New hawks the TMOTC way.

[45 Bravo]

Here are a couple of links that show the operation and shooting of the rifle.
http://youtube.com/watch?v=juOQ9Ij3G1c

http://youtube.com/watch?v=n2W7KAdCMeU

[45 Bravo]

I believe anytime you spend behind the trigger makes you a better shot, no matter what the gun you are shooting, it trains the eye, finger and brain to work together..

As for seals, the Benjamin pump rifles of the 1950's & 60's (I have one)used leather pump cups, and to seal the air valve(where the high pressure air is stored) use lead seals, that was changed at a later date.

The Benjamin rifles are available today (new about $140) in .22 caliber, but require pumping after each shot.
They are still made from solid brass, and could be readily adapted to a somewhat larger caliber.

That being said, the only way to fling a .38-40 caliber ball with much force is by a pre charged pneumatic.
The best tinkers special out there is at the moment as far as I know is flying dragon air rifles out of Iowa.

Mike Melick modifies the  xs60c from co2 to pre charged pneumatic, and sells the rifle and a hand pump for about $200 shipped.
He also addressed some reliability issues of the design, he is a fantastic tuner, and a very nice person to deal with.

It is currently available up to .25 caliber
But with some tinkering it would be possible it do .32 caliber easy, but as your caliber goes up, your shot count per charge goes down.

The air tube runs under the barrel, so a wood stock would have to be constructed to cover that, similar to the full stocked long rifles,.

A octagon barrel could be fitted with a transfer port (moves the air from the valve to the barrel) think of it as a touch hole but under the barrel,
I will leave it up to you guys to figure if you want to muzzle load the balls, or breech load, the first would be easier.

this is the cheapest precharged pneumatic out there as a complete package..

It operates at a lower pressure than other PCP guns so it requires less pumps.

If you talk with him about your idea of a "muzzleloading air rifle" I know he will be intrigued and will offer a lot of input on what can be done.

Good luck..