In an effort to take my 458 as far as I can, I'm loading up some 500 gn Hornady with 3031 (with the loads off the "master"spreadsheet) and it is compressing the powder to get to a COL that fits in the mags(as the sheet said it would). The question is, how much can I compress and will the round ever run the risk of going off from over compressing. I have visions of a mini IED in my shop.  I guess I could
just make a lighter load, but what's the fun in that?-John

If memory serves, some of the extreme loads by Spongemaster were done by pre-compressing the powder before even seating the bullet.  This, to me, is rather dangerous as that may alter the typical particle size due to fracture, which can drastically affect burn rate and pressure.  I would NOT recommend this technique.  Based on some of the loads he sent me, he was pushing 135% load density ... powder spilling out of the case it sounds like.

Hadn't heard from him in a while

Vepr,

I will add to what Marty posted.  

One poorly understood phenomenon in internal ballistics is the theory about the powder column, the plug buring effect on the column and other such detail that is way over my head.  I have talked with the gentleman who has written extensively on the subjet.  Basically what can happen in a round is that the primer force can actually start the column of powder and bullet moving before ignition starts.  His testing is done primarily to address neck tension and crimp, shoulder design and their importance.  It might seem to me that radically compressing the charge could affect the burn characteristics of the powder by reducing air space and as Marty pointed, crushing the powder.  If this would indeed affect the burn rate, you could get the whole mass moving enough to force the bullet closer or into the lands just slightly before ignition.  This could send pressure through the ceiling, and parts into the air.  With that being said, I like a little compression in an autoloader, as this can help with preventing bullet setback upon feeding.

One thing that I know to be completely true is that any deviation from published pressure data, be it the primer, the case, the crimp or the bullet will affect the pressure in some mannor.  I know of a lot of folks who routinely mix parts from the reloading books, without starting the load workup sequence over.  This is usually not a serious problem as there is a bit of "room" built into the entire pressure system, but it could be.  I cannot fathom a situation where high compression would lead to the round going off in your press unless you were doing something so off as to crush the primer in some method.

Craig

As the others said you do not want to compress to the point that you fracture the grains.  The loads I worked up with 3031 and the 500 gr. FMJ Hornady were far from satisfactory.  Poor accuracy and over compression had me switching to H4198 where I got the same velocity with about 10 less grains of powder with much better accuracy and much less compression of the powder charge.
And old rule of thumb when using compressed loads (I am not sure I subscribe completely to this rule but here it is anyway) is that you seat a bullet, uncrimped, and measure the OAL.  Come back in one day and one week and re-measuer the OAL.  If the length has grown you are over compressing.  Of course, you MUST do this with no crimp applied.  Like I said, this is not really a good scientific measure of judging over compression but it is one of those very old tricks many of us old timers have used for decades.  Take it for what it is worth.  

Thanks guys, I only did up about 50, but I may unload them and try another powder.  I have no idea how spongeman did it, but I started with the low-mid load and it was TIGHT.  Back to the bench.  

Again, Spongeman told me about using a seater for a 44 to precompress the powder .... For the 500s, at least subsonic loads, I use Re7 .... IMR4198 is also used.  My program suggests Re10x, Norma 200, X-terminator for top loads, all coming in at 95%+ fill .... 3031 maxes out in fill before hitting max pressure, suggesting it is not the best choice, 2520 is the same

Thanks again Marty, I should have taken up knitting.

Back in some of my old days we used drop tubes and vibrating plates (really just an electric razor that the case sat on)  to get more powder in without having to much compression.  Hope that helps ya.  

These says as we have so many powders to choose from I'd just move to a better powder for you purpose.