Home Forums Hunting & Shooting Ask The Gunwriters 7mm Remington Magnum, Brian Pearce, HANDLOADER #323

7mm Remington Magnum, Brian Pearce, HANDLOADER #323



Friend Allan Jones, the editor
of the Speer Reloading Manuals
No. 12, No. 13 and No. 14, reported
that “there is a great deal
of velocity variation among sporters
in this caliber.” He continues
that “Rifles in the Speer’s lab’s
collection posted velocities varying
from 400 ft/sec under to 300
ft/sec over those posted by the
SAAMI pressure barrel with the
same ammo.” While I have not experienced
the incredible 700-fps
swings in sporter rifles that the
Speer lab has, I have seen 300-
to 400-fps swings with identical
loads between Remington, Weatherby,
Winchester, Ruger, Interarms
and Sako rifles, each featuring
24-inch barrels.

The best eye opener for me regarding the 7 mm Rem Mag was Ken Water's Pet Loads from January 1981. He referenced a couple of things that he thought were common to the cartridge that affected it round a lot. This was the first round I loaded for back in 1973, and had me scratching my head for quite some time. I have had several rifles (five if memory serves) and they have all been unique from the others. According to Waters there were (as I saw it) a couple of things that affected it. One he pointed to a wide variation in case capacities of different headstamps of brass, secondly he indicated a wide variation in chamber dimensions, as the critical headspace dimension was the belt, there seemed to be less regard for tolerances at the shoulder. Whether or not any of this still holds true, I don't know, but every time I load for a different 7 mm RM, I use a box of factory ammo and my chronograph as a reference point, along with loading notes and good data sources. So I took from that that if you had a particular 7 RM with an oversize chamber and a case with greater internal dimension than another 7 RM, they might be considered a "wildcat" each to the other.

Questions:

So factory loads must be pretty weak, just in case?

Treat each rifle independently don't shoot same loads in multiple rifles unless really conservatively loaded?

Barrel and or chamber specs all over the place causing this issue?




I've always read this about the 7 mag and I thoroughly trust the author so would probably be very diligent to segregate ammo if owned multiples.




Thoughts?





Mike

Which is why I used the word "variation". ...

With the added a/k/a pressure spikes, which was the bone I picked.

My understanding, from more than one major pressure lab, is that the 7mm RM shows a wider spread in pressures than most other cartridges of the same class, both high and low. Which is why the SAAMI Maximum Average Pressure (MAP) was set at around 60,000 PSI when piezo-electronic pressure-testing became standard.

Some other rounds have a SAAMI MAP of 65,000 PSI, which is as high as SAAMI will go, but they also have standard for what the highest-pressure round in a string measures, to allow a safety margin.

Yep, but in the .243 the pressure variation apparently doesn't occur with bullets under about 100 grains.

One problem with both rounds (again, info from pressure labs) is the short neck and shoulder angle. The combination results in relatively quick throat erosion, which in its early stages can also result in erratic pressure. Once the erosion extends farther down the barrel, pressures tend to decrease.

My hunting load is 11 grains over max published by Hodgdon, but it with a moly bullet and 3.34" OAL, so it is more like 10 gr over max equivalent. There is still good safety margin to stay below extractor groove growth.

I have hunted with 3 bolt action 7mmRM rifles that I rebarreled, re stocked, re triggered, pillar bedded, rescoped, etc , and one Ruger #1 and one Browning 1885 that have original barrels.
I headspace at 0.215" to the belt, not the 0.220" minimum called out by SAAMI.