Small reloading speed bump worthy of review

This is not the fault of the gun, design, or even the brass. It's just an odd set of circumstances that provide the advanced custom loader a bit of a challenge. Not only is the pursuit of what the problem is an adventure but also in the choices to remedy.

I have enjoyed the likely under rated .401 GNR a bunch. With factory 10mm bullets it is a barn burner! Out of my 6" revolver is it an easy task to get a 180-200 gr. jacketed bullet to 1600+ fps. I have also manufactured my own custom jacketed bullets by pushing .410" slugs through a LEE sizer to make them .401". I used an odd mix of old Winchester and R-P brass I had on hand. But of late I acquired a new batch of Starline virgin .44 Magnum brass. Suddenly I had some seating issues with my loaded ammo. The common factor to the problem was obviously the switch in brass. Now to dissect why.

Better to find at the range or load bench than on a distant trek.

I pushed 210 gr. .410" XTP bullets through a LEE .401" sizer which precisely altered them to 0.401". In the process they become elongated by about 0.020"! The solid jacket web deflects around the lead core and the lengthening occurs, it has to go someplace, you know. Interestingly the HP at the mouth also enlarged around 0.025" in diameter from the original unmodified spec. bullet. My other brass worked fine with the .401" bullets so what gives? To double check I pulled the de-prime pin from the sizer and re-ran the loaded ammo through the FL sizer. It reduced the neck thickness notably, like 0.005" and those fully seated fine. However this made the bullets loose and able to be spun in the crimp with the fingers. I could not remedy this by crimping tighter. However it seemed not to matter in shooting.

Close inspection you can see the lengthening and slightly larger HP cavity opening. But you have to look close, you are dealing with thousandths of an inch.

One more interesting point is that factory 10mm jacketed bullets are generally 0.399" diameter. Not 0.401" as many size their cast bullets. The actual "hole" in a 10mm barrel is truly 0.390" at it's most narrow! I measured it with pin gauges. I will investigate as to how much this helps with the thick neck brass issue. LEE will custom make you any size push through custom sizer for $38 and about a 6-week lead time. Perhaps a 0.399" push through sizer would fix everything and I could ignore the thickness of varied brands of brass altogether? I think it's worth $38 to see. (plus it will make more proper custom bullets for my 10mm Auto) A pulled bullet from a loaded round "FL re-sized" mic'd right at 0.400"! This was just below the crimping/neck seat line. Below that the bullet of course remained at 0.401".

ONE thousandth! Could that be one dimension that is creating heartburn? The Starline is 0.012" at the neck (X2 sides) plus the .401 bullet = 0.425. The re-sized loaded ammo mic's about 0.420" at the neck with a seated bullet. I'll have to look into the thickness of other brands of brass to see if that is a large factor. Could my old brass be hammered thinner a bit and allowing the "plunk" drop in seating. I'm still playing and having fun. If you encounter a problem don't chew your hat. Look at it as an adventure to seek the issue and find a solution. A simple change in components (that vary occasionally) can solve everything and the one to blame is often the monkey in the mirror.