Date not even close

You must not be very familiar with old Western movies. It would have been very difficult to make a movie with all accurate percussion firearms in 1955. In the 1950s there were no companies making reproduction models of blackpowder arms. All the guns used were original. The 1873 model Colt and the 1892 or 1894 lever action Winchesters were still in production and readily available. You will even see John Wayne using them in the 1840s in The Comancheros. Italian company Uberti began to make reproductions of older firearms in the 1960s but prior to that there just were not enough blackpowder firearms around in good enough shape at reasonable prices to be used in Western movies.

If you want to enjoy older westerns you will just have to accept that they did the best with what was available to them at the time. I am sure if the repro percussion guns HAD been in production in 1955 this movie would have used them.

Reasons of plot.

The Tall Men is based on a novel by Heck Allen/Clay Fisher. Presumably the novel set the date in 1866, and made the Allison brothers veterans of the Civil War, and had them foolishly decide to drive the cattle along the Bozeman Trail during Red Cloud's War instead of taking the safer Bridger Trail. "Real men don't use the Bridger Trail" I guess.

I think that it is more historically accurate to give the date of 1866 to a movie set during Red Cloud's War and use anachronistic guns than to give the date of say, 1874 to that movie and use correct 1874 guns and anachronistically have Red Cloud's War happen during a time of peace.

In The Savage (1952) Corporal Martin says he joined the army more than 20 years earlier during the Civil War, thus putting the date in or after the period 1881 to 1885. James Aherne was 11 when he was adopted by Miniconjou Sioux chief Yellow Eagle in 1868, and thus would have been about 18 to 20 and old enough to fight as a warrior during the Great Sioux War of 1876-77. But everyone asks him if he will fight against the white men in the coming war. So either the Great Sioux War happened in The Savage but James Aherne was too sick or injured to fight and so nobody knows how he would choose, or else the Great Sioux War never happened, which would explain why the Sioux seem undefeated in The Savage (1952).

Thus I think that The Tall Men was more historically accurate than The Savage which puts peace when there was war and war when there was peace.