When I purchased a 280AI I pretty much solidified the need to make my own modified cases for use with the Hornady COAL length tool.
A little research showed that I would need a 5/16x36 die which I picked up from amazon for $11. I also found that a 19/64 drill was the correct size so I set out and bought a big drill kit from AIH that had the appropriate size mixed in amongst numerous others that will hopefully come in handy for other projects. A quick amazon search shows the drill bit can be had for under $10 on its own.
I started with once fired cases from the rifles I plan to test and tossed them on a small press I keep loaded with a lee universal decapping die.
I had read a number of posts on how to hold the casing to drill it and I tried them out.
First up was using a inertia bullet puller. It worked pretty well for this purpose but would slip a bit.
With the primer pockets drilled out to 19/64ths I set up the 5/16x36 die and started running it in. It worked for the first little bit then the die kept catching and would spin the case in the collet.
Once it became apparent that process was a losing fight I decided to try another trick I had found on the interwebs. I grabbed out the micrometer and measured my case then pulled out the drill bit set and started measuring them till I found a close match for the 7-08 case.
I grabbed a small scrap of 2x2 and drilled a hole through it then put it on the miter saw and cut it lengthways through the hole. I slapped the new wood jig in a vice and clamped the case down in it.
This proved much more effective and I was able to finish threading it easily. The finished case threaded right onto the Hornady tool and works just like my factory made ones.
In the future I will skip the bullet puller idea and go straight to the wood jig for both drilling and threading. A drill press with a vice would help this process meet much tighter tolerances but I was able to achieve satisfactory results using a hand held cordless drill.