So chaps - it's reloading week in neutron's household, so another day, another question for the gurus!
I've got a 150gr deer round for my .308 as follows:
Reloder 15 46.5gr 2.725" Murom Primer 24" barrel 2820 fps
It's pretty good, has killed deer without incident (i.e. no runners - yet) and chronos at an average of 2820fps, +/- 12fps, so it's pretty consistent. It sometimes seems a little the wrong side of MOA, but I'm fairly sure that's me!
The data for this round still confuses me though. The maximum / minimum charge values for the load are wildly different, depending on who you ask.
Alliant, who make the powder, give the following MAX load:
Reloder 15 49.0gr 2.700" CCI Primer 22" barrel 2919 fps
Sierra on the other hand, give the following MAX load
Reloder 15 44.8gr 2.750" Fed 210M Primer 26" barrel 2800fps
Other manuals give maxima in the range of 46.0gr and I started to see pressure signs at 47.5gr and stopped at that point.
I recognise that I can only know what works for my rifle by testing it myself, but this is a theoretical obeservation and question: that Alliant figure of 49gr is massively high compared to everything else I've found. I'm also pretty certain that 49gr is a compressed load at that length / charge, given that my ever-so-slightly-longer 46.5gr round is very nearly compressed already.
Could someone help me to unpick this disparate data and help me understand better what the effects of compressing powder are, if that is what explains the data?
In this case, it looks rather like a little compression changes the burn characteristics or affects the pressure somehow, making what would seem to be an enormous (over-)charge apparently safe in some rifles.
I'm wondering if this is to do with the dispacement of some of the air that would otherwise have been between the powder grains: with the "filler" air displaced by extra powder, the volume of gas which will be heated and expand in addition to the gases produced by combustion will be smaller - could this account for lower pressures and an apparently safe higher charge?
Hopefully some of that makes sense. Any / all comments appreciated - I'm just trying to better my understanding and compressed charges is something I next to nothing about.