Home Forums Hunting & Shooting Custom Rifles & Wildcats Interesting thread on the short mags..........

Blaine,

Don't get me wrong, I don't have any problems with the traditional belted magnums. I just bought a Win. M-70 in .300 WSM and a Remington M-700 AWR in .338 RUM. I just want to find out for myself what these are all about and capable of. - the9.3Guy!

Don--

Perhaps a more accurate term would be "wide pressure swings" rather than spikes. It seemingly occurs with about every hunting weight bullet in the 7mm Remington Magnum.

In general, the labs expect maximum pressure swings of 2000-5000 psi with most cartridges, but of course look for the lower as a sign of a consistent load. It's pretty easy to find this sort of spread in most cartridges.

But the 7mm Remington Magnum will swing 8000 or even 10000, and often after several identical rounds have stayed within 4000 psi of each other. And oddly enough, the 7mm STW, which has the identical shoulder angle and short neck, doesn't show the same swings! This indicates that the shoulder angle/short neck theory probably isn't correct.

The problems with the .243 don't show up as much until the throat becomes a little worn--and again, only with bullets in the 100-grain weighr range. Once again, they don't know why.

Despite all this, the 7mm Remington Magnum and .243 Winchester have reputations as accurate cartridges, at least among hunters. I have used both a lot with excellent results. But nobody picks them as target rounds, and the erratic pressure swings are one reason. Oddly enough, if you neck either case up a little, to .30 from 7mm in the case of the magnum, and from 6mm to 6.5mm in the case of the .243, the problem goes away. Why? Nobody knows.

If you do some math with one of the loading manuals, you'll find that almost any "modern" cartridge shows some bullet protrusion below the neck to various degrees. The 200-grain Partition in the .30-06 seats almost 2/10ths of an inch below the neck, yet nobody complains about it. Same deal with the 175 Partition in the 7x57.

The big difference between these and most magnums is that, in the magnums, there's a lot more powder space left AROUND the base of the bullet than in the standard cartridges.

Acy--

You're probably OK. The latest pressure-tested handload data for the cartridge indicates you can approach 3100 fps in the standard magazine anyway. Nosler lists 3082 with IMR4831 as their fastest load, but in a 26" barrel.

Miking cases doesn't work worth a hoot. I did some research on this last year, first working up loads using all the standard home-grown indicators from bolt-lift to miking, in a .22 Hornet, .270 Winchester and .30-06. Then I had the identical loads (brass, primer, powder and bullets all from the same lot) tested in a lab. Sometimes miking worked OK and sometimes it was absolutely useless--probably because brass varies from brand to brand in hardness, head thickness, etc.

I also found out that many common indicators of high pressure don't work so hot. One load that caused very sticky bolt lifts tested just fine in the lab, while another that worked slickly in my .270 was 2000+ psi over SAAMI max!

A much better indicator is your chronograph. If you're getting so much extra velocity from a given case that it seems unreal, it probably is. You're probably right at the top safe velocity for a "long-seated" .300 WSM.

I posted this question about wild pressure swings at AR and Jack Belk commented that he has found that the 243 Win has blown up more rifles than any other cartridge and that's excluding obvious mistakes such as overloads and plugged barrels.

I have used the 243 Win with for 200 yard competative target shooting with success and so has David Tubbs who won the national championship with a 243 Win. However I used only short bullets as I previously reported. The 7mm RM also won the Wimbleton Cup but those cartridges could have had special throats.

You need at least one reactionary post in this discussion, here it is, if I can't get within 200 yards so I can shoot it with a 220 grain softpoint in a Krag with a Lyman 48 I won't pull the trigger.

Miki--

Kind of off the subject but well put, Miki.

My own .30-40 is a High Wall with Axtell tang sight. It is also deadly out to 200, if I do my job! Doesn't rip up meat like the mangle-ems either. I am also partial to the .300 Savage. Have two, one with 2x Noske scope for "long shots," the other with Redfield peep.

In regard to measuring case heads consistently, for a quick check at the range I recommend what is called a thickness "Snap gage"they are Graduated in .001's you set nominal size and the gage snaps on fired case to show + or _ on indicator, the case can be rotated also to show out of round. In the shop it can be done easily to 50 millionths. They are avalible from Travers tool co .(ttc.com), Rutland and others, I think maybe Brownells may have them.