The Great 7MM Rem Mag Ballistic Experiment.
- Thread starter chuck nelson
- Start date
Seems to be quite a bit of guessing going on here?
Picking a bullet, and seating it to mag length, then having chamber and throat cut to same, is hardly a recent discovery. However, I will agree with my good friend Chuck, that the 7mmRM has long suffered from a short throat, and diminished case capacity, due to having to seat bullets further in the case.
There is no hard, fast rule when speaking about pressures. Seat a bullet deeper in a case, without adjusting powder charge, and most likely pressure will increase. Seat a bullet touching, or just off of the lands, with no other adjustments, pressure will most likely increase. Cut a throat longer, to seat a bullet longer, to put more powder in the case? Pressure will most likely increase. There is no free lunch. Velocity comes with a pressure price, every time.
Roy W. pioneered the huge, cavernous throat, to reduce pressures in rather large cases. Nothing new there either, but usually accuracy suffers with such massive jumps to the lands.
Not really sure what Chuck was trying to accomplish here, other than to show off another k-mart mauser build, type a rather entertaining story, and let us all know that he, and he alone, has guessed that he has discovered how Roy W. got all that speed from those cases.
R.
Moving a bullet closer to the lands could reduce pressure, as long as the bullet was still far enough away from the lands, as to not increase the pressure. I hope that make sense?
Also, seating a bullet deeper in a rifle case could reduce pressure, depending on variables, of course.
The rifle case versus the pistol case has more to do with powder burn rates and throat/cylinder/chamber issues than it does with a linear pressure relationship.
R.
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Oh Please.
I realize that me posting pictures of my rifles just absolutely throws you over the edge. It angers you beyond anything I can imagine. Put me on ignore. For everyones sake.
The comedy is non stop! First the entertaining write up in the OP, and now this! What's next? Are you going to start smiling in pictures?
Wrong handed, 5 or 6 trick move k-mart mausers really don't get me excited, so I'm not really sure why you would think I would be angry?
Besides, if I put you on ignore, how could we continue to be good friends?
R.
I was out shooting my 7rm last weekend...just burning through some cheap Speer 130grs, I use H4831 which doesn't get me top velocity, but I sure can't ##### about the accuracy. They would probably make a good long range yote bullet in this gun, or deer if you are hitting it at 300m+. I imagine the 130's would be hard on the deers innards. I do have TTSX and Accubonds loaded up for the more serious stuff.
Anyway, one fella wrote a post in the reloading section about his 7rm acting funny with 7828. It was a warm load but still within guidelines, they shot well and I guess he loaded a bunch which took a few years to get around to shooting. When he brought some of the now seasoned ammo out, it was considerably over pressure. I can't recall if it was blow primers but something like that. When he pulled the bullets to reuse the components, the powder has swollen and was jammed into one big clump that had to be broken out.
I'm wondering if that's where the overpressure urban myth comes from and nothing really with the cartridge design itself? I mean there are very similar 30cal and 6.5mm magnuns based on the same case and we don't hear any of these stories there.
Anyway, one fella wrote a post in the reloading section about his 7rm acting funny with 7828. It was a warm load but still within guidelines, they shot well and I guess he loaded a bunch which took a few years to get around to shooting. When he brought some of the now seasoned ammo out, it was considerably over pressure. I can't recall if it was blow primers but something like that. When he pulled the bullets to reuse the components, the powder has swollen and was jammed into one big clump that had to be broken out.
I'm wondering if that's where the overpressure urban myth comes from and nothing really with the cartridge design itself? I mean there are very similar 30cal and 6.5mm magnuns based on the same case and we don't hear any of these stories there.
Chuck, one of the beauty features of CGN is that if there are no logical substance to these unnecessary attacks then why not contact a Mod. and let them deal with it at their level.
Why play hero when it's not necessary ?
I have, over the last forty years, messed around with short throats, long throats, and, in one rifle stamped "300 Lovelace", a deep throat. In essence the length of the throat reduced pressure with a given load and seating depth but velocity was reduced at the same time. In creasing the powder charge restored pressure and velocity. Seating the bullet out increased case capacity while seating closer to the lands shortened the pressure curve. The upshot of all of this was, although velocity didn't change much, cartridge behaviour did change. A 257 Roberts in a standard action, chambered with a longer throat, shot better and seemed more flexible. My own 257, in a Mauser action, is cut with a .250" long throat. A 264 Winchester with a long throat also seemed to gain flexibility.
Increasing case capacity by any means has some effect on the pressure/velocity curve. If you are able to add some powder, burning that powder produces more gas and stretches the pressure peak over a longer time which will usually result in slightly higher velocities. Not a great deal, mind you, since the amount of case capacity gained is relatively minor.
Weatherby chambers cut with a short throat will often be unable to handle factory loads without loosening primer pockets. Adding some freebore reduces pressure and allows the use of this ammunition. If the diameter of the throat is kept close to bullet diameter, accuracy is not greatly affected, if at all. This was the problem with many of the early Weatherby chambers; the throats were not just long, they were oversized. 300's often had a freebore diameter of .312". If that diameter had been kept to 3085", accuracy would have been largely unaffected. The Lovelace was throated at .3080 x .500 deep and shot very well.
Increasing case capacity by any means has some effect on the pressure/velocity curve. If you are able to add some powder, burning that powder produces more gas and stretches the pressure peak over a longer time which will usually result in slightly higher velocities. Not a great deal, mind you, since the amount of case capacity gained is relatively minor.
Weatherby chambers cut with a short throat will often be unable to handle factory loads without loosening primer pockets. Adding some freebore reduces pressure and allows the use of this ammunition. If the diameter of the throat is kept close to bullet diameter, accuracy is not greatly affected, if at all. This was the problem with many of the early Weatherby chambers; the throats were not just long, they were oversized. 300's often had a freebore diameter of .312". If that diameter had been kept to 3085", accuracy would have been largely unaffected. The Lovelace was throated at .3080 x .500 deep and shot very well.
CN........I did an interesting experiment many years back that somewhat shadows what you are doing with your 7mmRM. I started with a factory 700 in 7 Wby with a 24" barrel, I don't recall, but I'm sure it had the 1/2" long Wby throat. I worked up loads with it and 160 gn Part until I got to max with just a hint of ejector mark and recorded a velocity of 3200 fps +/- a tad as I recall. I then rechambered it to 7mmRM and worked up the same loads using the same IMR 7828 from the same can and CCI 250 primers from the same box and 160 Parts from the same box. These loads were chronographed indoors under exactly the same conditions on the same day, and I used exactly the same 20 W-W brass I fireformed to the 7RM (which was their original headstamp). The 7mm RM chrono'd at 3020 fps with the same hint of an ejector mark, indicating more or less the same pressure. I do believe I also used the same seating depth. I can't explain the disparity and I did this experiment with no preconceived outcome in mind, it was as perfect for both cartridges as I could make it......even reusing the same 20 brass.
Then for S&G I chambered the same barrel out to 7mmSTW and worked up another 160 gn load using the same primers and bullets and fireformed and necked down 300 H&H W-W brass. I don't recall the powder I used but it would have been what ever the books listed as giving the highest velocity in the STW. I then worked up loads until I got the same hint of an ejector mark and these loads crono'd at 3130 +/- a tad fps as I recall, giving me a little more than 100 fps over the 7 RM, which was what the books said it should do. Again chrono'd indoors under the same lighting conditions through my Oehler 35P. The throat on the STW would have been shorter to SAAMI spec, but I don't remember what it was exactly, other than Bevan said it was to SAAMI and it was his reamer I used.
I don't claim to be able to explain the results but they are what they are and I just put them out here for all to peruse and draw your own conclusions. I tried to eliminate as many variables as possible and I believe the data is as accurate as I could possibly make it without pressure testing equipment. However I have been loading and reading W-W brass pressure signs for many years and believe that the individual pressures were very close in all three cartridges.
My only possible conclusion to all this was that maybe Roy did understand internal ballistics a little better than most give him credit for and maybe his "venturi" shoulder design really does change the way the pressure curve acts upon the projectile, and isn't just a marketing gimmick.
Why Not? (Ted) was privy to this experiment and saw the rifle in question and perused my chronograph data and load data, all he did was smile and say maybe old Roy wasn't just a marketing genius after all.........(not a quote but a paraphrase).
I found, in other experiments with my 7mm/300 Wby that extended flash tubes increased velocity but did not change the peak pressure for a give powder charge. It makes sense in that lighting the powder just below the base of the bullet (about 3/4 of the way up the powder column) would decrease pressure because one isn't trying to drive the 80 or so grains of powder out that little 7mm hole and down the bore. The fact is you have effectively reduced the projectile weight by 33% and extended the pressure curve further down the barrel and created a more efficient burn situation for the powder. The powder no longer moves, but is being held in the case and burnt as a stationary fuel. But man are they a pain in the a$$ to load and reload, let me tell you!!
Then for S&G I chambered the same barrel out to 7mmSTW and worked up another 160 gn load using the same primers and bullets and fireformed and necked down 300 H&H W-W brass. I don't recall the powder I used but it would have been what ever the books listed as giving the highest velocity in the STW. I then worked up loads until I got the same hint of an ejector mark and these loads crono'd at 3130 +/- a tad fps as I recall, giving me a little more than 100 fps over the 7 RM, which was what the books said it should do. Again chrono'd indoors under the same lighting conditions through my Oehler 35P. The throat on the STW would have been shorter to SAAMI spec, but I don't remember what it was exactly, other than Bevan said it was to SAAMI and it was his reamer I used.
I don't claim to be able to explain the results but they are what they are and I just put them out here for all to peruse and draw your own conclusions. I tried to eliminate as many variables as possible and I believe the data is as accurate as I could possibly make it without pressure testing equipment. However I have been loading and reading W-W brass pressure signs for many years and believe that the individual pressures were very close in all three cartridges.
My only possible conclusion to all this was that maybe Roy did understand internal ballistics a little better than most give him credit for and maybe his "venturi" shoulder design really does change the way the pressure curve acts upon the projectile, and isn't just a marketing gimmick.
Why Not? (Ted) was privy to this experiment and saw the rifle in question and perused my chronograph data and load data, all he did was smile and say maybe old Roy wasn't just a marketing genius after all.........(not a quote but a paraphrase).
I found, in other experiments with my 7mm/300 Wby that extended flash tubes increased velocity but did not change the peak pressure for a give powder charge. It makes sense in that lighting the powder just below the base of the bullet (about 3/4 of the way up the powder column) would decrease pressure because one isn't trying to drive the 80 or so grains of powder out that little 7mm hole and down the bore. The fact is you have effectively reduced the projectile weight by 33% and extended the pressure curve further down the barrel and created a more efficient burn situation for the powder. The powder no longer moves, but is being held in the case and burnt as a stationary fuel. But man are they a pain in the a$$ to load and reload, let me tell you!!
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