I have been reloading for 6 years now so I'm still fairly new to this in the eyes of many on this board. I have loaded for several chamberings in bolt action, lever, auto, and revolver.

I am puzzled over a fairly new rifle I am loading for. The rifle is an MRC action that was barreled by MRC and chambered in 7mm WSM. With this rifle using Winchester brass, Federal primers, CCI primers, Nosler, Sierra, and Hornady bullets, H-4350 powder, H-4831 powder, and Win. WXR powder I am getting a little over book max. velocity around 2 grains before max. load and seeing pressure signs around 1 1/2 grains before max. book load in just about every combo I have tried from different reloading manuals.

As you can see I've tried enough different load combos to be fairly certain that the loads are not "stiff" via the manuals.

No real problems with this, just have to start low and work my way up carefully over a chroney, looking for pressure signs as is always recomended. I just would like to know what causes it!

One more thing, my Hornady dies, even when set to size as much as possible will only size the cases down to the point were there is still slight resistance chambering rounds. I doubt this is part of the problem since I usualy size in this fashion anyhow but thought I would throw that bit of info in also.

Thanks in advance for any helpfull posts!

Steve, I just went through the same thing and I think I can tell you a bit about your "problem".



First of all, the WSM's are spec'd to shoot out of the factory short action guns that have short magazine boxes. The Remington and Ruger are 2.89" maximum. The Winchester is 3.08" and you have to leave clearance in all for proper magazine function and feeding. In one of my M70's, max COAL is 3.06" for smooth feeding. All your loading information is predicated on this.



In your MRC magazine, the max COAL is closer to 3.115" so that you can seat your bullets out quite a bit further. That gives you more powder chamber space in the cartridge. That additional little bit of room translates to higher velocity at the same pressures with a bit more propellant.



I've JUST started shooting a 338WSM on the MRC short action (57 rounds total so far.) I initially seated the bullets out to 3.11" and started load development for 225g bullets with loads I already had for another 338WSM on a M70 short action. Immediately the same loads chronographed almost 200fps SLOWER than the same load in the M70. To make a long story short, I'm able in a couple of cases to get MORE propellant in there with no pressure signs, than for loads for a 338 Win Mag and I'm now getting Win Mag velocity out of this WSM. The one developed load is for a 225 Acccubond. It shot a .51" three shot group @ 2850fps. That load in my Win Mag for .6" groups goes out at 2855fps. I've got velocity for a number of other bullets but haven't gone to accuracy and load development yet, but all the velocity I'm seeing so far says the gun will flirt with Win Mag velocity as I'm sure you'll find yours will with the 7mm Rem Mag. Your actually starting out completely with a "new" cartridge, especially for reloading purposes.



I found the right load by starting near the max COAL. I reduced the seating depth in .020" increments. I was astounded to see that each .020" of deeper bullet seating INCREASED the velocity by 20fps and I've yet to see ANY external pressure signs at some pretty respectable velocity. Obviosly I ended up with the bullets way off and away from the lands, which also helps reduce pressure.



I can't get enough of the slow burners in the case, the slowest powder that gives respectable velocity with the 225's, being H4350. That might equate the 160's in your 7mm. The same occurred with the previous WSM's in this caliber. I also found some of the faster burners seemed to do better for accuracy and velocity. I'm firing loads for 210g bullets tomorrow that in velocity tests were chronographing 2940fps with no pressure signs, but it was with N150, which is fairly close the H380 and RL15.



You probably have a pretty tight chamber in yours which definitely makes a difference in the maximums and would reduce powder requirement.



I doubt the die and sizing has any effect but it probably will help your accuracy by not setting the shoulder back too much.



Hope this helps.