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Thread: Compressed Load Behaviour

  1. #1

    Compressed Load Behaviour

    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.

    Many thanks,


  2. #2
    I suspect chamber dimensions, throat length, twist rate, atmospheric conditions etc may play a large part in these discrepancies in safe maximum pressure, hence the need to determine your own maximum in your rifle (as you have done). All these figures published are only a guide that will generally incorporate a safety factor to account for more adventurous handloaders.

  3. #3
    RL-15 in a .308 Win with 150-gr bullets, max can be anywhere from 44.0 to 47.5 grains, depending on a lot of factors:
    * brass volume. In the USA, lots of reloads are in USGI brass, which holds less
    * bullet shape. A flat base is shorter and not seated as deeply into the case, just the neck.
    * seating depth. A bullet seated 2.70 inches is going to generate more peak pressure than when seated at 2.80 inches.

    In my experience, 45.0 to 46.0 grains of RL-15 has been about 2,800 fps or better in my rifles, and the best accuracy, with almost every bullet.

  4. #4
    personally i always go with powder manufacturer charge data

    I dont pay much attention to min and max loads other than a vague reference point

    the thing that most people dont seem to grasp early on is the data is NOT for your rifle
    chances are you have different makes of brass, primer and even bullet

    I use min reference as a starting point whizz through single or double rounds to check for any pressure issues rather than accuracy
    once the charge range gets into a reasonable indicated velocity range I will run groups of 3-5 of each level to find the accuracy nodes
    rarely do I find them anywhere near max stated charge levels

    BUT..and its a big but. I have found several examples where MY MAX LOAD is well before theirs.

    the discrepancies between different data sources could be down to a 1000 and 1 things
    be aware different lots of powder have slightly varying characteristcis and the manufacturers will always err on the side of caution when publishing

    I do however run at least two examples of fairly heavily compressed loads
    78gr H1000 with a 208gr AMax is crunching before the bullet is even half way seated!
    20.8gr N133 with 45gr Scierra SP is compressed too

    don't assume because the powder is under pressure due to compression that this will lead to high pressure.
    the two are not linked

    in fact a reloader who has forgotten more than I will ever know about .222 said "fill a case with N140, seat your bullets and you won't go wrong!..."

    he wasn't joking

    If it is accurate and there are no issues pressure wise...keep going
    Last edited by bewsher500; 30-10-2015 at 17:16.

  5. #5
    I used RL15 in a 308 with 150 grain Spire Points. I've since moved to Partitions and changed the load but my Spire Point load was 49 grains and there were no signs of pressure. I used this load for quite some time, and still load it now and again, and had cases with a good number of firings on them - probably 10 or more - with no problems. The load was giving me a measured 3000fps.

    I also used 49 grains of RL15 under the Partitions but thought it wasn't so accurate. Truth is I didn't do much testing but I didn't see any pressure signs with the Partitions either. I ended up at 46.5 grains with the Partitions but, as mentioned, none of my testing was going to be statistically significant in any way and maybe 49 grains would have shot just as well.

    Both loads were using Lapua brass. I would say the 49 grain load is compressed but not excessively so.
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  6. #6
    Chaps - many thanks for your thoughts and your comments - much appreciated.

    I'm about to take the Mrs out for dinner and I shouldn't really be posting on here at this point for that reason: I'll read through again when I get back and ask some questions if I've got anything I'm not clear on. Interesting to read about the sweet spot for some of you being around the 46gr mark - I wonder if that is statistically significant...

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