If you don't have any 12 gauge 2 1/2 inch hulls for your short chambered gun, you can easily shorten 2 3/4 inch hulls and use a Hartin crimp.
Just shorten the hull 1/4 inch, use your powder, wads, etc. that you would use for your 2 3/4 inch loads but put an over shot card on top and crimp as you normally would. There's enough material left on the hull to crimp it securely. Of course the over shot card keeps the shot where it belongs. You can mark the card with your load, ie, 0.7 ounces.
Couple pictures below pretty much says it all. I did make a jig to position the hull precisely so that I would squarely cut off 1/4 inch using a serrated steak knife.
Jolly
