The question that is wondering me is why are both made? what does one do the other doesn't?
One can only speculate as to why the two types of hooked breech were developed. But the blame can probably be placed at the feet of the British.
The high-end gun market in England was very competitive in the 18th and 19th century. To compete, the most successful gun makers often came up with new innovations and refinements. Some of these were practical improvements and some were essentially marketing hype.
I've heard that the slant hooked breech was preferred because it did not put pressure on the bottom of the barrel channel at the breech when the barrel was tipped up to unhook it and lift out like the straight hooked breech supposedly does.
I have a number of 1970's vintage Hawken rifles with straight hooked breech, and I see no adverse affects to the barrel channel, so I'm not sure there is a problem there that needed a fix.
Another advantage of the slant hooked breech is that it acts similar to a wedge and locks up tighter from the recoil of each shot. This makes some sense and might mean that after the first shot, the barrel won't move anymore, giving more consistent accuracy.
As pointed out by
rich pierce and
D. Taylor Sapergia, if you are building a Hawken, the decision to use either straight hooked breech or a slant hooked breech should be driven by whether you are building an early J&S Hawken or a later S Hawken rifle. The straight hooked breech is most common on a J&S Hawken (there are exceptions) while a slant hooked breech in more common on a S Hawken.
If you don't care whether you build an early or a late Hawken, then I would suggest you focus on making a representative late S Hawken because the correct parts are more readily available. A convincing J&S Hawken requires some modification of available parts.
For a S Hawken with 1" barrel, I would suggest you use Track's breech plug
PLUG-RL-16-3. The breech & tang was designed by Ron Long. It's a scaled down version of his "Bridger" Hawken breech & tang.
I also recommend Track's
TG-HAWK-H-I trigger guard. It has a more correct shaped rear scroll for a S Hawken rifle.
You could use either
BP-HAWK-L-I butt plate or the "Bridger" butt plate,
BP-HAWK-JB-I. If you use the later, you should modify it as per
Herb by squaring the inside corner and removing some metal on the inside of the crescent. He has a post on here somewhere with pictures showing the changes to be made. It's all simple file work.
Another option is to order a set of late Hawken trigger guard and butt plate from Doc Carlson of the Upper Missouri Trading Co. I purchased a set from him at the CLA show three years ago, and they appear to have been cast from the same original Hawken that Art Ressel used for his kit castings.