There is an easy solution but it requires a little fiddling.
Make a case with a fairly tight neck, no powder or primer. Cover the outside of a bullet with black magic marker and seat it so that it sticks out too far -- you are making a dummy round. Chamber the dummy round. Open the bolt and extract the dummy round. The bullet may be stuck in the rifling, it doesn't matter. If it does, remove the case then pop the bullet out with a cleaning rod. When the bullet is seated in the case at the highest "scratches" made by the neck in the magic marker that is the jam length.
That is actually a little longer than the real max because the bullet will deform a little when you jam it in like this. You can "magic marker" another bullet and seat it .003 to .005 shorter and chamber it. Will probably see rifling marks in the magic marker. Measure the COL. If you make anything longer than this, you will have to force the bolt closed to chamber it.
There are a couple of problems here. First, if you measure 10 bullets from base to meplat, you will see a lot of variation. The solution is to measure COL using one of the "sinclair insert style bullet comparator"-type devices. The base to ogive measurements tend to be more consistent -- and -- that is the part of the bullet that contacts the rifling. So you are measuring what you want to measure, you really don't care how long the round is to the meplat but you do care how long it is to the ogive.
The second issue is easy to solve. If installed, the ejector will tend to push the dummy round off to one side. I cut coils off the ejector spring to reduce the tension. For this particular exercise, you could drive out the roll pin and just remove it. You still get some off-center forces from the extractor but it is a lot less.
If you make the neck too tight, you may be unable to close the bolt. You can fix that. Make sure that the upper edge of the neck is making scratches in the magic marker. If you bell the mouth of the neck, the MM scratches will lie to you. You can use machinist blue instead of MM, it is thicker. It is probably better to remove the firing pin assembly, that is just another spring applying forces to the whole assembly. I usually don't but to each his own.
I would repeat the procedure 3 to 5 times. You don't always get it right the first time. If the neck is really tight, the bullet get slightly distorted - use different bullets for each test. I find that this method give comparable results to my Stony Point tool.
If you have the tool, it is very easy to make your own cases for it. Bore out the primer with the correct drill bit. I have a Wilson case trimmer and case holders for all of my major cases. Use the Wilson case holder, a machinists vice, and a drill press - in a pinch you can hand drill the hole. Tap the hole - brass is very easy to tap. Make sure that the neck is the same or just a smidge bigger than bullet diameter. Done.
