Texas Sheepdawg wrote:
I'm confused now.
The part about the powders not having as much umph with lighter bullets or the shoulder datum point thing?
We've seen this in the 450b. A powder may be just on the edge of too slow for a given bullet weight but plenty fast as the weight of the bullet is increased. A powder where a 300gr 450b bullet falls in say the lower end of the sweet spot, may be too slow for a 200gr 450b bullet or vice versa. These powders I've been trying are qualified for the 30RAR, but may not be a good match for 125gr bullets, whereas they may do fine with 150's and 165's. I'm trying hard to get more speed out of the 125 SST that I would ever try to get out of a 150 or heavier bullet.
I had another range day today using the higher capacity fire formed cases and a slightly long COL, just .040 off of the lands for the 125 SST. I have not finished writing it up, but results have now disqualified two (or more) of the powders used in this first outing. You just can't pack enough powder in them, still fit the bullet and get the target velocity I was seeking, which was 2900fps or slightly more. Its just an experiment in determining the limits of the caliber for a given bullet weight. AA2200 or possibly Ramshot Xterminator are the two remaining candidate powders for the 125 SST and are like hens teeth around these parts. I did get a tip from a local who thinks he recalls seeing some Xterm down at Ahlman's, which is close to a 3 hour round trip from me. He said they had a lot of different powders not commonly seen in the immediate metro TC vicinity. Maybe they'll have AA2200 also. I suspect all 5 powder I tried in this experiment will have the energy to kick the 150's hard enough that I ill not need to load them to maximum capacity before seeing pressure signs.
If its the 1.57 Datum thing, that's a bottleneck reloading technique, wherein you do not need to fully resize the cases down to absolute minimum length like the factory loads. More like the perfect fit for your chamber. A common technique used in bolt action and single shot rifles. You do need to push the shoulder (which grows in length also) during fireforming, back a little bit just to have some wiggle room when they are chambering. Today for example, I confirmed that 1.57 was about as long as one would want to go with my particular chamber. I forget, but I think the factory loads which err on the side of caution, were something like 1.53 for added margin of error. Those few thousandths contribute to maximum case capacity. We see the same problem in the 6.5 Grendel. They crave more case capacity, but mainly because the .264 bullets run long as the weight and BC go up. I would have given my eye teeth for an additional grain of powder capacity with some of the powders I chronicled on the 65gremlin forum a few years back. They were just coming into their sweet zone when there was no more room left to add powder. AR-Comp comes to mind as it is extremely wide temperature compensated (-40 to +160f with only a 12fps change), perfect for these northern climes. Unfortunately, with the case capacity, it fell around 100fps slower than the hottest temperature sensitive ball powders. It was also consistent in terms of velocity and threw phenomenal ragged hole 5-shot groups 100yards using the 123gr Hornady Amax.
If those were not what was confusing, than tell me what was. We often trim the necks to make maximum case length, but some times, its better to push the shoulder back a little for the same result. Just no more than absolutely necessary for reliable chambering. It's late and I'm failing quickly before I over-explain answers that may not be for the confusion your are left with.
Elaborate for me and I will strive to lessen the confusion. For the record, my best result today came from H335, a ball powder. As it happens, its also one of the best choices for the 6.5G, albeit temperature sensitive. So, what may have been discouraging results today may be corrected when the range temps are a lot further above the low to mid 50's. Stll not seeing ay of the primers show the slightest sign of flattening. a further testimony to the fact that these powder choice are driving the caliber as far as it can safely handle. Again, more once I finish the Range Report write up.