While I understand your concern about a possible blockage after 6 hours it is too late to induce vomiting.
In normal dogs the stomach empties 2-3 hours after eating.
If the shells were broken and in possibly sharp pieces the I would not recommend inducing vomiting. In cases where dogs eat sharp objects the chances of tearing her esophagus or stomach as it contracts to force the pieces out is much too high. I hope she at least chewed them a little before she swallowed as that will make passage a little easier.
In this case we need to try and hope that we can get these to pass through her gastrointestinal tract. And that her stomach acid helps us by dissolving the fragments.
To help I recommend starting with a Vaseline (or any generic petroleum jelly) sandwich. Take a slice of whole wheat bread, slather with Vaseline as you would peanut butter on a slice of bread, cover with a second slice. Feed it to her bite sized piece by piece or she will make a huge mess. This adds fiber and hopefully makes the pieces slip rather than catch.
For the next few days, until the pieces pass, she'll need to be fed a bland diet with lots of fiber added in small meals several times a day, 4 to 6 meals is ideal. We do this so the sharp fragment pieces are surrounded and the gut isn't irritated contracting on just sharp edges.
A homemade diet for this is 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats drained off the meat, and 2/3 boiled, plain, white rice. Add 2 tablespoons of canned pumpkin, (not pie filling, just pumpkin), to each meal for fiber.
You'll need to check her stools frequently for bone pieces in the next few days.
Signs that things aren't going well, and she needs an immediate veterinary visit, are vomiting, a tense painful belly, lack of an appetite, a fever (more than 103.5F rectally) and lethargy.
If you aren't seeing fragments in the next day or two a veterinary visit and an abdominal radiograph to try to see where the fragments are is an option. They may not show up if they are very small. If it doubt at all that things aren't going well it is always best to have her checked.
Once the pieces are passed and she's feeling well then start mixing in her regular food, adding a little more regular and less bland at each meal. It should take a week to get her converted back to regular food.
Let me know if you have any questions.