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Judging by Alliant's data,.. TS-15.5 is roughly 1 gr less than Varget or close to Varget. Closer to IMR 4064 amd AA-4064.
I would say from those numbers that it would appear that 1-2 gr
more TS 15.5 would be required on a weight basis as compared to Varget. For example, I don't believe anyone would attempt to load 45.4 gr of Varget under a Juggernaut seated at 2.800 COL, or 44.1 gr Varget under a 200 Hybrid seated at 2.800 COL. Those would be unsafe and potentially dangerous loads. I'm not even sure you could actually get that much Varget into a case with those bullets seated at 2.800 COL.
The relative bulk density of a powder such TS 15.5 will play some factor in its use. Typical Varget loads used by F-TR shooters with 200s will often run at around 100% fill ratio, +/- perhaps 1-2% or so. If TS 15.5 really requires 1-2 gr greater charge weight than Varget to achieve a comparable velocity, that would suggest to me that it has a density (and shape) that will allow more powder to be put into the case as compared with something like Varget. Otherwise, one might run into compressed loads trying to achieve comparable velocity when having to use more TS 15.5 as compared to some other powder.
The bottom line is that there is nothing wrong with testing new components. Nothing at all. Many of us do that in the [continual] quest for some kind of improvement or advantage. But for those of us that occasionally delved deep down into the rabbit hole, there needs to be a good
reason to pursue/test some new component, and there needs to be some definable and obvious advantage in order to make a permanent switch. If some new component ends up performing just a little better or about the same as what one was using previously, what's the point in switching? Then you would have to spend more money and learn all the subtleties of using the new component all over again for little real gain. Lastly, the general availability of a new component is also an important consideration, especially in today's reloading component market. If a new component seems to provide some clear benefit, that benefit will largely be lost if one can't ever find the component in stock for purchase on a regular basis.
Over the last few years, another example of this exact question has been illustrated by F-TR shooters that made the switch from Varget or something similar to Vihtavuori N150. According to the burn rate charts, N150 has a slightly slower burn rate than Varget, and it seems to require about 1 gr or so greater charge weight to hit a given velocity. I have seen it used with great success by a number of F-TR shooters, so it definitely works. But the
real question is whether N150 is that much better than Varget? My guess is that they're pretty close in many people's hands. In other words, one is probably not "head and shoulders" above the other, and excellent (winning) results can be obtained with either one. I have recently been using some heavier (200+ gr) 30 cal bullets. It seemed like a no-brainer to test some N150 with these bullets. The reasoning or hypothesis would be that if N150 really has a burn rate slightly slower than Varget, it might be a better choice for heavier bullets. So I just purchased a couple pounds of N150 with the intention of testing it with the heavier bullets. Although I haven't started just yet, my readouts for those tests will be precision, velocity, and velocity ES/SD. I expect there will be a definitive answer to the original hypothesis. In other words, N150 will either prove to be a better choice than Varget, or it will largely be about the same. My testing should reveal which of those possibilities will turn out to be reality, in my hands, at least. If the N150 performs about the same as Varget, IMO there would be no good reason to switch and I will be out the cost of a couple pounds of N150; i.e. no big deal. On the other hand, if it really shines, I will consider switching to N150 for use with .30 cal heavies and start looking to pick up a few 8-pounders.
If one thinks some new component might be of benefit, making a query at at online shooting forum as the OP did is a good place to start. If there appears to be only limited information, it may be necessary to simply test it themselves. In that event, understand that the reason(s) for doing the tests are largely that you're looking for something
better than you already have, not just
different. Then make a fair evaluation of the results to answer the question. It usually doesn't take a lot of some new component to make a fair evaluation. That way, you're not out a lot of $$$ if it doesn't turn out to be better.
