How large GC's can i buy?

[Chicken Thief]

Have a cal. 12 rifled slug barrel coming and will order a 850 grain mould from Accurate Moulds
http://www.accuratemolds.com/bullet_detail.php?bullet=73-850P-D.png

He will do the gas chech shank for free so in short:

Do i want a gascheck and if so will i have to make them myself?

[waksupi]

I really doubt you will want to fire that at a higher velocity that 1700fps. It is going to kick the snot out of you, assuming it is not a crew served weapon. If you stay below that, you most likely do not need a gas check.

[JeffinNZ]

I once joked to Charlie Darnall that he should make a FC III for stamping out hub caps.

[Sonnypie]

If you have it shanked for Gas Checks, you could choose to not use them.
But if it is Plain Based, you would not have that choice.
How fast do you expect that howitzer to cast that 1/8 pound ingot?

(The biggest bore I ever saw in a hand-held weapon was an inoperable 4 gauge shotgun. That thing looked like it shot tennis balls!)

Your link looks a lot like a friends 69 caliber Navy muzzle loader mini-ball.

[madsenshooter]

Shouldn't need a gascheck, over powder wads will serve as the gas check.