I don't know if you have seen it, but there is a current thread going on about fillers. You may get more interest in your post there.
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Is a filler needed when using Bullseye Powder?
Yes, with jacketed's. I was just trying to get that particular rifle to shoot. It had a tight midsection of bore and a loose muzzle end so I lapped the midsection until it was even and counter-bored the muzzle end. I have other cast boolit shooters to play with at the moment so won't be pursuing cast in this particular rifle for a while to come but I do plan to eventually shoot cast out of it just to see if I can and maybe for range shooting. We have these military shoots every now and then and this rifle most closely resembles an original military rifle. They may not like my front sight mod though but the blade is still there. I just can't shoot with it as a plain blade.
https://i.postimg.cc/C1tz7Wmd/DSCF6083.jpg
I've actually raised the tube so that the blade doesn't protrude far enough to disturb the sight picture. It works real well for my eyes.
I don't use any filler or wad with bullseye loads in any cartridge.
So no issues with the Bullseye lying forward while firing down? It would certainly be more convenient not to use Dacron. I get good accuracy with a smooth sided two diameter 217 gr boolit over 5 gr of Clays which I believe is very similar if not about identical.
https://i.postimg.cc/3NfWSW7X/DSCF5138.jpg https://i.postimg.cc/y6LJYp9D/217gr-Cast-5gr-AS30-N.jpg
That's only 25m so not exactly bench rest accuracy but then it was with No4 peep sights and a front blade that I can't see too well. That's also a different rifle. I can't seem to find the target for the two-groove which makes me wonder whether I have actually tried them in it.
A lot would depend on the size of the cartridge case volume, what primer is used, whether drilled flash holes and the degree of "down". For example, in my Mini Mk X 7.62x39 with LRPs and drilled flash holes with 2.7 gr Bullseye under a 90 gr SWC I discern no noticeable difference in impact when shooting at lower targets such as downhill. In my 30-06 with 3.2 gr Bullseye under the same 90 gr SWC there can be a slightly lower impact if shooting steeply downhill at 50+ yards. Under normal carry position and bringing the rifle to shoulder I've not really noticed any effect of "powder positioning".
The reason I was asking about a filler with Bullseye is I want tp try around 8gr in the 450 Bushmaster with a 255 to 300 gr bullet. This would be from a Bolt action
I use 8 gr of Bullseye w/o wad or filler under 270 gr Rapine 460250s in the 45-70. I shoot them in my TDs and Siamese Mauser 450-400-70. Accuracy is excellent, recoil is mild and the fun factor is off the charts.