I am working with a friend's M99 250 Savage. I had two case head separations (reloads), so want to check to see if the problem is headspace. If I place tape on the case head a layer at a time, what thickness would begin to suggest a problem?

It should not lock into battery with one layer of tape on the case head.

You should have a headspace guage of course,,using a case of unkn dimensions has you starting from that point,,unkn.
But, lacking that, I would at least use a Factory loaded round to do the shade tree headspace test.
Pull the bullet from the round w/o damaging or altering the brass at all. You don't want the bullet to interfer in your 'reading'.
The reloads probably/most likely have their shoulder pushed back too far. You didn't say how many times the cases had been reloaded. If the brass is over worked, they will weaken and fail.

All 'tape' not being equal, and not starting with a known case length (a factory case instead of a headspace gauge), you are still wandering about in the dark a bit.

Is there a gunsmith/shop near by that can check the headspace for you to be sure. Case head separations are not something to take lightly.

Be careful and best of luck with your rifle.

What case are you going to use? How do you know that the case you're using is of the correct headspace length? you need to get a headspace Gage or start with a known good case.

Many years ago a gunsmith I admired greatly told me to use Scotch tape to check headspace. He told me to FL size a case, and put three layers of Scotch tape on the case head, then chamber the round. If it fit, try one more layer.

He told me the bolt should be tight with four layers. Any more than that was excessive. The way he explained it, Scotch tape was .00015 thick, and a FL die set the headspace back .006.

It made sense to me then, and makes sense to me now...

Remember, also, that the Savage 99 is a rear locking action that allows some case stretching. I load for a 99 in .308. If I load more than starting loads, I get case head separations on the third firing. Maybe I could be more diligent in not over-sizing my brass, but brass is cheap enough for as little as I shoot that rifle.

I would suggest, that the next time you are reloading for this rifle, that you use once fired brass, from this rifle. Back the sizing die off the shell holder about 1/8 turn. Then try chambering one of these, if they fit ok, shoot a few and keep track of the number of times you have reloaded these cases. See if that helps. Just because you have a little excessive headspace, it's not the end of the world in most cases. Neck sizing etc, usually solves many of these problems. Of course action type will dictate what you can can't do.

Jerry

I recently rebarreled a 99 in 250. The original headspace was loose, to be kind. I set the headspace on the new barrel within spec. When I took the gun out to test fire, I got several partial head separations with new old stock Winchester Silver Tips. I took the gun apart to check everything to find out the problem. It all was good. Finally I checked the ammo and the headspace was too short by as much as .020 in. Checking a box of Remington ammo I found the measurement was closer, although still a little short. When I shot that ammo there were no problems.
If I were starting to load for an original gun, I would fire form in one of two ways. Expand new brass to a larger diameter and size down the neck a little at a time until the bolt would just close on the new shoulder. Or, you can fire form the new brass with the cream of wheat method. Use about 10 - 15 gr. of a shotgun powder, fill with cream of wheat, then plug with wax. Before shooting, roll the cartridge on your lube pad to get full expansion. If the cases are not lubed, they will not set back on the bolt face, and the primer will protrude. You can even use 22-250 brass with these methods, which is much easier to find and works perfectly.