300 Win Mag Bafflement

Do they chamber?
You do not shoot them from a gage.

BTW All RCBS conventional seater dies have a crimp shoulder. You are probably crimping inadvertently.
I suspect you might normally be a LEE die user. Only earlier Lee dies from before the factory crimp dies have an internal crimp shoulder.
RCBS does not make or need a separate crimp die.

You might paint an offending case with a permanent black marker an see where it hits the gage.
You should also read RCBS set up instructions for their seater dies.
Basically you install the die with it backed away from the case a few turns.
Run the ram up full with a case in the shell holder. Then run the die down until it touches the case with the crimp shoulder. Then back the die up about 1/4 to 1/2 turn so none of your brass can ever hit the crimp shoulder. Then you will never bulge a case by accident. This applies to all die brands with an internal roll crimp shoulder in the seater.

Magic marker may or may not reveal witness marks on cartridge brass. It usually does, but it can lie to you. Witness fluid or soot, however, are the answer to determine your gauge issue.

I’m with the others, however, if it chambers without bolt resistance and fires, then you’re imagining a problem for yourself where one is not.

...and this is why I use a barrel for a gauge, and not a gauge.

Shmackey said:

Drops into my Lyman case gage like butter. Dropped powder, seated a 200-gr Sierra GameKing, and about a third of them hang up in the case gage right around the web.

Click to expand...

I took a look at a picture of a Lyman 300 Win Mag case gage, and the thing is non adjustable.

I have been using the adjustable L.E. Wilson type gages in setting up my dies. Here is a picture of one



And this is a video on how to set up the die



The base to shoulder distance of a belted magnum is not standardized and neither are the base to shoulder distances found in rifle chambers. So, the first thing I am going to comment on, is that you really don't know how much you have pushed the shoulder back, with your RCBS die, from the rifle chamber that the round will be used in.

These case gages are only measuring base to shoulder distances, they are usually cut large between base and shoulder and it is unusual to have a case so fat that they stick in the die. But it happens.

Then, something is going on as you seat the bullet. I think you are buckling the case when seating the bullet, that has happened to me. I was never able to measure it, never found the exact location where the interference fit happened, just observed the phenomena when using chamber reamer cut case gages. These case gages were cut with the same reamer as used in my match barrels, so they are dimensionally correct in length and width. The "cure" has been a small base die. Increasing the clearance between case and chamber resulted in less binding after inserting the bullet. You can try greasing the bullet or case neck before seating the bullet and see if that makes a difference.

I doubt you removed the firing pin mechanism and the spring loaded ejector, or, the extractor in testing your rounds. Spring tension from any of these components will disguise tight fitting rounds, which is why when you do a chamber check, all this stuff needs to be removed. Older mechanisms with claw extractors and easily removed firing pin assemblies provide much better feel when checking a round in a chamber. I regularly remove the firing pin assemblies in Win M70's, M98's, M1903's, when conducting round to chamber checks, and these actions don't have spring loaded ejectors either.

I also think you are buckling the lower part of the case shoulder when you are pushing the bullet into the case due to the necessary interference fit to get good tension. You say that you chamfered the case mouth. Did you do it on the inside as well? Breaking that sharp edge will help prevent this from being such an issue. I use a VLD reaming tool on the inside of my rifle brass and this seems to help keep good tension without collapsing the shoulder, especially with those that have thin necks like 30-30.

You are not trying to support the case you are trying to square the bullet in the brass when you seat it. Some here say that certain brands of dies are better than others and the competition or specialized dies are better yet, but I just use regular Lee or RCBS dies with good results. But you still may need to get a custom built insert/punch (or make your own) for your seating die to fit your bullet better. All companies will make them for you for a price. I have even waxed up the bullet and put JB weld in the cavity of the die and made a custom punch as a temporary answer. I use the VLD reamer as it gives a long gradual taper to the inside of the brass and seems to allow easier bullet seating. Also using slow steady pressure to seat the bullet seems to work better for me than placing the bullet and cranking the handle hardish. Some say using a bullet with a tapered base (boat tail) rather than a flat base works better as well but I have not had problems seating flat base bullets squarely in the neck. If you want to see if the bullet is in the brass straight (no runout), lay the finished round on a flat surface and roll it while watching the tip of the projectile. If there is no wobble or movement of the point the bullet is seated straight for practical purposes as you are not loading for 1000 YDS yet I assume.. you could measure your expander ball in the sizing die and see if it is around .002 smaller than the bullet. That is what I like my casings to end up sized to for good bullet tension when inserted.
So at the end of the day (and my long winded explanation) if the rounds chamber with no difficulty there is not really a problem with using the ammo in the firearm..