6.8 SPC with cast in AR-15

[dh2]

Since getting back in to casting boolits I have been going little by little to see how many guns I can on a cast boolit diet, doing my testing over a chronograph, I have had some rifles take right to them, My 6.8SPC is being a bit difficult, it is the first version of the 6.8 SPC, I started out with an old Lyman mold casting a 128gr. boolit pushed it with IMR4198 this did cycle the action very well ,bullet speed had a wide range, and all over the target at 25yards.
Next try was with a 114gr. mold from a group buy, I am pushing it with 24gr. of H332 all shots close to the 2144FPS average, Action cycles good, had to have the COL very short because it was going in to the rifling before the bolt was closed , but at 75 yards I get a group over 3 inches. I have been reading up on the thread " high velocity with cast." because of the speed a lot is said there about neck tension. I am using a Lyman M die to be able to set the cast boolit with out damage. 40 rounds last weekend and no leading in the barrel. This group is to me not expectable for this rifle. I am thinking from past experience when things get close slowly work up on the powder charge to see if it will tighten up. to me this is fast moving for a cast boolit. Is there some thing else I should be looking at?

[GabbyM]

I believe your frustrations with 6.8 SPC are typical. Sort of an oddity as to why they don't shoot cast boolits better. Especially the ones with a 1:11" barrel. One theory is the way throats are cut.

You have two issues that I see. First is velocity. Sadly I've never been able to run my 270 Win with 10" twist any faster than 1,850 to 1,900 f.p.s. 14.0 grains of Unique sends the little 122 grain LY 280468 sailing along happy. No powder position issues in the huge 270 Win case and shotgun powder. Clean burn and sub 2moa accuracy to 100 yards. one big hole groups at fifty yards. Shotgun powder however wouldn't cycle a AR.

Second issue is just a maybe. Since in general H-322 likes high pressure to burn well. Your 4198 should burn well. I've used it in 223 AR loads at 14.0 grains and it cycled. H4895 is an old favorite cast bullet powder. Would give you more gas than 4198. If you have a carbine gas system the 4198 will probably be enough gas pressure. After that my guess is when you work up a 1,825 fps load it will shoot accurately.

[7br]

I wound up putting an adjustable gas block on my ar15. I also had a custom mold made from Mountain Molds. The rifle is capable of 2" groups at 100 if you discount the flyer. http://castboolits.gunloads.com/showthread.php?242559-6-8-SPC-Range-test
Your mileage may vary.

[dh2]

I am using Lyman #2 alloy with gas checks and water dropped.