Are you saying that the wood for end caps need to come from wood cut across the grain as in not along say the length of a board, but perpendicular to it? If that is correct, I am very glad to know that before I cut some pieces of wood from some thick walnut boards I have.
Hmm. . . best way to explain is probably to go 'way back to the basics, to get our terminology on the same page.
Picture a tree growing straight out of the ground. If we cut the tree down by making a sawcut parallel to the ground, we are cutting
across the grain. If--starting at the cut and proceeding to the top of the tree--we saw the tree into boards, we are cutting
with the grain. (We could also saw the tree into slices across the grain, like cutting a salami.) All this comes into play when we start working the wood.
Depending on length (measured with the grain), slices of logs taken cross-grain (what I described earlier as being "like cutting a salami") have little structural strength. A 1" slice of wood cut like this, that incorporates the center of a log or branch, is going to split due to the way the wood dries out. The outer rings of wood dry--and shrink--before the interior. Since the wood lacks the structural strength to compress the interior wood, the outer rings split. These splits will often go to the center of the salami-slice of wood. This is why slices of branches cut cross-grain make poor end plugs. (Yes, for now I'm ignoring the interlocking grain of burls, and the interlocking grain of elm and the like.)
In my notes of observations of original powder horns, I've defined grain direction by looking at the end of the horn and noted what direction the grain showed across the opening. A "face-grain plug" shows face grain, an "end-grain plug" shows end grain. For this thread, I did a quick look through my file cabinet and found notes on about 60 non-commercially-made horns with strong attribution/history of use during the 1750-1790 period. None of those horns used "end-grain plugs." As I wrote before, I'm not saying it was never done, only that surviving horns with this grain orientation seem uncommon. There could be multiple reasons for this. Poor survival rate is one, another might be that the practice was uncommon.
Griz gave a good example of exceptions to this when he wrote that he had turned caps from 3x3's centered in his lathe. Turning wood between centers was a common period practice, and would result in a plug that I would categorize as an "end-grain plug." It also requires the use of a lathe, and some experience using same; the early horns I've seen done like this seem to have been professionally-made.
Of the horns I've examined that used plugs turned in this manner, there were several commonalities. None used the center of the tree, which would reduce the likelihood of checking or cracking. Judging by the amount of shrinking, as shown by the way the wood had dried out-of-round, almost all had been made from wood that was very dry to begin with. The vast majority (meaning "I-can't-think-of-an-exception-but-there-probably-is-one-somewhere") of such horns seem to have plugs where the "length" (the dimension measured with the grain) is greater than 1/2 of the diameter.
Griz also mentioned sealing the plug inside and out. This slows shrinkage due to drying, and gives the wood time to "adjust" without splitting (or at least, minimizes the likelihood of splitting). Of the hundred or so original horns I've seen with loose plugs, as well as a few plugs without horns, none have shown signs of being finished on the inside.
Having spewed all this verbiage, I should probably point out some of the many shortcomings with my notes. For one, I've made no attempt to include the many published horns unless I had the opportunity to examine them in-hand. I don't live in the area where most of the surviving horns from the 1750-1790 era are found, so the number of these horns I've had the chance to study is limited. And in my attempt to exclude fakes, I'm reasonably certain I've excluded original horns from study due to a lack of provenance.
