Hubby has a Thompson center 45 cal Hawkins black powder gun and the hammer wont stay cocked, what should he do?

Try pushing the trigger forward while cocking it, if it locks it's probably a bad trigger spring. Remove the hammer/lock assembly, it's only two screws, and have a look inside. The lock is very simple and should become obvious once you're in there.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/oscar_403cc49e0f00cda7

I hear black powder is quite corrosive and leaves a lot of residue, especially in calibres of less than half an inch.

I strongly suggest you seek experienced and specialised advice about your question to give you a better chance of enjoying your weapon safely for a very long time.
A traditional gunsmith would be a very good place to begin and making contact with a black powder club would also be a good move.

To be totally honest and I have been hunting with Black Powder guns for over 30 years way back before we even had the In-Line's we have today, I would continue to load the normal way by pouring the powder down the barrel then tapping your bullet in on top. Unless the instruction Manual for your gun says different. I have 6 In-Line Muzzleloaders but all my Breech Plugs are threaded so they screw in and non have any movement once tightened. My newest one is about 6 years old so I'm not up to date on the newer ones. Hope this helped. It's always to be safe when dealing with firearms.

Here's a trigger assembly:

http://www.trackofthewolf.com/imgPart/tr-tc-ne-used_0.jpg

And the backside of the lock:

http://www.trackofthewolf.com/imgPart/lock-tch-r_2.jpg

And how they interact:

http://www.rmcsports.com/flint.gif

NOTE:
The last pic shows a flintlock hammer and frizzen, but the internal mechanism (backside) are the same (shown)

When the trigger is pulled, the square plate on the trigger rises, lifting the back end of part #51, disengaging the sear, parts # 53 &54, allowing the spring (part#55) to drive the hammer.

A load for a 50 caliber rifle calls for 90 grains of FFG black powder or 90 grains of Pyrodex RS (maximum, start with a lighter load, and work your way to a good, accurate loading).

I'm not understanding the rest of your question.

Hello jps.... No problem at all with 150 grains. In fact, that is the spec. max. amount. The Thompson Center Omega Z5 was designed with simplicity in mind. It is easy to shoot and easy to clean. The breech seals the 209 primer from rain and snow and is easy to open with a simple forward motion of the lever on the trigger guard. The sealed breech plug design also prevents the crushing of Pyrodex pellets. The Z5 can handle a powder load of up to 150 grains for more power or longer range shooting.

Technical Information

Notes:

Exposed hammer

Sling swivel studs

Rubber recoil pad

Fiber optic sights

Blue chrome-moly steel with a silver weatherguard finish

Joe
Thanks for choosing Fixya!

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Hello,

I get my parts from www.gunparts.com , They have schematics for many diferent guns.

Hope this helps, Matt

If you need further help, I’m available over the phone at https://www.6ya.com/expert/matt_8dbc24bf722649ac

Check the O rings on the big metal striker, make sure that you have the spring on the back (where the stock would have went) in the right hole on the back plate. Make sure that your velocity is set to a reasonable level.

If you ran out of air, it won't stay cocked.
OR
The sear may have been displaced. That would require a disassemble. It is not that difficult, just do it on a white pillowcase and be sure not to let any springs fly all over the place.