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7MM Ultra Magnum

7mm_UM

Introduced in 2000, everything I noted about the 7mm STW, applies in spades to the 7mm Remington Ultra Magnum (RUM).

As with any high-performance round, barrel heating is a significant concern.  Allowing plenty of time for barrel cooling can significantly increase barrel life.  Conversely, overheating the barrel by rapid firing can render a barrel uselessly inaccurate after surprisingly few shots (see the 264 Win Mag text).  For a big game hunter this might not be much of a concern (see 7mm Rem Mag text).


When loaded with typical hunting bullets and the best modern propellants, the 7mm RUM offers not one iota of performance advantage over the 7mm Rem Mag if loaded to the same pressure.  But, of course, we cannot ignore SAAMI pressure specifications.  When safely loaded within SAAMI pressure specifications, the RUM has a 55-fps edge over the Rem Mag with the 195 Berger Elite Hunter.

While this difference might not seem like much, velocity is always king.  Velocity is why we gave up throwing fists and graduated to rocks; then gave up rocks and graduated to spears; then gave up spears and graduated to an atlatl; then gave up the atlatl and graduated to arrows; and then gave up arrows and graduated to bullets!

Refer to the 22-250 and 257 AI discussions to read about what can happen when we ignore SAAMI pressure specifications and load ammunition at pressure higher than the gun was proved safe with through the Proof-Testing protocol.

Should the reader get the silly notion this writer is immune to violating this important safety tenet, please consider the following:  In 1996, I was afforded the opportunity to spend more than a week in the Accurate Arms ballistic laboratory.  While there, I did many tests.  Among those was checking pressure of three of my long-time pet loads.  For various reasons, I was certain those combinations were entirely safe.

First load, a 270 Winchester, combining the 170-grain Speer Round Nose with N570 (a long obsolete 50 BMG propellant with a burn rate very similar to H870).  I was sure this load was mild for two reasons.  First, it seemed it should have been impossible to seat that bullet on enough of such a slow-burning propellant to generate significant pressure.  Second, because the fired Winchester cases showed no evidence of excessive pressure, the primer pockets were tight and the cases ejected from my Remington Pump without any resistance.  (Unlike a bolt-action rifle, the Remington Pump has zero primary camming force to pull a stuck round free.)

Second load, another 270, which combined N205 and the Sierra 130-grain SBT in the same 270.  I figured pressure might be a little high but, as with the first load, the fired cases (Remington this time) showed no evidence of excessive pressure and those ejected freely.

Third load, a 7mm Rem Mag, combining the 175-grain Sierra SBT with H870.  Again, I just knew it would not be possible to get enough H870 in that case to generate dangerous pressure with that bullet and the fired cases were perfectly normal and extracted freely.

I was cocksure of myself in all three instances — soon enough, I got over that, permanently!

Imagine my shock when I got the following pressures, without including the 5,000 to 7,000 psi offset that likely would have been necessary to account for the strength of the case (we had no way of measuring this for these well-used and very old cases but those values are conservative, for sure!).  Recorded average pressures: 170-grain 270: >69,000 psi; 130-grain 270, >72,600 psi; 7mm Rem Mag, >71,300 psi.

With realistic offset corrections, each of those loads generated very close to Proof Pressure.  For all practical purposes, every time I fired one of those rounds I was proof-testing my gun and for sure every time I fired one of those loads I was entirely circumventing any value of the factory proof testing.

So, when thinking about a guy who died (see the 257 AI discussion) and a friend who could have died (see the 22-250 discussion), I am wont to remember the following: there but for the Grace of God go I!
If you read the above, perhaps you can see that my claim about the importance of not ignoring the SAAMI pressure specifications is well-founded — never deliberately do so.  And, keep one other thing firmly in mind:  if you are getting higher velocity than published data suggests, you are almost certainly also exceeding safe working pressure.


The text associated with the cartridge description reflects opinions and conclusions of the author, M.L. (Mic) McPherson. Lee Precision and its employees do not necessarily either agree or disagree with any of his comments. We present these with due deference to his recognized expertise in the firearms field. His acumen extends to handloading and all aspects of ballistics - internal, external, and terminal.
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