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Old November 27, 2012, 04:35 PM   #31
Join Date: March 4, 2005
Location: Ohio
Posts: 13,714
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No. There's no free lunch. If the velocity is higher so is the pressure. If the standard primer lets you use more powder before there's a pressure sign, then it's because the portion of the powder ignited initially is lower. What a magnum primer does is increase the starting pressure in the case. The reason it's called "magnum" is that magnum cases are more likely to have the extra space in them that needs their extra pressurizing gas quantity. So as long as a load was developed with magnum primers to be operating at safe pressures, then it will be safe.

A good article on the topic by a former CCI employee is here.

There are a couple of things to be aware of with magnum primers. One is that the larger gas volume they make can sometimes unseat bullets in handgun rounds before the powder burn gets well enough under way. That can actually lead to lower pressure and velocity, and the consistency of it is usually poor. As a result, a chronograph shows higher velocity standard deviation.

On the other hand, if the charge volume leaves a lot of empty space and the powder has trouble getting up to starting pressure, the higher pressurization by a magnum primer can then improve velocity consistency over the standard primer. To find out, you just have to work loads up from a starting level with both types. I usually knock an extra 5% off a starting load for a magnum primer if the load data was developed with a standard primer.

In the case of the Tightgroup loads with the 300 grain XTP on Hodgdon's site, the primer used is a Remington 2½, which is not a magnum primer. On the Hodgdon site, when you look at load data, click on the "Print" button to see what the brass and primer were. Hodgdon's listed pressures for Tightgroup are pretty reasonable up to 9.6 grains and if they are correct, they should not cause sticking cases. Hodgdon claims 38,400 CUP for 9.6 grains, which has no exact equivalent to psi, but is probably in the 40,000 psi range. It's hard to tell though, as copper crushers can err by substantial margins. QuickLOAD, on the other hand, thinks that same load would be closer to 50,000 psi.

QuickLOAD is not great with straight wall cases, but your experience with sticky extraction suggests reality for your lot of Tightgroup lies somewhere in between QuickLOAD's numbers and Hodgdon's numbers. QuickLOAD thinks you'd have to go all the way down to 8 grains to stay inside SAAMI pressures, but without velocity measurements from you and the length of your barrel, I can only tell you what the defaults suggest.

The rule I gave you about the sticky extraction point is the basic method used by Elmer Keith and others to develop revolver loads above rated pressures. You can do it, but there's no need to stress the gun with higher-than-standard pressures when slower powders will get you the velocities you want with lower peak pressures. They will give you more muzzle blast because they keep the pressure up longer in the bullet's travel down the bore than fast powders do. That's how they can get to the same velocity with a lower peak pressure.
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Last edited by Unclenick; November 29, 2012 at 10:33 AM.
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