remington Magnum the 300 Winchester Magnum also has the same ills in a sloppy chamber which is why if one pays attention PTG offers a “non sloppy SAAMI” reamer for the .300 Win Mag (http://pacifictoolandgauge.com/conte…-july-4th-sale) .
Most will probably know all that I have pointed out or maybe not. Does this mean that the 7mm Remington Magnum or .300 Winchester Magnum a inaccurate cartridge that should be shunned? Heavens no both are fine cartridges that just need a little bit of attention by the HANDLOADER as the gent whom only shoots factory ammo will never see the problems and remark that all is good. Which for him that is a truth he only shoots that case once. Would I own one knowing all this……………… well I do LOL.
I will now set back and wait for the flame war to begin……..
- July 5, 2016 at 12:35 am #28758GoodsteelKeymaster
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You’re absolutely right Sarge, and that’s why you have to do a bunch of fancy footwork to get a factory rifle to play as well with a belt as it can with a standard rimless cartridge. Since a cartridge like the 30-06 or 308 headspaces on the shoulder datum line, it is much easier for the hand loader to establish well fitting ammo.
Rimmed ammo such as 30-30, 303 British, or 7.62X54R has a similar issues as the magnum cases do and often worse. Chamber pressures often being lower, it becomes a wash of equally janky conditions, all of which were designed for hunting/wartime precision from virgin factory ammo.
The biggest workaround for the hand loader is to treat the belted and rimmed/shouldered cases like a rimless case. Basically ignoring the sloppy headspace on the belt/rim, and using the shoulder instead.
Thats just fine, but it’s a shame, as the shoulder is by its very nature, a movable entity.
The concept of the belted magnum is genius by design but nearly useless in practice, unless you have an hombre like me building the rifle.
I use PTG reamers exclusively, and they are ground to bare minimum SAAMI specs. Add to that my system of chambering that produces a chamber that mimics the size of a PTG reamer within .0015, and the fact that I will upon request, leave the headspace gauge in the drawer and ream for a .002 clearance (less if you care to turn your belts for perfection) and you have a rifle that uses that missunderstood belt to excellent effect.
I believe that if built correctly, the belt can be a huge boon to the precision marksman simply because your ammo is held in perfect alignment with the chamber and headspaces on a rigid, immovable feature.
If care is taken to properly blueprint and accurize the action being used, that belt becomes the thing that points your ammo straight down the barrel, being that it is a semi-hard, precision, entity, pinched perfectly between a perfectly flat bolt face and a perfectly perpendicular belt feature in the chamber.
Like I said, I prefer .002 clearance, but much less is possible.
The same can be done with rimmed cartridges, but that’s a subject for another time.
- July 5, 2016 at 1:07 am #28759Sgt. MikeParticipant
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I set the head-space on the 7mm Mag at 2.125 inches at the shoulder, that allows about thou or maybe less in the chamber of my 700. The reloading dies was not setup where recommended and set a bit higher than recommended. Not that they (die manufacturer ) was at fault, but as adopted by SAAMI these are a tolerance nightmare causing some workarounds such as using the shoulder.
Kicking the 7mm around just wondering if the 7×57 (maybe the .280 Rem if feeding is a issue otherwise not really much difference) wouldn’t be a better choice once I get ready to rebarrel yeah I know the Bolt face needs to change
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