I was just reading through the new RetroBlackrifle, and I noticed that the C7 is listed with a 1/7 twist barrel?

Does anyone here have more information on that?

I always understood the rear sights, twist rate, and ammunition as being three elements of a complete system.  A1 rear sights are simple, and take advantage of relatively flat trajectory 55gr rounds, fired from a 1/12 twist rate barrel.  On the other hand, A2 rear sights are more complex, and are used with the more arc-like trajectory of the 62 grain rounds, stabilized in a 1/9 or 1/7 twist...

Do they use an A1, A2, or a unique rear aperture?

Personally, I've got an A2 aperture on my A1 upper, and do a 200m zero with the rifle so that I have an aperture size selection and then don't really worry about range adjustments, since the 200m 'Improved Battle Sight Zero' is accurate from something like 20 to 250meters.

All C7's were produced with a 1/7 twist.  Nato had already standardized on the 62 gr. SS109 round and 1/7 is the optimal twist to stabilize the tracer rounds used.

Can' t comment on apertures since they'd just been converted to flat tops when I joined up but the BUIS uses a large, ghost ring aperture with a smaller aperture used for more accuracy, I believe they're same plane.

Sam -

Is the C7 BUIS a commercial item??? Do you have a pic???  

+1

Not commercial, it's a big piece of crap that Diemaco made.  The orginal prototypes/early production ones were made out of metal (aluminum?) but the majority had the bodies made out of plastic, tighten the thing enough and it will break.  I'll try to remember to bring my camera to work and take a photo for you guys.  Very similar to a YHM BUIS but it was intended to be placed in front of the ELCAN which I believe is why it uses such a large aperture.

OK, how does it work if its infront of the Elcan?

The ELCAN needs to come off to use the BUIS or...?

ELCan has to be removed to use the BUIS.  

The use of "field sights" versus the A2 style sight has less to do with the trajectory of the SS109 than with the Marine Corps Rifle Team having more say in the service rifle features than the infantry.  The RO723 and 733 carbines have 1/7 barrels as well designed for M855, but they still utilize the "field sights."  Not 100% on the sperture of the C7, but I believe it was a standard A1 aperture.  Incidentally, Deimaco's detachable carry handle is also different and used field sights.  Then again, I've never had any problems with A2 sights, either, nothing a paint pen can't fix.

HTH,
~Augee