Most anything that is in high demand of which there is a fixed or declining supply will trend higher in price over the years as long as that demand remains in tact. The supply of these guns in great condition should trend lower over time if for no other reason than some of them will continue to deteriorate with age and use. At some point, however, these guns will succumb to antiquity, which will reduce their demand as a true hunting rifle in the years ahead. Can you imagine actually betting your safari on the performance of a 200 year-old gun? Future hunters at some point will face this and may decide against their use, which could reduce demand and ultimately reverse the pricing trend that you mentioned.
There are no absolutes in investing, and that includes 100 year-old guns.
If I was going to play the investment game in the gun market, I would prefer a market that enjoys a limited or fixed supply, but may not as soon run into antiquity. The perfect combination of those two facts would be Class III firearms (here's mine) whose supply was capped in May of 1986, yet still enjoy relatively new technology. Some of these guns can even be brought up to current standards, extending their life, without hurting their numismatic value. The annual growth rate of their values over the last 11 years has been approximately 16%/year.
All this aside, I prefer to buy guns for good times in the field; and if by chance I enjoy some appreciation, then I usually think that's nice, but it likely will be a benefit that someone that comes after me will reap.