There are well documented accounts of the Whitworth being used with effect at the siege of Battery Wagner during the Charleston, SC. campaign at ranges of 1500-1600 yards. There are several other accounts, including a Florida sharpshooter hitting a man as he walked at 1200 yards. The shooter had to compute the speed of the man's walk as well as the range. There are many other accounts too numerous to state here. The book The Confederate Whitworth Sharpshooters" by John Anderson Morrow is a very good reference for the gun and the men who used them.
I don't know who all makes this rifle. Parker-Hale is the only one I know of, and I'm not sure they still do. I have visited their website and it stated that they were sold to another company, however it also stated that they still made barrels. Maybe David Minshall or some of our other UK friends will see your post and have a better answer. I have a very early Parker-Hale and it is a fun, though punishing gun to shoot. I haven't had an opportunity to use it at very long ranges yet.
We did a thread a while back called Loooooooong Range Gun. You can go to it and get quite a few comments on this great weapon.
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