Thanks for the reply..
I am not going to use a 1:12 twist but a 1:8. I want the upward velocity flexibility the slightly larger "USA" case will provide yet still filling the primary subsonic role.
I am not planning on shooting the extra heavy bullets, but what I stock for my 308. At 1:8 I may be able to over spin the 125 TNT with the 7.62x39USA, but not 130 gr and up. I currently get 2800-3000 FPS with the 125 TNT in a 1:11.25 w/o bullet breakup.
What turned me on to all of this whole idea was the SSK 6.5 MPC round. I think it is perfect in the AR15 platform. It is the "answer" If I had a person to handload for me for free I would absolutely own an upper in 6.5 MPC. I would prefer it to the .223 by a long shot (pun intentional) I don't, so I won't.
The 7.62x39USA is similar to the MPC except it is .30 instead of 6.5. Full power loadings would be very nice in the 125 gr loads (but the BC is nowhere near the 6.5)
I currently shoot the 6.5 in a 260 Rem. and have LOTS of bullets and considered using the MPC for a sub sonic platform getting a fast twist in 6.5 but the .30 is so much better in sub sonic I can't see getting the 6.5 MPC unless the military goes with it. But by that time I will be blind and Ar15s will be contraband.
I am trying to reduce the # of calibers I load for. The velocity the 7.62x39USA is getting is very respectable for the small amount of powder. It is VERY efficient. (this is why the 6.5 MPC is so much better than the 5.56 NATO. Combine the efficiency and the better BC an you have a major winner)
The 7.62x39USA = 2555 FPS with 125 gr bullets in the AR. It should also be an excellent case for subsonic loadings. I think it has a dual role with more weight on the supersonic side unlike the Whisper which favors the subsonic side of the equation with heavy .30 bullets.
went to the range to test a couple loads in the 16" and 20" 7.62x39USA
16" 1:8 Olmypic .078 gas port carbine length gas system, fire-formed brass...
125 TNT 22gr lil-gun = 2,540 fps
125 TNT 22gr H110 = 2,480 fps
125 TNT 22gr 296 = 2,475 fps
20" 1:12 Noveske .102 gas port rifle length gas system, fire-formed brass....
125 TNT 26.0gr 1680 = 2,606 fps
I think I've hit a stopping point with the 16" around 2500fps...my favorite loads have been with the 4227 and lil-gun around 2350 to 2450fps.....I can probably push it a little faster but I'm not sure if the additional velocity is worth the increased pressure (wear and tear on the rifle).....2350 to 2450 is comfortable to shoot....the additional power it takes to reach the 2500 to 2550 mark is pretty noticeable in the BOOM factor....
The 20" rifle handled the 125's at 2600fps great.....I think the 1680 really shines in the 20" barrel.....it's funny, in the 16" barrel a charge of 22.5gr of 4227 and 25.8gr of 1680 produced almost identical fps (around 2440fps) ....but out of the 20" with the same loads the 1680 was over 100fps faster....no pressure signs with the 26gr of 1680 and still have a little room in the case.....
Granted this is with a 1:12 the 1:8 would be slower as it would raise pressures.
It would be nice to get the advantages of the 300 whisper in the 200 gr an under weights and the advantages of the larger case of the "USA" when shooting supersonic.
I am NOT going to be shooting over 200 gr bullets. The whisper is made for the biggest .30 bullets. I will shoot primarily 168 SMK, 175 SMK, 170 gr round nose, and for super sonic in the AR the 125 TNT.
My concern with the 300 whisper chamber is it's made for VERY long bullets which I will NOT be shooting. The loads I will shoot will have to jump quite a way before entering the rifling. The 7.62x39USA does NOT have this long of a distance before the rifling and will be better suited for my lighter weight bullets I will be shooting.
Maybe I am fooling myself trying to get the best of both worlds. A .30 in the AR that shines with subsonic 175 SMKs as well as being able to push respectable supersonic loads.
I have 1000s of bullets in 168 and 175 and a fairly good amount of 125s Also a good pile of 30-30 bullets. 150 and 170s
Will the 300 Whisper feed with 125s in the AR? Will the 300 Whisper be accurate with 125s in the AR? I have a lot of questions to get answered before I can make the choice.
Luckily some people have done a lot of the work already. I just need to ask the right people and see what they think.
The 7.62x39 Russian with a 1:8 twist would work, but I want to use the .223 as a parent case.. I have over 5,000 empty Federal cases.
I do not own a AK or any firearm that shoots a 7.62x39 Rus.
If I am going to have to make cases and hand load why not go with a case that will meet my specific needs. The whisper may not be the best solution for me. It may and I just don't know it.
As of now I feel the 7.62x39USA may be a better choice than the 300 Whisper, but practicality may push me to just go with a standard 300 Whisper.