Quote:
Originally Posted by stagpanther
I've done a fair amount of work with the 6.8spc case and wildcats derived from it--the case design is a very good one and if you can push the COL past 2.26--which is very easy to do even within the limits of existing weapons designs--you gain performance by being able to seat longer bullets to a longer COL while gaining additional case capacity for powder. With the 7.62 x 39 I've been able to significantly exceed the 2400 fps using a 125 gr bullet with the right powder and bullet combination, but the case design just isn't going to allow the same flexibility in longer bullets and/or longer COL. IMO it's strength--chamber and go bang every time no matter what no matter who made it or uses it--is also it's weakness in that it allows very little variation and still allow ultra reliability with the world's vast population of AK's. The stubby .310/.311 belly flops ballistically pretty quickly as the range stretches out (still wouldn't want to be hit by one though).
One of the biggest weaknesses I see in the vast majority of "hybrid" AR cartridge design barrels is the functionality of the feed ramps and barrel extension lugs--almost all of which rigidly adhere to the standard M4 type. This is what I attribute a lot of functional reliability issues too. If, for example, you look at the 458 socom (another "best of breed" design IMO, which, like the 6.8spc had SF guys have a hand in it's development, leading one to believe maybe they do know a thing or two about cartridge designs  ) you can see the bottom lugs on the extension are "faired" to one smooth ramp for the cartridge to reliably make it into the chamber. Admittedly this is because it's a single stack design; but with not too much work on things like barrel extension, magazine and bolt face designs a whole lot of functional and accuracy issues could be improved upon IMO. |
The 7.62X39 is supposedly one the best, if not the best, intermediate cartridges for chambering in a SBR... a weapon with a barrel length of from about 8 inches to around 14 inches.
This might simplify the reason ordnance would select the similar 6.8...for use in short barrels for better handling in urban environs, vehicles, etc.